Here is your research paper on human genetics, chromosomes and genes!
We inherit some physical and biochemical characters from our parents and ancestors. Transmission of inherited characters or traits through generations is known as the heredity. Genetics is that branch of bioscience which deals with the study of the underlying principles of heredity.
It has been established that the hereditary characters or traits are transmitted by the genes of the chromosomes. The expression of inherited characters is, however, modified by the environments in which an individual grows and develops.
A basic knowledge of human chromosomes and genes is, therefore, essential to understand the principles of genetics.
Chromosomes are deeply stained thread-like structures within the nucleus of each animal cell. Genes are borne by the chromosomes in linear series as parts of specific DNA molecule. Individual chromosomes are visible under the microscope only during cell division.
During interphase of the cell, the nucleus contains a network of chromatin threads or granules but not individual chromosome, because each chromosome becomes uncoiled into long thin thread which is beyond the resolution of the light microscope. But some chromosomes remain coiled at places and these are identified as chromatin granules in the interphase (Fig. 11-1).
The uncoiled portion of the chromosome is known as euchromatin which is genetically active; the coiled portion is called heterochromatin which is genetically inert. During cell division, each chromosome is tightly coiled along its entire length and becomes shorter and thicker. Eventually individual chromosomes are readily visible under the microscope (Fig. 11-2).
Therefore, the chromosomes are genetically inactive during cell division. All biochemical activities of chromosomes in the form of DNA replication, mRNA formation and protein synthesis take place during interphase which consists of three stages of the cell cycle—G1 (Gap 1), S (Synthesis), G2 (Gap 2) stages. DNA replication takes place in S stage and covers a period of about 7 hours.
Each chromosome presents a primary constriction known as centromere or kinetocore which is attached to the achromatic spindle during cell division and organises the formation of the chromosomal microtubule (Fig. 11- 3a).
In prophase of cell division, each chromo-some splits longitudinally into two chromatids except at the centromere (Fig. 1 l-3b). The free ends of the chromatids are known as the telomeres, which when intact, do not permit fusion with the chromatids of adjacent chromosomes. The chromatids of some chromosomes present secondary contsrictions near one end, and the segment of chromatids distal to constrictions form satellite bodies (Fig. 11- 3c, d). The secondary constrictions are believed to organise the formation of nucleoli.
Types of Chromosomes (Fig. 11 -4):
The centromeres occupy variable positions in relation to their pair of chromatids. Accordingly, the chromosomes may be called metacentric when centromere is in the middle, sub-metacentric when the centromere is slightly shifted from the middle, acrocentric when it is situated close to the end, and telocentric if the centromere occupies the end of the chromosome. The telocentric chromosomes are not present in human being, unless pathological. Most of the acrocentric chromosomes exhibit satellite bodies on their shorter arms separated by the secondary constrictions. The shorter arms of chromatids are symbolised by p and longer arms by q.
Number of Chromosomes:
The number of chromosomes is constant in a species. In human, the number is 46 (diploid) in all somatic cells and immature germ cells, but 23 (haploid) in number in mature germ cells or gametes. The exact number of 46 chromosomes in each somatic cell of normal human being was first detected by Tjio and Levan (1956) with the advent of tissue culture.
Some hereditary disorders are associated with the alteration of chromosomal number. When the number is increased by multiple of haploid (23) chromosomes (other than diploid number), the condition is known as polyploidy. If polyploidy, say triploidy or tetraploidy, affects all somatic cells, survival rate is poor. Polyploidy at zygote stage may be due to fertilization of an ovum by more than one spermatozoa.
Under normal conditions, polyploidy may be found in some liver cells and in the mucosa of the urinary bladder. This may occur in telophase of mitosis, when after the formation of two nuclear membranes enveloping diploid number of chromosomes, the cytoplasm fails to divide and the two nuclear membranes fuse enveloping double number of diploid chromosomes.
Aneuploidy is a condition where the chromosome number is altered by one or more, but not by multiples of haploid. Most of the hazards of chromosomal number take place at anaphase. After splitting of the centromeres, one or more chromosomes fail to migrate properly due to abnormal function of achromatic spindle. The phenomenon is known as the nondisjunction.
As a result, both members of a particular pair go to one daughter cell which receives extra chromosome (trisomy), and the other daughter cell is deficient in that chromosome (monosomy). Sometimes after splitting of the centromere one member of the newly formed chromosome separates as usual forming normal chromosome complement in one daughter cell, whereas the other member fails to reach the opposite pole of the spindle resulting in deficiency of that chromosome (monosomy) in the other daughter cell. This is known as anaphase lag.
Non-disjunction may take place in mitosis or meiosis and it may involve sex chromosomes as well as autosomes. Autosomal non-disjunction is less viable, particularly when is affects large chromosomes. Our body is more tolerent to the trisomic cells than the monosomic one. The monosomic cells degenerate early. Turner’s syndrome of female with 45, XO chromosomal constitution is possibly the only example of viable monosomic individual. If non- disjuncion takes place in first cleavage division of the zygote, then all cells are aneuploid and the individual shows mosaicism with half of the total cells being trisomic and other half monosomic.
When non-disjunction occurs in meiosis I, all four gametes are abnormal (two with 24 chromosomes, and two with 22 chromosomes). If it takes place in meiosis II, two gametes are normal and two abnormal. When fertilization takes place between normal and abnormal gametes, all cells of the organism derived from that zygote are aneuploid. Nondisjunction in gametogenesis is sometimes observed in elderly females (35 years and above). Possibly the primary oocyte which starts first meiotic division in prenatal life, completes the process just before ovulation after a prolonged interval of about 40 years more. Delayed completion of first meiosis of oocyte might favour non-disjunction.
Arrangements of Chromosomes:
Fortysix chromosomes in each somatic cell of normal human being are arranged in 23 pairs. Twenty two pairs are known as the autosomes, the genes of which regulate the body characters; the remaining pair is known as the sex chromosomes which regulate primarily the sex characters. One member of each pair is paternal, and the other member maternal in origin.
The pairing takes place between the identical chromosomes which are identical in length, position of the centromere, banding pattern, and distribution of genes. The paired chromosomes are known as the homologous chromosomes (Fig. 11-5).
In female the two sex chromosomes are identical in length and are symbolised by XX. In male, paired sex chromosomes are unequal in length and are symbolised by XY. The longer one is represented by X, and the shorter one by Y. During pairing of the male sex chromosomes, both have homologous and non-ho-mologous parts (Fig. 11-6).
The genes or cistrons, which are parts of specific DNA molecule, are contained within the chromosomes in a linear series. They form the functional units of hereditary characters. The position of a gene in the chromosome is called its locus which is mentioned with reference to the centromere.
The genes do not change the loci, except in alternation of chromosomal morphology or in recombination due to cross-over in meiosis. The genes occupying the identical loci in a pair of homologous chromosomes are known as allelomorphs or alleles (See Fig. 11-5). The allelic genes regulate different specific physical and biochemical characters, through the formation of RNA and biosythesis of proteins.
In chromosome preparation from mitotic cell culture (after arresting cell division at metaphase), the homologous pairs of chromosomes are not visualised. The homologous pairs are only matched during karyotyping from the enlarged photomicrographs. In zygotene stage of the prophase of first meiotic division, however, the homologous chromosomes are found in pairs establishing point to point relationship; this phenomenon is known as synapsis.
Sex Chromatin or Barr bodies:
During interphase, somatic cell of normal female presents a heterochromatin plano-convex body beneath the nuclear membrane. This is known as sex chromatin or Barr body. It was first detected by Barr and Bertram in 1949 in the nuclei of the phrenic nerve cells of female cat. Out of two X chromosomes in normal female, one of them is highly coiled and the other member highly uncoiled. The highly coiled genetically inactive X chromosome forms the Barr body, which is plastered beneath the nuclear membrane (Fig. 11-7).
These bodies help in the nuclear sexing of the tissues. Barr bodies are easily found in those cells, which possess open-faced nuclei. Usually Barr bodies are studied from the cells of buccal smear, or by observing ‘drum stick’ bodies attached to the nuclei of polymorph nuclear leucocytes.
The number of Barr bodies in a cell is equal to the total number of X chromosomes minus one. In a normal female with two X chromosomes the number of body is one. In triple X sydrome (XXX) the number is increased to two; in a female with Turner’s syndrome having only one X chromosome (XO), Barr body is absent. In male with Klinefelter’s syndrome having XXY chromosomes (trisomy), Barr body is present.
The presence of Y chromosome in male is detected as intensely fluorescent body (F- body) within the nucleus, when a buccal smear is stained with flurochrome dye and examined under fluorescence microscope. Since this technique is costly and the slide quickly deteriorates, it is usually not employed to study the sex chromatin status.
Chemical structure of Chromosomes:
On chemical analysis each chromosome is found to contain DNA, small amount of RNA, histone and non-histone proteins, and metallic ions. DNA is the most essential and stable molecular constituent of chromosomes.
Recent studies have revealed that each eukaryote chromosome contains a single continuous double stranded DNA molecule. Most of the DNA molecule exists in the chromosome as a highly coiled or folded structure. DNA in active state of transcription is most extended and becomes euchromatic; inactive DNA region remains highly coiled and becomes heterochromatic. The degree of coiling of DNA varies with the rate of protein synthesis in the different phase of cell cycle.
Two types of permanent hetero-chromatic regions are observed in human chromosomes;
(a) Facultative heterochromatin ’affects inactive X chromosome of normal female. In early embryogenesis of female, both X chromosomes are actively involved in the development of ovaries; thereafter one of X chromosomes becomes permanently inactive and forms a heterochromatic Barr body.
(b) Constitutive heterochromatin is observed in the primary and secondary constrictions of chromosomes. Repetitive sequence of DNA bases, rich in guanine and cytosine, are said to be present in constitutive heterochromatin and in satellite bodies. Repetitive DNA in some parts of chromosomes possibly codes for intrinsic molecules in the form of ribosomal RNAs, transfer RNAs and regulatory proteins.
Histones are basic proteins rich in arginine and lysine. These proteins are aggregated as spheroidal particles along the DNA strand which is coiled around each particle and forms a complex body known as nucleosome or v- body (Fig. 11-8). Each nucleosome is made up of four pairs of histones which are arranged in two symmetrical groups. Experimental evidence suggests that the association of DNA with histone represses gene activity.
Non-histone proteins are acidic and form many enzymes, e.g. DNA polymerase and RNA polymerase. Some of non-histone proteins disengage the histones from nucleosome and derepress gene activity.
Procedure of Chromosomal analysis:
For cytogenetic study of chromosomes, cells are chosen which grow and divide rapidly in culture. The most commonly used tissues are skin, bone marrow and peripheral blood.
The principles of chromosome preparation from peripheral blood are as follows:
(a) About 1-2 ml. of blood is withdrawn from a vein, heparinised and treated with phyto- hemagglutinin, extracted from red kidney bean.
The phytohemagglutinin (PHA) stimulates the lymphocytes (particularly T-cells) to proliferate by mitosis and selectively allows agglutination and sedimentation of mature erythrocytes.
(b) Aliquot of the plasma with suspended lymphocytes is now transferred to culture bottles under sterile condition containing TC199 (Difco) as the culture medium. Incubation in culture bottle continues for about 3 days at 37°C with the addition of streptomycin and penicillin as preservatives.
(c) Colchicine is now added to the culture and kept for about 2 hours. Colchicine arrests the cell division at metaphase, by preventing the formation of microtubules of achromatic spindle. At metaphase, the chromatids united by centromeres are maximally contracted.
(d) Cells are collected by centrifugation of the contents of culture bottle. Hypotonic solution of sodium citrate is added to the cells and incubated for about 20 minutes. The hypotonic solution allows the cells to swell and to disperse the chromosomes.
(e) The hypotonic medium is discarded by centrifugation. Now fixatives of mixture of ethanol and acetic acid are added to the pellet of cells, and are gently shaken to form a cell suspension.
(f) Small drops of cell suspension are placed across one end of chemically cleaned slides. The slides are allowed to dry at room temperature.
(g) Staining—For conventional study of chromosomal pattern, Giemsa stain is widely used with good results (Fig. 11-9).
Precise identification of individual chromosomes is now made possible by noting pattern of bands on chromosomes after applying any of the four different staining techniques:
When fixed metaphase chromosomes are stained with quinacrine hydrochloride or quinacrine mustard, certain chromosome bands appear as fluorescent regions under fluorescent microscopy. These Q- banding patterns (fluorescent) are unique for each chromosome. Presumably the regions of Q-bands are more rich in adenine (A) and thymine (T) bases of DNA than the interband regions. A particularly large Q-band is evident in the distal part of the long arm of Y chromosome, even during interphase.
The fixed chromosomes are subjected to mild treatment with proteolytic enzymes (trypsin) before staining. The enzymes are capable of denaturing the protein in the chromosomes. When stained with Giemsa after such treatment, a pattern of dark staining G-bands may be seen on the chromosomes under a light microscope.
The G-banding and Q-banding regions of chromosomes closely correspond. Comings (1974) has suggested that proteins remaining after denaturation might prevent the staining material from getting into certain regions of DNA. It is possible that less protein might be associated with A-T rich DNA; this explains the concordance of G and Q-bands.
The G bands and Q-band regions are rich A-T base pairs; they correspond with the heterochromatin regions of chromosomes where DNA replication takes place slightly later. The interband regions are rich in G-C base pairs.
This is the reverse of G- binding, where inter band regions are demonstrated by Giemsa stain after heating to 87°C. The R-binding is complementary to G-binding.
After harsh treatment of fixed chromosomes with alkali, acid or salt, the Giemsa stain reveals a stained region, the С-band, close to the centromere. The C-banding is, however, not evident in Y chromosome.
Chromosome banding helps to locate certain abnormalities of chromosome structure, such as deletion and translocation of specific regions of chromosomes.
It is a process of arranging the chromosomes in order. Enlarged photomicrograph of a chromosome ‘spread’ is taken from the stained slide. Individual chromosomes are cut out from the photograph, matched with homologous pairs, and are arranged in a sequence, the longest chromosomes being placed at the beginning and the shortest at the end.
Individual chromosomes are identified according to their length, position of centromere, length-ratio between their arms and presence of satellite bodies on their arms. (Fig. 11-10) Banding pattern adds further to the identification of individual chromosomes. (Fig. 11-11).
Classification of human chromosomes:
According to ‘Denver System’ of classification (1960), human chromosomes including sex chromosomes are arranged into seven groups from A to G, in order of decreasing length.
(1) Group A:
It includes pairs of 1, 2, 3 chromosomes. Each of them is long and metacentric. However, chromosome 2 placed in Group A is the longest sub-metacentric chromosome.
(2) Group В:
It consists of pairs of 4 and 5 chromosomes, which are fairly long with sub- metacentric centromeres.
(3) Group С:
It is a large group and includes pairs of 6 to 12 chromosomes; X chromosomes also belongs to this group. Most of them are of medium size and sub-metacentric. Banding patterns help identification of individual chromosomes.
(4) Group D:
13 to 15 chromosome pairs belong to this group. All of them are medium sized and acrocentric. A satellite body is attached to the free end of the short arm of each chromosome.
(5) Group E:
It includes chromosome numbers 16 to 18. They are fairly short sub- metacentric chromosomes.
(6) Group F:
19 and 20 chromosomes in pairs belong to this group. Each of them is short and metacentric.
(7) Group G:
It includes 21 and 22 pairs of chromosomes; Y chromosome belongs to this group. Each of them is very short and acrocentric, 21 and 22 chromosomes present satellite bodies on their short arms. The distal ends of long arms of Y chromosome present fluorescent bodies after staining with a flurochrome dye.
Points of Observation:
(a) 1 to 3 chromosomes of group A, and 19, 20 chromosomes of group F are metacentric.
(b) 13 to 15 chromosomes of group D, and 21, 22 and Y chromosomes of group G are acrocentric. Five chromosome pairs comprising 13, 14, 15, 21, 22 possess satellite bodies; hence called sat-ch.ro- mosomes. Sat-chromosomes are concerned with orgainsation of the nucleoli.
(c) Rest of the chromosomes are sub- metacentric.
Gene Localisation on Chromosomes:
Gene localisation on particular human chromosomes, although difficult to determine, may be assessed by pedigree analysis, by studying patients with a chromosome deletion and by studying segregation of ‘marker’ genes in families with a particular hereditary disorder. Marker genes are frequent in the general population. The autosomal marker traits include the blood groups and certain serum proteins.
The X-linked marker traits include colour-blindness, the Xg blood group and in some cases glucose- 6-phosphate dehydrogenase deficiency. Pedigree studies have demonstrated close linkage between the gene loci of ABO blood group and nail-patella syndrome, and between the Duffy blood group and one form of congenital cataract.
Gene mapping on individual chromosomes is further improved by using restriction enzymes (endonuclease) which are synthesized by many bacteria. Restriction enzymes divide the DNA into fragments of variable length by cutting between the specific sequence of bases, the sites of which are different for different enzymes. Such restriction fragment length polymorphism (RFLP) acts as DNA finger print, and is detected by adopting Recombinant DNA Technology.
Analysis of DNA structure by RFLP, makes it possible to determine which parent is the source of a defective chromosome. This helps in genetic counselling, in investigation of crimes and determining paternity. About 50,000-100,000 genes are estimated to be present in the total human genome with 3 billion base pairs. As of early 1993, over 2500 loci have been assigned to specific positions on the human genetic map.
Abnormalities of about 450 of these genes have been linked to human diseases. Some of the important gene localisation on autosomes are placed herewith.
1 – Dufy blood group, Rh factor, histone proteins, congenital catract, retinitis pigmentosa.
2 – Red cell acid phosphatase. Kappa light chain of immunoglobulin.
5 – Hexosaminidase-B
6 – Major histocompatibility complex (HLA), spino-cerebeller ataxia, adrenogenital syndrome.
7 – Collagen structural gene.
9 – ABO blood group, nail-patella syndrome.
14 – Heavy chain of immunoglobulin
15 – Hexosaminidase-A 17- Thymidine Kinase
19- Polio and echo virus sensitivity
20 – Adenosine deaminase
21 – Down syndrome gene; a gene for Alzheimer’s disease;
22 – Genes for lambda light chain of immunoglobulin
The X chromosome appears to contain the loci for glucose-6-phosphate dehydrogenase, haemophilia A, colour vision and Becker muscular dystrophy on the long arm, and the Xg blood group, ichthyosis vulgaris, ocular albinism and X-linked mental retardation loci on the short arm.
The Y chromosome contains male determining ‘SRY’ genes, a component of TDF (testis determining factor). Presence of a single Y chromosome induces the development of testes; the foetal testes liberate testosterone and mullerian regression factor, which by local action allow the differentiation of the mesonephric tubules and ducts to develop into the duct system of testes and at the same time help regression of the paramesonephric ducts (mullerian system). Thus Y chromosome by train of events induces the development of male gonads, the sex ducts and external genitalia expressing the male phenotype.
But in ‘testicular feminisation’ syndrome with XY chromosomes, the individual appears to be perfect female with breasts and female external genitalia, but with intra-abdominal testes. Due to genetic defect of Y chromosome, the mullerian system becomes unresponsive to the effects of male hormones liberated by the foetal testes.
A normal male presents XY chromosomes constitution; but when an individual possesses more than one X chromosomes with single Y chromosome (47, XXY; 48 XXXY), the subject is phenotypically male with dysgenesis of seminiferous tubules (Klinefelter’s syndrome). Therefore, Y chromosome presents potent male determining genes, irrespective of number of X chromosomes. But the presence of extra X chromosomes in Klinefelter’s syndrome imparts reduced fertility and makes the individual somewhat mentally retarded.
In addition to male determining genes, Y chromosome contains genes for hairy pinna and H-Y (histocompatibility) antigen. The length of Y chromosome varies from person to person and follows the principle of Mendelism. Due to the presence of H-Y antigen, male grafts are occasionally rejected by females of the same strain.
A normal female possesses XX chromosome constitution. In early embryogenesis both X chromosomes are genetically active and induce the development of ovaries. Thereafter, one X chromosome becomes heterochromatic and genetically inert, and persists as sex chromatin or Barr body (Faculative heterochromatin). Foetal ovaries do not secrete any hormone. Therefore in absence of testes (with or without ovaries) Wolffian system (mesonephric) regresses and Mullerian system (paramesonephric) differentiates into female sex organs and female external genitalia.
On rare occasions an individual with XX chromosome constitution appears male in phenotype; this suggests the presence of testis-determining genes in one of the two X chromosomes which are of Y origin. This rare inheritance is possible in an individual due to cross-over in gametogenesis on the paternal side. It is curiously observed that persons with 45, XO chromosome constitution may remain alive, but 45, YO combination is non-viable.
Structural Alteration of Chromosomes (Fig. 11-12):
It means loss of a segment of chromosome, which may be terminal or interstitial. Interstital deletion resulting from two breaks is followed by a union of the broken ends. In ‘cri du chat’ syndrome, terminal part of the short arm of chromosome 5 is deleted.
Exchange of segments between non-homologus chromosomes is known as translocation. The process of translocation requires breaks of both non-homologous chromosomes, followed by repair leading to an abnormal arrangement. A translocation may not always produce abnormal phenotype, but it can lead to formation of unbalanced gametes and carries a high risk of abnormal progeny.
Reciprocal translocation between two pairs of non-homologous chromosomes may be heterozygous when only one of the chromosomes in a pair is involved, or homozygous when both members of a chromosome pair have exchanged segments with one another. Sometimes translocation involves three breaks and a broken part of a chromosome is inserted into a non-homologous chromosome, whereas other non-homologous chromosome presents interstitial deletion.
Robertsonian translocation or centric fusion is a special type of translocation in which the breaks occur at the centromeres of the two chromosomes and whole chromosome arms are exchanged. In a man, it usually involves two acrocentric chromosomes, e.g., between groups D and G, 21/22 or 21/21. In D/G translocation long arm of G chromosome is fused with long arm of D chromosome, and the fragment formed by the fusion of the short arms of the two chromosomes is lost.
Mother of a translocated Down’s syndrome is usually a carrier of D/G translocation with only 45 chromosomes. She produces four types of gametes—one with normal D chromosome, one with normal G chromosome, one with translocated D/G chromosome like carrier mother, and one with D/G chromosome and a normal G chromosome.
The offspring derived from the last variety of gamete will have 46 chromosomes but will be trisomic for chromosome 21 with manifestation of Down’s syndrome. Therefore, a carrier mother with D/G translocation will have a risk of getting a child with Down’s syndrome. When a mother carries a translocation involving both chromosomes 21, all her children will have Down’s syndrome.
A part of a chromosome is detached and later unites with the same chromosome in inverted position. The genes are not lost but placed in altered loci.
The centromere of a chromosome, due to abnormal anaphase (mitosis or meiosis), splits transversely instead of longitudinal splitting. This culminates in the formation of two chromosomes of unequal length, each presenting metacentric chromosomes with duplication of genes. The resulting chromosomes derived from transverse splitting of the centromere are known as iso-chromo- somes.
It is a process of addition of a portion of chromosome from another homologous chromosome with duplication of genes. Duplication effects of genes due to isosplitting of one X chromosomes are sometimes observed in Turner’s syndrome.
Ring chromosome is observed when a chromosome is deleted at both ends, and then the deleted ‘sticky’ ends are adhered to each other in the form of a ring. The manifestation of ring chromosome depends on the deletion of specific genes.
Symbols Used in the Cytogenetic:
p—Short arm of chromosome
q—Long arm of chromosome
+or -Sign: When placed before an appropriate symbol, it means addition or missing of the whole chromosome. For example, trisomy 21 Down’s syndrome may be represented as 47, XY+21.
When + or – singns are placed after a symbol, these indicate increase or decrease of the length of chromosome. For example, cri du chat syndrome affecting a male child with deletion of short arm of chromosome 5 is represented as 46, XY, 5p-
In philadelphia or Ph’ chromosome, reciprocal translocation occurs between long arm of chromosome 9 band 34 and long arm of chromosome 22 band 11. Therefore, karyotype of this disease is-t (9; 22) (q34; ql 1).
The notation is further refined to indicte particular bands on any specific chromosome.
Diagonal line across the chromosomes or their number indicates mosaicism, eg. XY/XX; XO/XX; XY/XXX; 45/46/47.
Genes are the units of heredity and are composed of part of specific DNA molecules. As mentioned before, genes are arranged in linear series within the chromosomes with precise sequence and number of DNA bases, different for different genes, and having a defined beginning and a defined termination. Since a single chromosomes contains one double helix of DNA molecule in a tightly coiled form, numerous genes or cistrons are borne by a single DNA molecule.
The position of a gene in the chromosome is called locus, which is measured with reference to the centromere. Usually the genes do not change the loci, except in recombination during crossing over or in alteration of chromosomal morphology.
The genes occupying identical loci in a pair of homologous chromosomes are called the allelomorphs or alleles. Broadly speaking, allelic genes regulate different physical and biochemical characters of an individual. Considered from molecular level, one pair of allelic genes regulates synthesis of one polypeptide chain.
When allelic genes regulating a particular character or trait, say height, work in the same direction (both tall or both short), they are called homozygous; when working in opposite direction (one tall and the other short), the alleles are heterozygous. Most of the hereditary traits are polygenic and produced by the complex interaction of numerous genes and influenced by environment. Sometimes, a pair of allelic genes may influence more than one character; this is known as pleiotropy.
Chemical structure of DNA (Fig. 11-13):
It has been established in 1953 by Wilkins, Watson, and Crick on X-Ray diffraction that the DNA molecule is composed of two strands of polynucleotides arranged in a double helix. Each strands consists of a backbone of altenate pentose sugar (D-2-deoxyribose) and phosphate molecule, and the two strands are held together by hydrogen bonds between the nitrogenous bases, which are attached to the sugars as side group and point towards the centre of the helix.
The bases are of two kinds, purine and pyrimidine. A purine in one strand always pairs with a pyrimidine in the other strand. Purine bases include adenine (A) and guanine (G); pyrimidine bases include thymine (T) and cytosine (C). Base pairing is specific under normal condition (when in keto form)—adenine pairs with thymine having two hydrogen bonds and is represented by A =T; guanine pairs with cytosine by three hydrogen bonds and represented by G=C.
This shows that during de- naturation of DNA, separation of the two strands at A=T level is quicker than that of G=C level. However, when the bases are in enol form, adenine may pair with cytosine and guanine with thymine. This is the basis of mutation of genes.
The two strands of DNA molecule are complementary to each other. If the base sequence of one strand is known, the base composition of the other strand can be formulated. Sequence of bases and number of nucleotides of DNA are specific, and are different in different genes. Thus innumerable forms of DNA exist in the genes and store diverse genetic information.
Functions of DNA molecule:
DNA molecules possess the following potentialities:
(1) Self Replication
(2) Biosynthesis of RNA and proteins
Self Replication (Fig. 11-14):
During nuclear division the two strands of DNA molecule separate, and each strand acts as a template and organises the formation of a new complementary strand from a pool of nucleotides as a result of specific base pairing. In this way when the cells divide, the genetic information’s are transmitted unchanged to each daughter cell. Both strands participate in the process of DNA replication, which takes places in S-phase (synthesis) of cell cycle. Replication involves several enzymes, such as DNA polymerase, DNA ligase and specific endonuclease.
Biosynthesis of RNA and Proteins:
DNA molecule also acts as a template for the synthesis of RNA, and the latter conveys the genetic message and deciphers the synthesis of specific polypeptide chain of proteins by linear linkage of amino acids. Therefore, the central dogma of molecular genetics includes DNA→RNA by a process of transcription, and RNA→ proteins by translation.
RNA (Ribose nucleic acid) differs from DNA basically in three ways: it possesses usually a single stranded polynucleotide chain; pentose sugar is D-ribose; out of four organic bases three are similar to DNA (Adenine, Guanine, Cytosine), and the fourth one is uracyl instead of thyamine. Therefore, during transcription from DNA to RNA adenine pairs with uracyl (A=U). RNA exists in three forms—messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Polygenic DNA molecule acts as a template for all three varieties of RNA. Unlike DNA replication, only one of the two strands of DNA molecule acts as a template for RNA.
Polynucleotide chain of mRNA is formed within the nucleus by the side of any one strand of DNA molecule with the help of RNA polymerase. During synthesis of RNA, the two strands of DNA separate (Fig. 11-15). Strand selection of DNA, for RNA synthesis, takes place with the help of RNA polymerase I for rRNA, polymerase II for mRNA and polymerase III for tRNA. Messenger RNA thus formed conveys genetic message with complementary base sequence, and moves into the cytoplasm through the nuclear pores.
A number of cytoplasmic ribosomes (containing ribosomal RNA and proteins) are attached to the polynucleotide chain of mRNA. The ribosomes are the sites where polypeptide chains of proteins are formed by the linear linkage of different amino acids.
The amino acid sequence and number are specific for different proteins; these are determined by precise reading of the base sequence of mRNA in 5′ end to 3′ end direction. Twenty (20) amino acids are involved in the biosynthesis of proteins. Before the formation of peptide linkage, the amino acids are activated and attached to one end of specific transfer RNA molecule (tRNA). Base sequence of tRNA carrying activated amino acids identifies complementary base sequence of mRNA and is attached to the latter by hydrogen bonds until a polypeptide chain of protein is formed.
Therefore mRNA, rRNA, tRNA and a number of enzymes are actively involved at different steps of biosynthesis of protein. The complicated process of biosynthesis from the polynucleotide chain of mRNA to the polypeptide chain of protein is known as translation (Fig. 11-16). The polynucleotide chain of mRNA may be monocistronic or polycistronic.
Since bases of DNA or RNA and amino acids of proteins are arranged in linear sequence, there must be some co-relation between nitrogenous bases and amino acids. DNA or RNA presents four (4) bases, and primary structure of proteins is composed of twenty (20) amino acids. After laborious experiments Nirenberg and Matthaei in 1961 established that a sequence of three (3) bases of mRNA (and therefore of complementary DNA) codes for one amino acid.
Since three consecutive bases are specific for one amino acid, the possible number of combinations of four bases taken three at a time would be 43 or 64. Such triplet of nucleotide based is called a codon. Finally, all 64 codons are discovered specifying different amino acid. However, three codons such as UAG, UGA, and UAA do not code for any amino acid; hence these three are called nonsense or terminal codons and signal the termination of polypeptide chain.
Three unpaired bases attached to one loop of tRNA are known
as anti-codons which fit with the complementary codons of mRNA. Whereas codons are read from 5′ end to 3’ end direction, anticodons are read from 3’ to 5′ direction; as stated earlier, tRNA carries activated amino acid at one end of the chain.
The Genetic Code on mRNA, and the amino acids for which they code.
Codon obey some principles:
(a) Codons are non-overlapping and follow a strict sequence along the polynucleotide strand of mRNA.
(b) They are universal and applicable to all organisms.
(c) Degenerative codons — When two or more codons stand for the same amino acid, they are said to be in degenerative form. GUU, GUC, GUA, GUG code for Valine; UUU, UUC code for phenyl alanine; UUA and UUG stand for leucine. In most cases, the first two bases remain unaffected and alteration of third base produces degeneration.
(d) Ambiguous or mis-sence codon specify different amino acids. Under normal condition UUU stands for phenyl alanine, but in presence of streptomycin it may code for leucine or isoleucine.
(e) Initiation or starting codon—AUG codes for methionine and acts as a start signal in the synthesis of polypeptide chain. The sequence of amino acids in the polypeptide chain is known as the primary structure of protein.
The free amino group at one end of the chain is known as the N-terminal end, and the free carboxyl group at the other end of the chain is called the С terminal end. Each amino acid in the chain is called a residue. The N terminal residue is considered as the first number, and the C- terminal residue as the last number of the amino acid sequence.
Methionine in the initiation complex is formylated by specific enzymes so that the peptide bond does not take place at the N-terminal end. Two codons, AUG and UGG stand only for a single amino acid; AUG for methionine and UGG for tryptophan.
(f) Terminal or non-sence codon. Three codons such as UAG, UGA and UAA do not code for any amino acids. The terminal codons signify the termination of polypeptide chain.
Current concept of gene organization:
1. As mentioned earlier, a gene is a part of specific DNA molecule which regulates the synthesis of one polypeptide chain. A typical gene is composed of a strand of DNA that includes a transcription unit and a promoter region.
The transcription unit consists of several segments of exons that dictate the formation of proteins, separated by segments of introns that are not translated into proteins. A pre- mRNA is formed from the DNA, and then the introns are eliminated in the nucleus by a process of post-transcriptional splicing, so that the final mRNA which enters the cytoplasm is made up of exons only.
The promoter region lies on the 5′ end side of the transcription unit of the gene. It contains various DNA segments which precede the transcription unit from 3′ end to 5′ end side in the form of Specifier, Quantifier and Regulator segments. The base sequence of specified segment includes TATA (popularly called TATA Box), which ensures that the transcription starts a proper point. Z-DNA is a segment of promoter region, that may determine tissue-specific expression.
2. Post-translational modification. After polypeptide chain is translated through mRNA, rRNA and tRNA, the final protein product is modified by a combination of reactions that include hydroxylation, carboxylation, glycosylation or phosphorylation of amino acid residues. A larger polypeptide is converted to a smaller form by cleavage of peptide bonds; thereafter the protein is folded into its complex configuration.
A typical eukaryotic cell synthesizes about 10,000 different proteins during its life time. Proteins synthesized by the genes may be one of three types—enzymes, structural proteins and regulatory proteins.
3. The cytogenetic analyses in hydatidiform mole, a tumour or trophoblastic, suggest that the abnormal ovum loses its own nucleus and is fertilised by two sperms. Thus the zygote contains two male pronuclei, possessing between them at least one X chromosome, In complete molar pregnancy trophoblastic membranes develop, but embryos do not appear, Genomic imprinting suggests that the maternal chromosomes regulate embryoblast development, and the paternal chromosomes regulate trophoblastic development.
During crossing over in meiosis, there is exchange of genetic material between homologous chromosomes. This leads to recombination or shuffling of genes. One of the two events might be observed in crossing over. The two different genes that were originally located on the same chromosome of a particular chromosome pair, might be separated from each other and thereafter are distributed to both homolgous chromosomes; or one of the two genes originally located in each homologous chromosome might be brought together on the same chromosome.
When two different genes are located on the same chromosome pair, they are said to be linked. Crossing over is more likely to occur between the genes on a particular chromosome which are far apart than the genes which are close together. One can assess the relatives distances between genes on any chromosome by determining the frequency with which crossing over takes place between these genes. Genetic distance between two loci on a particular chromosome is expressed in centimorgan (cM). Two loci are 1cM apart, if there is a 1% probability of cross over between them in meiosis. On an average 30 to 35 cross over per cell are estimated to occur during meiosis in males, and perhaps twice as many during meiosis in females.
By determining the frequency of recombination due to cross-over among the progeny, it is possible to frame a linkage map in human with the grouping of genes on particular chromosomes. (Vide supra, in gene localisation on chromosomes)
Recombination of DNA fragments can be studied experimentally by allowing fusion of cells from two different species and then placing in culture. The fused cell-hybrids contain chromosomal constitution from both species and exchange segments of DNA as they regenerate and divide. All these regeneration processes involve random exchange of DNA sequences, and eventually protein synthesis is significantly changed from the pre-fused ancestor cells.
In 1972, Jackson et al. described the biochemical methods for cutting DNA molecules from two different organisms, using restriction enzymes, and recombining the fragments to produce biologically functional hybrid DNA molecules.
Subsequently, scientists successfully inserted the genes for both chains of insulin into some strain of Escherichia Coli, and after isolation and purification, the A and В chains were joined by disulphide bonds to produce human insulin. With the discovery of ‘Recombinant DNA’ technology, a number of essential substances such as, human insulin, interferon, human growth hormone, calcitonin and many others are produced commercially.
A change of a base pair of the DNA molecule is known as the gene mutation (point mutation). Since genes are responsible for the synthesis of protein through transcription from DNA to RNA and translation from RNA to protein, the mutation may have the following diverse effects on the corresponding protein:
(a) The changed triplet codon may code for the same amino acid without any alteration of the resulting protein. About 20 to 25% of all possible single base changes belong to this type.
(b) In about 70 to 75% cases a single base mutation may code for a different amino acid and result in the synthesis of an altered protein which produces reduced or complete loss of biological activity.
(c) In about 2 to 4% cases of single base mutation, the triplet may signal the termination of a peptide chain which is unable to retain normal biological activity.
(d) On rare occasions, more than a single base in the DNA sequence may be involved in a gene mutation. As a result the level of a particular enzyme may be reduced because it is not synthesized or synthesized with reduced activity. Sometimes, a gene mutation may lead to increased synthesis of enzymes with increased activity.
(e) In some cases of genetic disorders a specific protein may be synthesized, but the protein remains functionally inactive. This happens in most cases of haemophillia.
Normally base pairing in replication or transcription takes place in keto form, where the combinations are A=T (in DNA), A=U (in RNA), G=C. But in a gene mutation base pairing occurs in enolfrom, in which combinations are A=C, G=T (in DNA), G=U (in RNA). Such unusual bair pairing is known as tautomerisation.
Mutation may be spontaneous or induced by various chemical or physical agents, e.g. mustard gas, radiation from X-rays, gamma rays from radium and other radio-active atoms. Mutant genes may be inherited or appear at random. One of the typical examples of a gene mutation is observed in sickle-cell anaemia, where the beta chain of adult haemoglobin containing 146 amino acids possesses Valine, instead of glutamic acid, in the 6th position.
RNA directed DNA synthesis:
It has been suggested by Temin in 1972, from the study of RNA viruses that the flow of genetic information occasionally occurs in reverse direction from RNA to DNA with the help of reverse transcriptase. Such viruses are known as retroviruses, which when introduced into the host animal cell incorporate with the specific region of strand of nuclear DNA by a process of recombination.
This forms the basis of the study of oncogenes. Certain regions of DNA in normal cells serve as templates for the synthesis of RNA and the latter in turn acts as template for the synthesis of DNA, which is subsequently incorporated with the nuclear DNA. The resultant amplification of certain regions of DNA helps in embryonic differentiation and possibly in the pathogenesis of cancer.
Types of Genes:
1. Dominant gene expresses its physical or biochemical trait, when the allelic genes are either homozygous or heterozygous for the trait. This follows Men- delian patterns of inheritance and can be observed from the pedigree record of the family. Tallness is caused by dominant gene. The genetic constitution of a tall individual may be T:T or T:t (T for tallness, t for shortness). Most of the dominant traits are expressed in heterozygote state (Fig. 11-17).
Genetic disorders caused by mutation of autosomal dominant genes possess the following characteristics:
(a) The trait is passed on from one generation to another. It has a vertical transmission. Each affected person usually has an affected parent. Sometimes the disorder may appear suddenly in one generation. This may result from a fresh mutation; or if the parent with abnormal gene died in early life before the disease could manifest, the history of parent’s affection may be lacking. This is so in Huntington’s chorea, where the disease is expressed in mid-adult life.
(b) When one of the parents is affected, the risk of having an affected child is 50%.
(c) Since the trait is autosomal, both sexes may be equally affected. Some autosomal genes are expressed preferentially in one sex. These are called sex- limited genes. Gout and pre-senile baldness predominantly affect the males.
(d) If the affected individual marries a normal person, half of their children will be affected.
(e) The degree of expression of abnormal trait may vary in different members of the same family. For example, in poly- dactyly some member shows a small wart-like appendage on the side of the hand, whereas other member exhibits a complete extra-finger. Sometimes a gene, when non-penetrant, may not express it at all. If a child and a grandparent have the same disease and the middle generation does not show any manifestation, the condition is said to have skipped a generation.
(f) The unaffected mambers of a family do not transmit the trait further.
2. Co-dominant genes:
When both the allelic genes are dominant but of two different types, both traits may have concurrent expression. In ABO blood groups, A gene and В gene are both dominant; when they occupy identical loci in homologus chromosomes, AB blood group is expressed (Fig. 11-18).
3. Recessive genes:
Expresses the trait only in homozygote state that means when both alleles are recessive for that trait (Fig. 11- 19). Therefore following the Mendelian principles the genetic constitution of a short individual is t:t (t for shortness).
Diseases caused by mutation of autosomal recessive genes present the following characteristics:
(a) The disease is transmitted by a couple both of whom are carriers of one abnormal gene, but are themselves healthy because the other allele is normal.
(b) The pattern of transmission appears horizontal in pedigree analysis because often siblings are affected, whereas the parents are normal.
(c) The risk of having an affected child (with the double dose of the abnormal gene) to a carrier couple is 25%. Therefore, most carrier couples, if advised properly, would not take the risk of having another affected baby unless prenatal diagnostic facilities are available for the trait.
(d) Most of the metabolic abnormalities are inherited as autosomal recessive traits. The heterozygote status of a carrier couple (having one affected child) may be detected biochemically in several inborn errors of metabolism. The enzyme levels in the heterozygotes are about 50% less than the control.
(e) Since the condition is autosomal, both sexes are likely to be affected equally.
(f) The parents of individuals affected with autosomal recessive traits are often related, since the marriages between close blood relatives (cousin marriages) are more likely to carry the same genes from a common ancestor. The rarer a recessive disease, greater is the frequency of consanguinity among the parents of affected individuals.
(g) If two persons homozygous for a recessive condition were to marry and have children, all their children would be affected. But this is not so in every case. In one family both parents were albinos (recessive disorder), but their children were normal; careful examination of the father revealed that he had a different type of albinism from his wife.
4. Carrier Gene:
Heterozygous recessive gene acts as a carrier which may be expressed in subsequent generations. When both parents are heterozygous tall (T:t), the possibilities of the height of the offspring may be such that out of four children three are tall and one short, in the proportion of 3:1. One tall child is homozygous, and the other two are heterozygous.
5. Sex-linked Genes:
The genes located on the X chromosome or Y chromosome are known as the sex- linked genes. Mutation of X-linked genes is more common, and is mostly expressed as recessive traits.
Х-linked recessive traits (Fig. 11-20):
Haemophilia, partial colour blindness, glucose- 6-phosphate dehydrogenase deficiency, Duchenne’s muscular dystrophy are examples of X-linked mutant recessive genes. These traits exhibit the following characteristics:
(a) The female (XX) becomes carrier of the disease when one X chromosome contains an abnormal gene, whereas the allelic gene of other X chromosome is normal. So the females do not express the disease in heterozygous state. On the other hand, when the abnormal gene involves the non-homologous part of single X chromosome of a male (XY), the disease is expressed in that individual because the defective gene has no corresponding allele in Y chromosome to counter-act. Hence, the affected male is called hemizygous. Broadly speaking, in X linked recessive traits the females are the carriers and the males are the victims of the disease.
(b) When mother is a carrier and father is healthy, 50% of the sons are affected by the disease and the remaining 50% are normal; 50% of the daughters are carrier of the disease and the rest are free. Therefore, when a boy is haemophilic, his mother should be a carrier and 50% of his sisters are carriers of the disease. However, the healthy brother or carrier-free sister of a haemophilic individual does not transmit the disease to the next generation.
(c) When mother is a carrier and father is haemophilic, half of the sons are affected and half are healthy; half of the daughters are affected and half are carriers. This shows that females may be affected in such parental combination, but the possibility is remote because the haemophilic male usually dies early before attaining parenthood. The above combination further shows that there is no male to male transmission.
(d) If the affected males do not reproduce, the pedigree pattern of an X-linked recessive trait tends to be oblique because the trait is transmitted to the sons of carrier sisters of affected males.
(e) On rare occasions, a female may exhibit X-linked recessive trait. This can be explained as follows:
(i) She might be a Turner female (XO);
(ii) Physically female appearance is due to testicular feminisation with XY chromosomes;
(iii) Affected female may have carrier mother and affected father; or a carrier mother and a normal father with fresh mutation affecting X chromosome.
X-linked Dominant Traits:
These are observed in Vitamin D resistant rickets and Xg blood group. The characteristics of dominant traits are as follows:-
(a) An affected male transmits the disease to all his daughters, but to none of his sons.
(b) Both males and females are affected, but the disease is less severe in females.
This is also known as the holandric inheritance, where only males are affected. The affected male transmits the trait to all his sons, and to none of his daughters. Male to male transmission is suggestive of Y-linked inheritance.
Hairy pinna and H-Y histocompatibility antigen manifest holandric inheritance.
Symbols used in Pedigree Chart (Fig. 11-21):
Autosomal Dominant Inheritance (Fig. 11- 22)
Some examples of autosomal dominant traits—
ii. Osteogenesis imperfecta;
iii. Brachydactyly, polydactyly, syndactyly;
iv. True pophyria with port-wine urine due to porphyrins;
v. Sex limited, gout and baldness affecting predominantly pre-senile males;
vi. Huntington’s chorea, appearing at about 50 years or after;
vii. Angioneurotic oedema;
viii. Familial hypercholesterolaemia;
ix. Diabetes insipidus;
x. Marfan’s syndrome, manifested by elongated extremities, dislocation of lens of eyes and cardio-vascular abnormalities;
xi. Nail patella syndrome, manifested by dystrophy of nails, absence of patella and nephropathy;
xii. Multiple neurofibromatosis;
xiii. Polyposis coil.
Some examples of X-linked recessive traits-
i. Haemophilita-This is due to functionally defective antihaemophilic globulin.
ii. Partial colour blindness-It is expressed as inability to distinguish between red and green.
iii. Duchenne’s muscular dystrophy.
iv. Glucose-6-phosphate dehydrogenase deficiency-It is manifested by haemo- lytic anaemia when treated with primaquin, phenacetin, nitrofurantoin, some sulphonamides and acetyl salicylic acid.
v. Testicular feminisation.
vi. Hunter’s syndrome-It is due to deficient enzyme induronosulphate sulphatase, and is manifested by feature of Hurler’s syndrome except corneal clouding.
Autosomal Recessive Inheritance (Fig. 11- 22) 23):
Some Examples of autosomal recessive traits—
(1) Inborn errors of metabolism;
i. Acatalasia, due to deficient enzyme catalase; it leads to oral sepsis;
ii. Albinism, a complete depigmentation of ‘ skin due to deficiency of tyrosinase;
iii. Alkaptonuria, in which affected persons excrete dark coloured urine due to the presence of homogentisic acid. It is caused by the lack of enzyme homogentisic acid oxidase;
iv. Galactosaemia, due to deficiency of Galactose-I-phosphate uridyl transferase, and is manifested by vomiting and diarrhoea as a result of intolerance to galactose; this is followed by mental retardation, cataracts and cirrhosis of liver;
v. Hurler’s syndrome, caused by deficient enzyme iduronidase, and manifested by mental retardation, skeletal abnormalities, hepatosplenomegaly and corneal clouding.
vi. Phenylketonuria, due to lack of phenylalanine hydroxylase and manifested by mental retardation, fairy skin and epilepsy;
vii. Tay-sachs disease, due to lack of hexosaminidase, and manifested by mental retardation, blindness and neurological abnormalities.
i. In sickle-cell anemia, beta chain contains valine in the 6th position, instead of glutamic acid. Heterozygous sickle cell-traits are more resistant to the attacks of malaria.
ii. Thalassemia major is expressed in homozygotes, and thalassemia minor in heterozygotes.
i. Some of the immunological disorders may be due to autosomal recessive traits.
X-linked Recessive Inheritance (Fig. 11-24):
Genetic Factors in Some Common Diseases:
Early onset diabetes (juvenile-I.D.D.M) is more genetically predisposed than late onset diabetes. Some investigators suggest that is possesses autosomal recessive inheritance, while others believe that is has multifactorial inheritance. In genetically predisposed individuals, prediabetics are recognised by raised serum level of islet-cell antibodies.
There are two schools on the modes of inheritance; one school suggests that it possesses multifactorial inheritance, whereas other school believes that it is due to mutation of single dominant gene.
Ischaemic heart diseases:
Early onset ischaemic heart disease is due to familial hyper- cholesterolaemia which is inherited as an autosomal dominant trait. In the majority of the affected individuals the condition is multifactorial with a heritability of about 65%.
Duodenal ulcer is more frequent in individuals with О group of blood and in non-secretors of ABO substance. Forty per cent of the peptic ulcers possesses hereditary predisposition.
It is inherited on a multifatorial basis with a heritability of about 85%. Some believes that it is inherited as autosomal dominant traits.
Some Terminology used in Genetics
The genome indicates the full set of genes, haploid in the gametes and diploid in the somatic cells of an individual.
It means the genetic constitution of an individual which is fixed at the time of fertilization. The genotype of a tall individual may be T:T (homozygous) or T:t (heterozygous) which may be assessed by pedigree analysis.
It means physical or biochemical expression of the genotype. Phenotype is potentially variable and is the result of interaction between the genotype and the environment in which the individual develops and grows. It may so happen that an individual with T:T genotype is short in height. This is presumably due to some endocrinal or nutritional disorders which suppress the action of genotype.
Sometimes a change in the environment produces a new phenotype, which closely resembles the appearance causes by a specific genotype. Such form of phenotype is known as phenocopy.
Jumping Genes or Transposons:
These are groups of genetic elements that really can move from place to place and by doing so modify or suppress the function of target genetic region. The jumping genes include pseudogenes, the retroviruses and oncogenes, and possess DNA sequences that jump. Each jumping gene has a short, similar, terminal repeat of bases at either end.
Each has the property of recognition of a specific sequence on the target DNA and generates a direct repeat of the same. Coming to the target sequence, the movable genes produce asymmetrical breaks on the opposite strands of the DNA duplex and then are integrated in the target site.
The transposons control mutation and recombination, and may be responsible for amplification of genes. The functional status of the jumping genes is still inconclusive.
Whenever an individual or a couple with genetic disorder seeks advice, the genetic counsellor is confronted with three problems;
(a) To establish the precise diagnosis (genetical or environmental) by clinical examination and laboratory investigations;
(b) To discuss the prognosis and value of any possible treatments;
(c) To determine the risk of recurrence of the disease in a family and to investigate the carrier detection, if any.
Chromosomal studies and karyotypes are indicated in the following conditions;
(i) In infants with congenital anomalies involving more than one system;
(ii) In abnormal sexual development;
(iii) Infertility, recurrent abortions etc.
When chromosomal defects (numerical or structural) are detected with abnormal phenotypes, treatment, if any, is symtomatic and not curative.
Discussion about the Risk of Recurrence in a Family:
(1) If both parents have normal chromosomes, although the child is affected by chromosomal abnormality (say, Trisomy 21-Mongol), the parents may be assured that the chances of recurrence of the same condition affecting future children are less, because the cause of this abnormality is non-disjunction in gametogenesis particularly involving elderly mother, and the phenomenon is mostly accidental.
If, however, the karyotype of Mongol baby shows translocation between G and D chromosomes (46) and the karyotype of the healthy mother shows balanced translocated chromosomes, the parents should be informed that similar Mongol baby might develop more frequently in subsequent pregnancies.
(2) In affected person with heterozygous autosomal dominant gene (say, Achondroplasia), the risk of recurrence among the offspring is 1 in 2 (50%), provided the dominant gene is fully penetrant.
(3) In autosomal recessive disorders, when both parents are healthy with heterozygous recessive gene for the same trait, chance of recurrence (say, phenylketonuria) among the offspring is 1 to 4. All of us are carrying about 3 to 8 detrimental recessive genes, but chance of expression of autosomal recessive disorder is rare, except in consanguinous marriages. When a phenylketonuric father gets married to his first cousin, the chance of the affected child is about 1 in 12, whereas in marriage with unrelated person the chance is about 1 in 10,000.
(4) In sex-linked recessive disorder (say, haemophilia) when a boy is affected, his healthy mother should be a carrier and 50% of his sister are carrier of the disease. The carrier detection is a important task of the genetic counsellor. When an X-linked affected male (say, partial colour blindness) begets children, all daughters are carriers and all sons are normal.
(5) Sometimes individual seeks advice whether he will be affected by diabetes mellitus, since both of his parents are suffering from diabetes (autosomal recessive disorder.)
In such event, apart from blood glucose analysis of the individual, the anti-islet cells antibody titre of his serum will provide information whether he is pre-diabetic. He is then advised to follow dietary restriction.
Detection of Carriers:
The carriers may be detected by the following methods:-
(1) Low level of catalase in acatalasia;
(2) Elevated serum level of creatin kinase in Duchenne’s muscular dystrophy;
(3) Reduces factor VIII in haemophilia A;
(4) Reduced factor IX in haemophilia B;
(5) Reduced erythrocyte glucose-6- phospate in G-6-PD deficiency.
(1) Prenatal determination of foetal sex by sex-chromatin study;
(2) Lecithin-sphingomyelin ratio of amniotic fluid for detection of foetal lung maturity;
(3) Alpha-fetoprotein level of amniotic fluid for detection of anencephaly and open spina bifida.
Use of foetoscope for collecting foetal blood from umbilical vessels helps prenatal diagnosis of sickle-cell anaemia and beta thalassaemia.