MEIOSIS & MENDEL'S LAWS IMPORTANT CONCEPTS - CHARACTER: Feature of an organism, i.e. seed color, flower color, height - TRAIT: Now called a gene. Stretch of DNA associated with a given trait. - DIFFERENT FORMS OF GENES (TRAITS) EXIST Yellow seed trait = yellow seed gene Green seed trait = green seed gene - DIFFERENT FORMS OF GENES OF ONE CHARACTER: Now called an allele. If two different genes for the same character exist, then there exist two alleles for that character. (ie eye color) EACH ORGANISM HAS 2 GENES FOR EACH CHARACTER Each chromosome has genes for many characters. Each of the two chromosomes has a gene for a particular character. ARRANGEMENT OF GENES OF AN ORGANISM - HOMOZYGOUS: Two genes for a character are the same allele. E. g. allele for a red shirt on each chromosome. (YY) - HETEROZYGOUS: Two genes are different alleles E. g. a red shirt allele on one chromosome and a yellow shirt allele on the other chromosome. (Yy) PHENOTYPE: How the character is expressed or seen by the viewer Example: if a Red Shirt is seen it is a RED phenotype If a Yellow shirt is seen, it is a YELLOW phenotype GENOTYPE: Designation of the alleles carried on the chromosomes of an organism for a given character. (ie YY, Yy, yy) DOMINANT ALLELE: Visible whenever it is present, need only 1 dominant allele to give dominant phenotype RECESSIVE ALLELE: Visible only in absence of dominant allele, usually need 2 recessive alleles to give recessive phenotype Alleles are designated with a letter. The dominant allele is usually a capital letter with the first letter of the character as its designation. E. g., Yellow = Y The recessive allele is usually the lower case of the dominant allele letter. E. g., Red = y Example: YY = two yellow alleles = homozygous for the yellow allele Yy = one yellow allele one red allele = heterozygous yy = two red alleles = homozygous for the red allele Mendel's laws A. LAW OF SEGREGATION 1. The two genes (i.e. alleles) of a character separate into different gametes during meiosis. 2. Show heterozygote, 1 pair of chromosomes a. Meiosis I: Chromosomes separate after replication YyYy Yy and Yy b. Meiosis II: Gametes form, one allele in each gamete, Yy Y and y Yy Y and y i. Therefore, after meiosis: equal distribution (50% all the time) c. Therefore get gametes in equal proportions 3. A controlled breeding cross can be made with a male and a female Assume a cross between a dominant homozygote and a recessive homozygote. a. Parental cross (P): phentypes are yellow x red. Dad ? YY and Mom ? yy y y Y Yy Yy Y Yy Yy so that all gametes from partents of this cross are Y and y. results: All offspring are yellow (phenotype) All offspring are heterozygous Yy (genotype) [These offspring are called the F2 generation] No matter how many offspring there are from this cross, they are all alike as to genotype and phenotype. If one makes a controlled cross between two young from the above cross, it is called an F1 cross. F1 cross Y y Y YY Yy y Yy Yy Now: count all the young with the same phenotype 3 Yellow young / 4 possible young = 3/4 yellow (phenotype); 1 Red young / 4 possible young = 1/4 Red (phenotype) Now let's look at the genotypes that are present: YY (genotype) : 1/2 Yy (genotype) : 1/4 yy (phenotype) Homozygous Y : heterozygous : homozygous y B. LAW OF INDEPENDENT ASSORTMENT 1. During meiosis: Genes of one character segregate to gametes independently of genes of another character. a. This means that the genes (and chromosomes) for shirt color character are not influenced by genes (and chromosomes) of a character on a different chromosome. This type of cross is called a two factor cross (dihybrid cross). 2. Dihybrid cross (Two factor cross) The genes for each character are on different chromosomes An example of a dihybrid cross in which the genes for the two characters are heterozygous in both parents: Y allele = yellow shirt y allele = red shirt A allele = brown shoes a allele = blue shoes i. YyAa x YyAa Each parent now has 2 pair of chromosomes. One pair of chromosomes has the gene for shirt color. One pair of chromosomes has the gene for shoe color. Show meiosis and possible gamete formation YyAa YY / yy / AA / aa Four possible gametes, all in equal amounts i. Four possible phenotypes, unequal frequencies YYAA / yyaa / YYaa / yyAA There are four possible gametes (1) Dominant phenotype A with dominant phenotype B (Y-A-) (- in above genotype indicates that it does not matter what the second allele is, the phenotype is dominant for each character) (a) Frequency is 9/16 (2) Dominant phenotype A with recessive phenotype b (Y-aa) (a) Frequency is 3/16 (3) Recessive phenotype a with Dominant phenotype B (yyA-) (a) Frequency is 3/16 (4) Recessive phenotype a with recessive phenotype b (yyaa) (a) Frequency is 1/16
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