Meiosis generates diversity using two mechanisms: independent assortment of chromosomes and crossing over between homologous chromosomes. Linked genes occur on the same chromosome, thus they cannot adhere to the Principle of Independent Assortment. However, recombination can still occur.
For recombination to occur, it is necessary for there to be crossing over between the two homologous chromosomes, with a chiasma between the two loci. Thus, the closer together the loci, the less likelihood of a chiasma forming, due to there being less chromosome length for it to happen on
For example, imagine that a goat has two loci on the same chromosome:
1) Coat colour locus: where the "B" allele gives a white individual, and is dominant to "b" which gives a black individual in the homozygote.
2) Horn shape locus: where "C" gives straight horns, and is dominant to "c" which produces crooked horns in the homozygote.
If a pure breeding white, straight-horned goat was bred with a pure-breeding black, crooked-horned goat, the cross would be represented like this (note the change in notation in the genotype, denoting linkage):
Below is a depiction of the chromosome pair of interest in the nucleus of the F1:
Linkage is most easily detected using a test-cross. The results of a test cross will show a deviation from the 1:1:1:1 ratio in phenotypes of the offspring, expected where the alleles are undergoing independent assortment.
Here, the F1 are dihybrids, and in contrast to the typical dyhybrid cross, recombination of these alleles in the gametes depends upon crossing over, rather than independent assortment.
In the formation of gametes, crossing over may or may not occur in such a way as to recombine the parental alleles. Thus any "batch" of parental gametes will contain some gametes that have undergone allele recombination, and some that have not.
Where a crossover does not occur, or occurs but not between the two loci, only the original arrangement of alleles will occur in the gametes. In the case above, the resulting gametes would be only BC and bc.
Where a cross over does occur between the two loci, recombination occurs. The probability of this event depends upon the distance between the loci.
Crossing Over, with recombination. The two homologous chromosomes are shown in different colours so that the rearrangement of genetic material can be followed.
We are unable to predict the exact proportion of phenotypes that will occur from the test-cross. We do expect, though, that the offspring of a test-cross where the genes are linked will show a higher proportion of parental phenotypes. This is due to the fact that crossing over has to occur between the two loci to recombine the alleles.
Possible gametes produced, note that the proportions of each of these gametes cannot be determined before the cross has been performed. This is due to the fact that we do not yet know what proportion of gametes will have undergone recombination .
We observe the phenotypes of the testcross offspring and their relative proportions in order to determine the degree of linkage of the two loci. The individuals who resulted from recombination of the loci will have phenotypes that differ from that of the parents. Thus the percentage recombination is calculated by dividing the number of recombinant individuals by the total number of offspring.
This is obviously not the 1:1:1:1 ratio of phenotypes expected in test cross offspring where the loci are not linked.
Percentage Recombination: 4+5/66+68+4+5 = 6.2%
Thus, the loci are sufficiently far apart to produce 6.2% recombinant gametes. If we performed another test cross, we would expect approximately 6.2% of the offspring to have a phenotype that differed from the parental types.
Chromosome mapping uses this figure to indicate the physical distance between loci on a chromosome. For the purposes of chromosome mapping, 1% recombination is regarded as 1 map unit, and in this case the distance between the two loci would be represented thus:
Note that the bar must be in proportion, and the units are centiMorgans (cM).
This notation reinforces the fact that the further apart two loci are on a chromosome:
- the greater the chance of a chiasma between them, and so
- the greater the chance of recombination of alleles occurring, and thus
- the greater the chance of recombinant gametes being produced.
Knox et al., pages165-166
Lecture notes and Web lecture notes for Lect 31.