Linkage and Gene Mapping 1 a Why does more crossing over occ

Linkage and Gene Mapping

1. a) Why does more crossing over occur between two distantly linked genes than between two genes that are very close together on the same chromosome?

b) What is the maximum frequency of recombination from single crossover events that can be observed between 2 genes? Why?

c) For two genes that are genetically unlinked but physically linked because they are on the same chromosome, what is their recombination frequency in mu?

2. In a test-cross between an individual who is AaBb, and an individual who is aabb, the following types and numbers of offspring were obtained:

56 AaBb           

17 Aabb

19 aaBb

53 aabb

a) Are the A and B genes showing independent assortment?

b) Is the A allele on the same or different homolog as the B allele in the AaBb parent? How do you know?  

Solution

1a) Distantly linked genes consist of enough room for break and exchange of chromosomal fragments rather than in closely linked genes. Due to this reason, recombination frequency is more in distantly linked genes or the genes that are apart than in genes are located closely on same chromosome.

b) The maximum recombinant frequency in single crossover observed between two genes is 50%. This is because half of the chromosomal fragments (non-sister chromatids) are exchanged during crossingover and remaining half (sister chromatids) are inherited as same. Otherwise half of parental chromosomes undergo crossingover and half inherited as same. Due to this reason, maximum recombination frequency observed is only 50%.

c) The recombination frequency in genes that are physically linked but not genetically linked and located on same chromosome would be 50%. This is because genes linked distantly undergo 50% recombination. As a result, the recombination frequency in mu is = 0.5 x 100 = 50% or 0.5 mu.

2)

AaBb x aabb

The genes A and B are independently assorted, this is because four possible gametes are obtained from the above cross and they are Ab, Ab, aB and ab. If A and B are located on same chromosome only 2 gametes AB and ab are possible because they are not independently assorted. Therefore, genes A and B are on different chromosomes and are independently assorted.

b)

The A and b genes are located on different chromosomes and because of this, they are independently assorted forming AB and ab gametes from AaBb parent.