Question

You discover a novel unicellular, sexually reproducing organism. The ‘A’ (or ‘a’) locus is 38 map units away from the ‘B’ (or ‘b’) locus. Suppose that ‘A’ encodes cilia that help the cell to move (and is dominant to ‘a’ which is incapable of moving) and that ‘B’ encodes ability to perform photosynthesis (and is dominant to ‘b’ which is incapable of photosynthesis).

1. If you cross an individual that has ‘AB’ together on a single chromosome and ‘ab’ on the homologous chromosome with another individual that has the genotype ‘aabb’, what fraction of the offspring organisms can move and perform photosynthesis, what fraction can move but not perform photosynthesis, what fraction can’t move but can perform photosynthesis, and what fraction can’t do either?

Answer 1

The cross is between AaBb and aabb

Gametes formed by AaBb are AB, Ab, aB and ab out of which AB and ab are parental type while Ab and aB are recombinant type.

Gametes formed by aabb are ab.

..............AB................Ab.............aB..........ab

ab.......AaBb............Aabb........aaBb.......aabb

So, in this cross AaBb and aabb are parental type while Aabb and aaBb are recombinant type.

The distance between two genes is 38 cM. The distance between two genes is calculated by the total proportion of recombinant progeny.

Since gametes can combine in independent manner so, the proportion is equally distributed in both genotypes.

The fraction of recombinants (Aabb and aaBb) is 0.38.

The fraction of Aabb (can move but not perform photosynthesis) is 0.19

The fraction of aaBb (can't move but can perform photosynthesis) is 0.19

The fraction of parental type (AaBb or aabb) is (1-0.38) or 0.62.

The fraction of AaBb (can move and perform photosynthesis) is 0.31

The fraction of aabb (can't do either) is 0.31