Question

In pegasus genetics, horn length, feather color, and tail length genes are located on the same chromosome. Each gene has two alleles: wild type, which is dominant, or mutant, which is recessive.

           H is dominant and results in long horns; h is recessive and produces short horns.

            F is dominant and results in rainbow feathers; f is recessive and produces white feathers.

            T is dominant and results in a long tail; t is recessive and produces a short tail

The H gene and F gene are separated by 7 map units; the F gene and the T gene are separated by 28 map units.

H-----------------------------F--------------------------------------------T

|----------- 7 m.u--------| |-------------28 m.u.-------------------|

If you crossed a pegasus homozygous for the wild type allele of each gene with a homozygous recessive pegasus for each gene, the F₁ generation would entirely consist of pegasi that had the wild type traits, but were heterozygous for each gene.

a) If you were to cross these heterozygous F₁pegasi to recessive homozygous pegasi, how many flies would you predict to have the double cross-over phenotype? (You will show all of your work and phenotypic classes in the next problem. Just show the NUMBER of double cross-over pegasi predicted by the map unit data here (both categories of double crossover phenotypes should be included). Assume there are 800 progeny. Round to two decimal places x.xx)

b) If you were to cross these heterozygous F₁pegasi to recessive homozygous pegasi to map linkage (trihybrid testcross), how many pegasi would you predict to observe in each of the eight predicted phenotypic categories with respect to these three genes (given the map units above) if there were 800 total offspring?  Show your work and show all phenotypic classes. (1 point each class, 4 points for work)

Answer 1

We know that the linkage distance is calculated by dividing the total number of recombinant gametes by total number of gametes. Expected double cross over frequency is the frequency of single crossover in first region multiply by the single crossover in second region.

Number of expected double crossovers= expected DCO frequency × Total number of progeny.

Please find the hanswritten solution: