1 An individual is heterozygous for 3 genes Aa Bb and Cc He

1) An individual is heterozygous for 3 genes (A/a B/b and C/c). He is crossed to an individual that is homozygous recessive for the same 3 genes (a/a b/b and c/c). The following offspring were seen. Which of the following phenotypes represent a double crossover event? (Hint - are you seeing all of the phenotypes in the next generation that you expected to see?) Select all that apply.

+++

154

+b+

322

a+c

318

abc

148

++c

76

ab+

82

2) Use the same information in the question above. What is the gene order?

The gene order is B-C-A

The gene order is A-B-C

The gene order is B-A-C

3) Using the same information as in the previous problem,  What is the recombination frequency between each gene pair? Round your answer to 2 decimal places. For example, report 12.2345% as 0.12.

recombination frequency between A/B =

recombination frequency between A/C =

recombination frequency between B/C =

******According to the hint, we don\'t see two phenotypes that we are expecting which are +bc, and a++. Are those the ones that represent double crossovers, and if so, how do we calculate the recombination frequency between each gene? Would very much appreciate some help.

Solution

1)

Individuals with double cross over are always low in number.

Here no double crossover took place as two sets of phenotype shows zero individuals.

In this case last two lowest number of individuals are +bc=0 & a++=0

These two phenotypes show double crossover.

2)

Phenotype with highest no of individuals are parental.

In this case first two highest number of individuals are +b+=322 & a+c=318

These two are parental.

A double crossover event always moves the middle allele from one sister chromatid to anther.

If we compare parental with double crossover then we can see that allele ``c`` is moving.

So it must be in the middle position.

So the order of the genes are B-C-A