In cod, the genes for short fins ( sf ), rough scales ( rs ) and spotted body (

Question

In cod, the genes for short fins (sf), rough scales (rs) and spotted body (sb) are located on a single chromosome separated by the following map distances:

sf ---(10.5)---rs-------(26.5)------sb

A female with short fins and a rough scales was mated to a male with a spotted body (assume that these traits are mutant phenotypes, and that the mutant alleles are recessive).

The resulting phenotypically wild-type females were mated with males that had the mutant phenotype for all three traits, producing 900 offspring.

Question: Calculate the number of expected double crossover progeny. Enter your answer to as a whole number, rounded to the nearest integer.

Explanation / Answer

sf ---(10.5)---rs-------(26.5)------sb

Possibility of single crossover event between the genes sf and rs = 10.5/100 = 0.105

Possibility of single crossover event between the genes rs and sb = 26.5/100 = 0.265

From the above data, we can easily calculate the frequency of double crossover among these genes

0.105 x 0.265 = 0.027825

If 900 offsprings are produced, then the expected number of double crossover progeny would be,

0.027825 x 900= 25.0425

The expected number of double crossover progeny is 25.

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