Genetics Question - Testcrossing

[icedoverfire]

Homework Statement

An individual with genotype AA Bb Cc DD Ee is testcrossed. Assuming independent assortment of all gene loci, what fraction of the progeny are expected to have genotype Aa Bb Cc Dd Ee?

Homework Equations

Crossing AA Bb Cc Dd Ee X aa bb cc dd ee

The Attempt at a Solution

With Gene A, the only possible outcome is Aa, so the probability is 1.
With Gene B, the outcome Bb is present in 1/2 of the cases.
With Gene C, the outcome Cc is present in 1/2 of the cases.
With Gene D, the outcome Dd is present in all cases, probability = 1.
With Gene E, the outcome Ee is present in 1/2 of the cases.

So multiplying all those together you get that 1/8 is the fraction with the expected genotype.

...

My book does not have the answer for this problem. I am double checking my work.

Thanks,

-ice

 

[nate808]

blitz

Hello iceblitz,

Your solution is correct. The expected fraction of progeny with genotype Aa Bb Cc Dd Ee is 1/8 or 12.5%. This is because each gene loci is independently assorted, meaning that the outcome of one gene does not affect the outcome of another gene. Therefore, the probability of each gene combination can be multiplied together to get the overall probability of the expected genotype. Good job on double checking your work! Keep up the good work in your genetics studies.

 

[Blueshift5]

cream

Your solution looks correct. To further explain, a testcross is a genetic cross between an individual with a dominant phenotype (in this case, AA Bb Cc DD Ee) and an individual with a recessive phenotype (aa bb cc dd ee). This cross is used to determine the genotype of the dominant individual by observing the phenotypes of the offspring.

In this case, since all gene loci are assumed to assort independently, the probability of each offspring having the desired genotype (Aa Bb Cc Dd Ee) is simply the product of the individual probabilities for each gene locus. As you calculated, this results in a 1/8 chance of the offspring having the desired genotype.

It's always a good idea to double check your work, especially when the answer is not provided in your textbook. Keep up the good work!