A plant of genotype C/C ;d/d is crossed to c/c ;D/D,and the F₁ is testcrossed.If the genes in question are unlinked,the percentage of double homozygous recessive individuals in the offspring of the testcross will be: A)less than 25%. B)about 25%. C)more than 25%,but less than 50%. D)about 50%. E)more than 50% but less than 75%.

Question

Quizplus · Accepted Answer

The parental cross is C/C ;d/d x c/c ;D/D, which results in all heterozygous F1 offspring (C/c ; D/d). Testcrossing the F1 to a double homozygous recessive individual (c/c ; d/d) will result in four possible gametes from the F1: c/d, c/d, C/d, and C/d. Each of these gametes has an equal chance of combining with either c or d from the testcross parent. Therefore, the possible offspring genotypes and frequencies are:

c/c ; d/d = 25%
c/c ; D/d = 25%
C/c ; d/d = 25%
C/c ; D/d = 25%

Only the first genotype is double homozygous recessive, so the percentage of double homozygous recessive individuals in the testcross offspring is about 25%.

A Plant of Genotype C/C ;D/d Is Crossed to C/c