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 linked,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 genotype of the parent plant can be written as C/C d/d S1U1P1.S1S1P0/S1U1P1.S1S1P0. The genotype of the parent it is crossed with can be written as c/c D/D S1U1P1.S1S1P0/S1U1P1.S1S1P0. 
The F1 generation will have the genotype C/c D/d S1U1P1.S1S1P0/S1U1P1.S1S1P0, which is the result of the independent assortment of the genes. 
When the F1 generation is testcrossed with a double homozygous recessive individual (c/c d/d S1S1P0/S1S1P0), the possible gametes produced by the F1 are CD or Cd (from the D/d parent) and S1U1P1.S1S1P0 or S1S1P0 (from both parents). 
The possible offspring of the testcross are: 
- C/d S1U1P1.S1S1P0/S1S1P0 (16.67%)
- C/d S1U1P1.S1U1P1/S1S1P0 (16.67%)
- C/d S1U1P1.S1S1P0/S1U1P1 (16.67%)
- C/d S1U1P1.S1U1P1/S1U1P1 (16.67%)
- c/d S1U1P1.S1S1P0/S1S1P0 (16.67%)
- c/d S1U1P1.S1U1P1/S1S1P0 (16.67%)
- c/d S1U1P1.S1S1P0/S1U1P1 (16.67%)
- c/d S1U1P1.S1U1P1/S1U1P1 (16.67%)
None of these possible offspring are double homozygous recessive, therefore the percentage of double homozygous recessive individuals in the offspring of the testcross will be less than 25%.

A Plant of Genotype C/C