In maize,the genes W and D are so tightly linked that virtually no crossovers occur between them.A dihybrid W d/w D is testcrossed to w d/w d.The percent of progeny with W-/D- phenotype will be:
A)0.
B)25.
C)50.
D)75.
E)100.

Question

Quizplus · Accepted Answer

We know that the genes W and D are tightly linked, meaning they are located close together on the same chromosome. This makes it very unlikely for a crossover to occur between them during meiosis. Therefore, they will almost always be inherited together in the same gamete.

In the dihybrid individual W d/w D, the two dominant alleles (W and D) are located on the same chromosome, while the two recessive alleles (w and d) are on the other homologous chromosome. When this individual is testcrossed to a homozygous recessive individual (w d/w d), the gametes produced by the dihybrid individual will be W d and w D in equal proportions.

When we look at the resulting progeny, we see that there are four possible gamete combinations:

- W d/w d
- W D/w d
- w d/w d
- w D/w d

Of these four possibilities, only one (W d/w d) has the W and D alleles together, and therefore exhibits the W-/D- phenotype. Because this is only one out of four possible gamete combinations, the percentage of progeny with the W-/D- phenotype will be 25%, and not 0%, as there would be some recombinants due to chromosomal segregation.

In Maize,the Genes W and D Are So Tightly Linked