Why is mild to moderate expression of an Xlinked recessive t

Why is mild to moderate expression of an X-linked recessive trait not observed in heterozygous female Drosophila?

Solution

In X-linked recessive trait, females will get the mutant phenotype if they receive both copies of the defective gene or females who are homozygous for the gene mutation. If they have one dominant and one recessive allele, they will not show any mutant phenotype.

SSex-linkedgenes are located on the on the X chromosomes, for example wild type of drosophila will have red eyes, and white color eyes are recessive. If a female drosophila possesses X^RX^r(heterozygous condition), drosophila eyes will be red in color, if gets both the recessive alleles X^rX^r(homozygous condition), then only her eyes will become white. That is the reason why heterozygous female drosophila will not express the mild to moderate X-linked recessive trait. Due to the presence of two copies of X chromosomes, only one copy of wild allele is enough for them to get normal phenotype.