Problem:

We have discussed how recessive phenotypes are more likely to be expressed in males in the corresponding gene is found on the X-chromosome. For example, one-type of color blindness is caused by a recessive X-linked allele, and this trait is much more common in males than females. But we have also discussed how, in placental mammals, one copy of each X-chromosome is randomly silenced in female cells (X-inactivation). This raises somewhat of a paradox. Why don't half of heterozygous females exhibit color-blindness just like hemizygous males that carry the recessive allele?

Required:

Question 1: Based on your understanding of X-inactivation in placental mammals, provide an explanation for why heterozygous females generally do not exhibit colorblindness despite the random silencing of one X-chromosome.

Question 2: How would your answer change if we were talking about a marsupial mammal (in which the paternally inherited copy of the X chromosome is inactivated in every cell)? Would you expect half of heterozygous females in a marsupial species to exhibit color blindness? Why or why not?

Please read all the questions carefully and answer them.