We have discussed how recessive phenotypes are more likely to be expressed in ma

Question

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?

a. 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.

b. 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?

Explanation / Answer

Based on the given data,

a)

X inactivation is a process by which one of the two X chromosomes in the female mammals is turned off so as to compensate for the difference in dosage of the X chromosomes in males and females. Mary Lyon proposed the mechanism of X inactivation as a means of dosage compensation in mammals. In female mammals one of the X chromosomes gets inactivated.

As females contain two copies of X chromosome, if one copy of X chromosome inactivated or undergoes silencing, then the other copy would be functional and the color blindness in females is random and if it occurs in a female, the female would be a carrier as the gene exhibits heterozygous condition.

b)

Unlike the random X-inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally derived X chromosome. Then, one copy is inactivated as the other being functional and hence, all the females are heterozygous and would be carriers of the disease. If the paternally inherited copy is inactivated and the other copy of X chromosome is being affected then the heterozygous marsupials are affected with color blindness.

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