Suppose a mutation greatly increases the affinity of a rod’s sodium channels for cgmp, such that once cgmp binds to a channel, it remains bound for much longer than normal. what would be the effect of this mutation on the rod’s sodium channels?

Out of the following given choices;

A.      The sodium channels would open and close randomly, regardless of the presence of light.

B.      The sodium channels would remain closed in the light and the dark, even when cGMP decreases after light stimulation.

C.      The sodium channels would remain open in the dark and close rapidly to the light.

D.      The sodium channels would remain open longer, even when cGMP decreases after light stimulation.

The answer is D. cGMP maintain the sodium channels open when there is no light stimulation. This ensures that there is high intracellular Ca2+ concentration due to the Na+/Ca2+-K+ exchanger. When cGMP is hydrolyzed in light stimulation cascade, the channels close causing hyperpolarization of the rod cells.

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.