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Answer: Girls need to inherit two affected chromosomes to develop the disease, and it may also happen that the affected chromosome is inactivated in the eye (if it has only one)
Explanation:
Norrie's disease is an inherited eye disorder affecting males, which is characterized by abnormal or immature development of the retina (light-sensitive nerve tissue lining the back of the eye) and possible deterioration of the iris (colored part of the eye) and eyeballs. X-linked recessive inheritance is one of the ways in which a genetic trait or condition is passed from parent to child when there are mutations in a gene on the X chromosome from either the father or the mother. Females possess two X chromosomes, while males possess one X and one Y chromosome. In males, the presence of a mutation on the X chromosome causes the disease, since it is the only X they possess. In females, both X chromosomes need to be affected if it is a recessive condition.
So, the disease is inherited in an X-linked recessive manner, which means that almost all affected individuals are male. However, the sons of affected males will not have the mutation since they only inherit the Y chromosome, while all their daughters will be genetic carriers of the mutation because they inherit the affected X chromosome, although without clinical symptoms but being carriers. On the other hand, the mother will pass one of her two X chromosomes (which may or may not be affected) randomly to both her male and female offspring (if they are male, they will be affected and if they are female they will be carriers unless they also inherit an X chromosome from their father). So the only way for females to develop the disease is for their two copies of the gene to be mutated if both parents have the mutation.
But female carriers could be affected by inactivation of their X chromosome with the correct sequence. X chromosome inactivation is a random process that happens separately in individual cells during embryonic development. That is, one cell may inactivate the paternal X chromosome, while others may inactivate the maternal X chromosome. This is because a dosage compensation is necessary, since the Y chromosome in males is not homologous to the X, it does not have the same genes, so if females would express both X chromosomes, they would have twice as many genes as a male. All the descendant cells of each of these original cells will maintain the same pattern of X chromosome inactivation.
In females, one X chromosome is inactivated in each tissue in an arbitrary manner. Then, in addition to having to inherit two affected chromosomes, it may also happen that the affected chromosome is inactivated in the eye (if it has only one).
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