[The Lone Ranger]

There are actually quite a few mutations that can lead to macular degeneration, so it can be an autosomal disorder (in which case, it's inherited the same in males and females) or a sex-linked disorder.

The genetics of such things are a bit more complex than we're generally taught in school, because there are multiple forms of dominance, and it's not always true that a "dominant" allele completely masks the effects of a "recessive" allele. Still, for simplicity's sake, let's assume that this is indeed the case.


So yes, if the trait in question is sex-linked, that means the gene in question is located on the "X" chromosome but not the "Y" chromosome. Since a woman has 2 "X" chromosomes and a man has only 1, such traits will be inherited differently in males and females.


[The distinction between "genes" and "alleles" is sometimes a bit confusing. The gene for a trait corresponds to the region of a particular chromosome that codes for that trait. For example, we might talk about a gene for eye color. Alleles are variants of a gene -- so we might talk about an allele for blue eyes vs. an allele for brown eyes; those would be allelic variants of the eye color gene.]



Anyway, if the trait in question is sex-linked and caused by a recessive allele, the only way a woman could have the condition is if she inherited a recessive allele from each of her parents. (This would mean that her father definitely had the condition, and her mother was at least a carrier.)



If a woman has the condition, then each of her sons is essentially guaranteed to inherit an allele for that trait from her, since each of them inherits his single "X" chromosome from her. And since a son doesn't have a second "X" chromosome that might carry a dominant allele, he will have the condition.

Similarly, each of that woman's daughters will inherit a single recessive allele from their mother, and so will be a carrier at the very least. Whether or not a daughter has the condition depends upon what she inherits from her father. If the father doesn't have the condition, then each of his daughters will receive a dominant, normal allele from him, and so will be a carrier, but won't have the condition. If the father does have the condition, then each of his daughters will receive a second recessive allele from him, and so will have the condition.


So in other words, if the condition is the result of a sex-linked recessive allele, then a woman can only have the condition if she inherits the defective allele from both parents. If she inherits the defective allele from one parent but a functional allele from the other, she will be a carrier but won't have the condition.

If a woman is a carrier and her husband does not have the condition, there's a 50% chance each of their sons will inherit their mother's defective allele and so have the condition. There's no way (barring mutation) a daughter of this couple could have the condition (because each daughter will inherit a dominant allele from her father), but there's a 50% chance that each daughter will inherit a recessive allele from her mother and so be a carrier.

Thus, sex-linked recessive conditions are much more common in males. Given that a male inherits his single "X" chromosome from his mother, if that chromosome happens to carry a recessive allele, there's no way that the man's father could contribute a dominant allele for that trait that would mask the recessive allele's effects.




If a trait is caused by a sex-linked dominant allele, then the situation is basically reversed. In this case, a female has two opportunities to inherit the allele in question, whereas a male has only one.

A man who carries a dominant sex-linked allele will give it to each of his daughters -- and to none of his sons. A woman who carries one copy of the dominant allele has a 50% chance to give it to each of her children -- whether they're male or female.

So yes, sex-linked dominant conditions are more common in females than in males. (Deleterious conditions caused by dominant alleles are rare, however. This is because natural selection can easily weed dominant alleles out of a population, if they have harmful effects. It's much harder to rid a population of harmful alleles if those alleles are recessive.)



Under normal circumstance, since a father does not give his son an "X" chromosome, there is no way that a son can inherit a sex-linked trait from his father. Normally. It is rare, but a translocational mutation can transfer a gene from the "Y" chromosome to the "X", or vice versa. So it is possible (though extremely rare) that a father could pass an allele that is normally sex-linked to his son.