Analyzing Sex-Linked Traits

Earlier in the course we covered how to analyze the inheritance of multiple genes at a time. The basic idea is that when the genes segregate independently you can solve the progeny genotypes separately for each gene and then apply the AND rule to combine them and get the overall probability of any given genotype.

We also covered how to study linked genes. This deals with situations when the genes we are studying are on the same chromosome and do not segregate independently. Since the AND rule requires that the probabilities are independent we cannot apply the AND rule to study linked genes. We saw how to calculate Recombination Frequency (RF) and the how to apply RF to predict progeny genotypes.

When we study sex-linked genes we have to be careful about a couple of factors related to the analysis of multiple genes and linkage. These are covered in the sections below.

• Combining sex-linked and autosomal genes

  This is the same as dealing with multiple genes. The fact that one gene is sex-linked and the other is autosomal means that they are on different chromosome and so they segregate independently. Therefore, you can deal with the genes separately and then apply the AND rule to combine them.

• Dealing with multiple sex-linked genes

  This is the same as dealing with linked genes. If you have multiple X-linked genes (in the case of mammalian genetics Y-linked genes are rare) then you can calculate RF and apply it as covered previously.

• Sex and sex-linked genes

  This is the most difficult thing to manage. An important thing to keep in mind is that autosomal traits and the sex of an individual are independent. Therefore, you can apply the AND rule when considering their co-inheritance.

  For example, albinism is an autosomal trait. If you have a monohybrid cross, what is theprobability that you will get a son who is albino? Inheriting the aa genotype for albinism and the XY chromosome content for being male are independent. Therefore:

  Pr(son AND albino) = Pr(Son) x Pr(Albino) = 1/2 * 1/4 = 1/8

  However, because sex-linked traits are caused by genes on a sex chromosome,the probability of inheriting the trait and the sex of an individual are NOT independent. Therefore, you cannot apply the AND rule.

  For example, a woman is known to be a carrier for hemophilia A. She has a child with a man who isnot a hemophiliac. What is the probability that they have a daughter who has hemophilia?

  Pr(daughter AND hemophilia) IS NOT GENERALLY EQUAL TO Pr(daughter) x Pr(hemophilia)

  (In some cases the this equation might hold by chance but it is not GENERALLY true.)

  Dealing with sex-linked traits and the sex of an individual requires a bit more of an involved route. You need to consider the full phenotype - in terms of sex AND whatever physical ratio you are dealing with - of each genotype. Once you determine the proportion of individuals with a given genotype you then know the proportion with the resulting phenotype.