Question

why are sex-linked traits more common in males than in females

Answer 1

Genetic traits, in general, are controlled by genes. Each gene has at least two different forms (called alleles) that it can take. Most genes have two copies in each cell. They may be the same or different. In some cases, one allele dominates over the other in determining the trait. So, if the individual has one of each allele, the expression of the trait will be determined by the dominant allele.

Sometimes, a particular expression of a trait is recessive, which mean that both copies of the gene must exhibit the recessive (not dominant) form of the gene. Therefore, if an individual had one dominant and one recessive allele, he would not have the trait.

Sex-linked traits are a special case. They are controlled by genes on the sex chromosomes, X and Y. The female has two X chromosomes and the male has one X and one Y. Both males and females are equally likely to have a recessive allele on one X chromosome. However, if a female has a recessive allele, there's a good chance that the OTHER X chromosome will have a dominant allele, nullifying the effect of the recessive allele on the first X chromosome. But, a male does not have another X chromosome. So, if he has the recessive allele on his one X chromosome, he will have the trait. Period.

We'll use a statistical example. Say there is a 50% chance that a particular gene on the X chromosome will have a recessive allele. The chance of a male having the trait is simply 50%, because if he has the recessive on his one X chromosome, he's got the trait. But, a female has to have the recessive gene on BOTH X chromosomes. The odds of that happening are .5 x .5 = 25%.

Of course, there are also some sex-linked traits on the Y chromosome. But females don't even have a Y chromosome. So, only males can have traits that are controlled by genes on the Y chromosome.

Answer 2

a recessive allele on the x chromosome will always produce the trait in a male