Everything I've explained so far has dealt with genes on chromosomes 1-22, ie all the chromosomes that aren't the sex chromosomes. So, what happens if a gene is on one of the sex chromosomes?
You probably know that females have two X chromosomes and males have one X and one Y chromosome*. Now, the Y chromosome is actually rather smaller than the X chromosome, and contains very few genes other than those required to cause a person to be male. (yes, female is in some ways a default.) However, the X chromosome does actually contain quite a lot of genes whose absence causes a disease.
This is why there are so many more boys than girls with colour vision deficiencies**- at least one of the genes required for normal colour vision in humans is situated on the X chromosome, which means boys have only one chance to get a working copy, but girls have two. This also means that a woman with a colour deficiency is guaranteed to pass it on to her sons, but cannot have a daughter with it unless her father also has it. The daughters of a colour-deficient father are, of course, guaranteed to be at least carriers of that allele, while his sons cannot.
THe other famous sex-linked condition is haemophilia, a disease in which sufferers' blood does not clot properly due to a mutation in one of the genes that codes for a protein which signals the blood clot formation process. At this point, you may remember that haemophilia is a condition which *only* affects boys. Actually the truth is rather less pleasant: girl foetuses with haemophilia are non-viable.
On that lovely note, I will stop for today, and tomorrow look at some of the weirder effects that can be caused by sex linkage.
*I am aware that this is a very simplistic view. This is a series about genetics rather than gender.
**The popular term for red/green deficiency and indeed several other less common forms is "colour-blindness", which is actually a rather misleading term; actual complete lack of colour vision is extremely rare, whereas the inability to see one colour while able to discern others perfectly well is rather more common.