Advent Science Day Eighteen
Sex Linkage
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.
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.