Numerical abnormalities of chromosomes

papajimsmooth

Why are numerical abnormalities of sex chromosomes better tolerated than numerical abnormalities of autosomes?

IrishKnight

While I am no expert (but give it a few years) I think it is down to dosage affects. Basically smaller chromosomes have less genes that larger ones, which seems right when you think about it. So when there is a loss or a gain of a one of the small chromosomes, there is less of an affect. Downs Syndrome is the most famous example of an autosomal trisomany.

As for the sex chromosomes, the same can be applied, the Y chromosome contains very few genes so extra Y's can be tolerated by the body, but there might be some developmental retardation.

X chromosomes on the other hand are longer and contain more genes. However, in females who are XX, only one of the X's is active, while the second becomes inactive and becomes what is known as a Barr body. This is to prevent an overexpression of X chromosome genes. Therefore, a XXX female will inactive two of those. XXX females appear normal but might have some developmental retardation.

Basically, it all comes down to dosage affects...I think.

papajimsmooth

Thank you