Sex Chromosome Disorders

Recall that in meiosis some gametes can be formed with the wrong number of chromosomes. That’s how someone could be born with Down’s syndrome; a gamete had an extra chromosome. The problem of non-disjunction can also affect the sex chromosomes. Normally a gamete would contain one such chromosome. If non-disjunction occurs then a gamete could contain two X chromosomes or even two Y chromosomes. It’s also possible for a gamete to contain NO sex chromosomes. Whether that allows for a live birth depends on the sex chromosome that’s affected.

For example, this punnett square shows non-disjuction of an egg gamete. When the cell splits for the final time in meiosis it could contain two X chromosomes or perhaps no X chromosome at all. If one of these gametes were fertilized normally with a sperm cell, the resulting offspring possibilities are shown.

Note that any fertilized egg lacking at least one X chromosome would never develop. You can’t get by with just a Y chromosome. There are genes on the X chromosome that are necessary for life. However, there is one option shown where the offspring would only have one X chromosome. This is perfectly fine. After all, every male on the planet only has one X chromosome…. Another option would be a female offspring with 3 X chromosomes or a male with two X chromosomes (XYY). All of these offspring would develop to adulthood but there are certain differences from the normal male/female phenotype.

maleDisjunction.jpg

 

 

 

 

 

 

 

 

An XXY male has Kleinfelter syndrome. This is characterized by reduced fertility due to underdeveloped testis. This affects about one out of very 1000 males. Another example is XYY syndrome which includes some learning problems but little other phenotypic change. Some behaviorists have in the past suggested that males with an extra Y chromosome are more aggressive than others. No study has ever shown an increase in aggression though this outdated concept still persists.

femaleDisjunction.jpg

In the female side, it is possible for a girl baby to be born with just one X chromosome. This is Turner Syndrome, which exhibits certain physical traits including short stature, undeveloped secondary sex characteristics and often learning disabilities. Another possibility is Triple X syndrome. Other than being a bit taller than average there are no obvious physical or mental differences between these women and those with two X chromosomes.