Although X and Y were originally identical, the Y chromosome has decayed during its evolution, so only a few genes remain. Now, the X and Y chromosomes are very different in genetic content. A 525 kb region on Xp11 has an overall density of transposable elements of 89 per cent - the highest found so far. Factor VIII and IX Blood clotting factors.
Mutations cause haemophilia A and haemophilia B, respectively.
The X chromosome has five 'fragile' sites, due to multiplication of repeated sequences which render the chromosome unstable. Protein binds methylated C bases in DNA and regulates expression of genes. Mutations cause accumulation of fatty acids - adrenoleukodystrophy, characterized by mental deterioration progressing to dementia, slurred speech and abnormal functioning of adrenal gland. Condition was subject of the film Lorenzo's Oil. The gene for the disorder helps control the activity of the immune system.