Sex-linkage

Sex inheritance in humans:

• As females have two X chromosome, all the gametes are the same in that they contain a single X chromosome
• As males have one X chromosome and one Y chromosome, they produce two different types of gametes - half have an X chromosome and half have a Y chromosome

Haemophilia:

• The X chromosome is much longer than the Y chromosome
• This means that for most of the length of the X chromosome, there is no equivalent homologous portion of the Y chromosome
• Those characteristics that are controlled by recessive alleles on this non-homologous portion on the X chromosome will appear more frequently in a male
• This is because there is no homologous portion on the Y chromosome that might have the dominant allele, in the presence of which the recessive allele does express itself

• The X chromosome carries many genes
• An X-linked genetic disorder is a disorder caused by a defective gene on the X chromosome
• One example in humans is haemophilia, in which the blood clots only slowly and there may be slow and persistent internal bleeding

• One of a number of cases of haemophilia is a recessive allele with an altered sequence…