SEX LINKAGE

=Sex-Linked Conditions= Sex linked traits are determined by genes found on the sex chromosomes (X or Y chromosome). Gene found on the sex chromosomes do not always show a normal pattern of inheritance. '''Y-Linked dominant conditions''' Genes found on the Y-chromosomes are only expressed in males. '''X-linked recessive conditions''' Some inherited conditions are much more common in males (e.g. red-green colour blindness and haemophilia where the blood can’t clot). These conditions are due to recessive allele found on the X chromosome. In females both X chromosomes must carry the recessive allele before the condition is expressed. Because males only have one X chromosome, they always express the characteristic if they have it on their one X chromosome (it effectively has nothing to be recessive to). [image:http://i.imgur.com/eX78ps4.png?1] '''X-linked dominant conditions''' Dominant genes on the X chromosome are expressed in both sexes. However, in males the dominant trait is always inherited from their mother (affected males will always have affected mothers)