Some chromosomes not linked to sexual function are located on the sex chromosomes (X and Y). The Y chromosome is significantly shorter (50 million bp) than the X chromosome (153 million bp), thus the Y chromosome does not have many genes. This means that most sex-linked genetic conditions are X-linked. Because of the way in which sex chromosomes are inherited, their patterns differ from autosomal inheritance. Men are more likely to be affected by sex-linked conditions because they only have one X chromosome (XY) - they are always homozygous for the disease. Females have 2 (XX) so if they only have 1 diseased allele, they will be heterozygous and thus less affected by the condition. If they are heterozygous, they are called carriers and there is a 50:50 chance that they will pass the diseased allele on to their offspring. When looking at patterns of inheritance, there are things you must keep in mind: Only females can be carriers (heterozygous). Females cannot inherit a diseased allele of a recessive X-linked condition from an unaffected father. Affected females will always have affected sons - but their daughters will be carriers if the father is unaffected. Haemophilia and red-green colour blindness are the two most common sex-linked conditions.
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