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Sex Linked Genetic Conditions Biological Science 4 2011 What decides whether an individual is male or female? The male parent’s gametes determine the sex of the offspring. This occurs during the production of secondary spermatocytes- half the receive an X chromosome while half receive a Y chromosome. The Y chromosome is a truncated X chromosome. Because the Y chromosome is truncated, there are many genes with an allele on the X chromosome but no corresponding allele on the Y chromosome. There are two variants of sex-linked genetic conditions- either X linked dominant or X linked recessive. X linked dominant genes only require one copy of the allele for expression, whether in males or females. X linked recessive genes: In males, one allele gives expression In females, two alleles are required for expression. Colour Blindness 7% of males are colour blind, but only 0.5% of females. Colour blindness doesn’t mean that the affected individual doesn’t see colours at all (that is Daltonism, and is very rare). Colour blind individuals cannot distinguish between either red & green or red & violet (less common). The human eye has two types of receptors that detect light: Rods detect differences in the intensity of light Cones detect colours There are 3 types of cones, which detect red, blue & green wavelengths of light For example, someone with red/green colour blindness will have problems distinguishing between these colours, which all have the same intensity. Normal subjects see an 8 Colour blind subjects see a 3 Normal subjects see a 6 Colour blind subjects can’t see any number Normal subjects see a7 Colour blind subjects can’t see any number Normal subjects see a 35 Colour blind subjects see either a 3 or a 5 depending on the type of colour blindness Colour blind subjects can see numbers, as they detect differences in intensity Normal subjects can’t see any numbers! I don’t know what the numbers actually are! What are the possible advantages of colour blindness? Colour blind males may be able to see camouflaged animals more easily, which would give them an advantage when hunting. The allele for colour blindness is sometimes called Xc. It is a recessive condition. In carrier females, the normal allele cancels out the affect of the Xc. In affected males, there is no normal X to cancel out the Xc. What if the father is colour blind? -all daughters are carriers -all sons are normal Parents X X Xc X Xc X Xc Y XY XY What if the mother is a carrier? -1/2 the daughters are carriers -1/2 the sons are colour blind Parents X Xc X XX X Xc Y XY XcY Other X-linked recessive conditions include: Haemophilia Lack of clotting factor VIII Increased bleeding & bruising Gout Metabolic problem leading to increased amounts of uric acid in the body This causes joint inflammation & kidney stones X linked Dominant Conditions These are much rarer. Examples include inherited rickets, a form of kidney disease and a skin disease called Incontinentia pigmentia. The frequency is similar in males and females. However, the absence of male-tomale transmission distinguishes X linked dominant diseases from autosomal dominant diseases, in which both sexes are also equally affected. If the father has the condition, then none of his sons will be affected, but all daughters will be affected. If the mother has the condition, then all of her offspring have a ½ chance of having the condition. Queen Victoria’s legacy Queen Victoria was a carrier for haemophilia. Her daughters married into royal families throughout Europe. Rules for X-linked conditions X-linked recessive Males have the condition Females are carriers If a male has the allele If a female has the allele All daughters are carriers All sons are normal ½ daughters are carriers ½ sons have the condition X-linked dominant If a male has the allele All daughters have the condition All sons are normal If a female has the allele ½ offspring have the condition (whether sons or daughters)