Sex Linked Genes

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•  Some genetic diseases are sex-linked

Sex linkage refers to when a gene controlling a characteristic is located on a sex chromosome (X or Y)

  • The Y chromosome is much shorter than the X chromosome and contains only a few genes (50 million bp; 78 genes)
  • The X chromosome is longer and contains many genes not present on the Y chromosomes (153 million bp ; ~ 2,000 genes)
  • Hence, sex-linked conditions are usually X-linked - as very few genes exist on the shorter Y chromosome

X and Y chromosomes

X and Y comparison

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•  The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes

Sex-linked inheritance patterns differ from autosomal patterns due to the fact that the chromosomes aren’t paired in males (XY)

  • This leads to the expression of sex-linked traits being predominantly associated with a particularly gender

As human females have two X chromosomes (and therefore two alleles), they can be either homozygous or heterozygous

  • Hence, X-linked dominant traits are more common in females (as either allele may be dominant and cause disease)

Human males have only one X chromosome (and therefore only one allele) and are hemizygous for X-linked traits

  • X-linked recessive traits are more common in males, as the condition cannot be masked by a second allele

The following trends always hold true for X-linked conditions:

  • Only females can be carriers (a heterozygote for a recessive disease condition), males cannot be heterozygous carriers
  • Males will always inherit an X-linked trait from their mother (they inherit a Y chromosome from their father)
  • Females cannot inherit an X-linked recessive condition from an unaffected father (must receive his dominant allele)

Inheritance of an X-linked Recessive Disease Condition

X-linked inheritance

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•  Red-green colour blindness and haemophilia as examples of sex-linked inheritance

Red-green colour blindness and haemophilia are both examples of X-linked recessive conditions

  • Consequently, they are both far more common in males than in females (males cannot mask the trait as a carrier)

When assigning alleles for a sex-linked trait, the convention is to write the allele as a superscript to the sex chromosome (X)

  • Haemophilia:  XH = unaffected (normal blood clotting) ; Xh = affected (haemophilia)
  • Colour blindness:  XA = unaffected (normal vision) ; Xa = affected (colour blindness)


Haemophilia is a genetic disorder whereby the body’s ability to control blood clotting (and hence stop bleeding) is impaired

  • The formation of a blood clot is controlled by a cascade of coagulation factors whose genes are located on the X chromosome
  • When one of these factors becomes defective, fibrin formation is prevented - meaning bleeding continues for a long time
  • Different forms of haemophilia can occur, based on which specific coagulation factor is mutated (e.g. haemophilia A = factor VIII)

Red-Green Colour Blindness

Red-green colour blindness is a genetic disorder whereby an individual fails to discriminate between red and green hues

  • This condition is caused by a mutation to the red or green retinal photoreceptors, which are located on the X chromosome
  • Red-green colour blindness can be diagnosed using the Ishihara colour test

The Ishihara Colour Test
(Click on the image to compare normal vision and colour blind vision)

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