6.2.5

• sex-linked - gene present on one of the sex chromosomes
• Sex linkage in humans

  sex in humans is determined by one of the 23 pairs of chromosomes

  other 22 pairs are called autosomes, each of the autosomal pairs is fully homologous - they match for length + contain same genes at the same loci

  • sex chromosomes are XY chromosomes
  • X + Y chromosomes are not fully homologous

  → a small part of one matches a small part of the other, so that these chromosomes can pair up before meiosis

  figure 1 + 2

  
• The human X chromosome has over 1,000 genes influencing traits and metabolic functions, mostly unrelated to sex determination, with no partner alleles on the Y chromosome
• if the female has one abnormal allele on her X chromosome, she will probably have a functioning allele of the same gene on her other X chromosome

  if a male inherits, from his mother, an X chromosome with an abnormal allele for a particular gene, he will suffer from a genetic disease, as he will not have a functioning allele for that gene

  • males are functionally haploid, or hemizygous, for X-linked genes

  → they cannot be heterozygous or homozygous for X-linked genes

  Sex-linked characteristics in humans include haemophilia A + colour blindness
• Haemophilia A

  unable to clot blood fast enough + injuries may cause bleeding or an internal haemorrhage
• Haemophilia A 2

  one of the genes on the non-homologous region of the X chromosome codes for a blood-clotting protein called factor 8

  → a mutated form of the allele codes for non-functioning factor 8

  • a female with one abnormal allele + one functioning allele produces enough factor 8 to enable her blood to clot normally when required; however she is a carrier for the disease

  →if such a female passes the X chromosome containing the faulty allele to her son, he will have no functioning allele for factor 8 on his Y chromosome, resulting in haemophilia A

  figure 3

  
• Colour blindness

  one of the genes involved in coding for a protein involved in colour vision is on the X chromosome, but not on the Y chromosome

  • a mutated allele of this gene may result in colour blindness - inability to distinguish between red + green
  • a female with one abnormal allele + one functioning allele will not suffer from red-green colour, but a male with an abnormal allele on his X chromosome + will therefore suffer from red-green colour blindness

  → the inheritance pattern is the same as for haemophilia A - that of a recessive sex-linked disorder

  figure 4

  
• Sex linkage in cats

  one of the genes, C, for coat colour in cats is sex-linked, on the non-homologous region of the XX chromosome

  • the allele C^O produces orange fur
  • the allele C^B produces black fur

  these alleles are codominant, as cats with the genotype X^CO X^CB are tortoiseshell + have patches of black + ginger fur

  → both contribute to the phenotype, but orange allele is only expressed in cells where the X chromosome bearing the black coat allele is inactivated + vice versa
• Sex linkage in cats 2

  male cats may be either black or ginger but not tortoiseshell, as they only have one X chromosome

  Inactivation of X chromosomes in female mammals

  may appear that females have twice the number of X-linked genes being expressed as do males however a mechanism prevents this disparity

  • in every female cell nucleus, one X chromosome is inactivated

  → determination of which member of the pair of X chromosomes becomes inactivated is random + happens during early embryonic development