sex linkage

• • genes that are found on the X and Y chromosomes are inherited differently to genes found on autosomal chromosomes (all chromosomes except the sex chromosomes)
• • autosomes are fully homologous - the length of each chromosomes is the same and they contain the same genes at the same loci
• • X and Y chromosomes are very different in size. The X chromosome has genes that are not found on the Y chromosome. Therefore, they are not fully homologous
• • the genes that are found only on the X chromosome are said to be Sex/X linked because their inheritance is influenced by whether you are a male or female
• females = homogametic (XX)

  males = heterogametic (XY)
• sex/x linked disorders

  • the X chromosome carries over 1000 genes needed for many characteristics including metabolic functions. if a female has one abnormal allele on one X chromosome, there is a chance she could have a functioning allele on the other X chromosome
  • if a male has inherited one abnormal allele on his X chromosome, there is no chance there will be functioning allele on his other sex chromosome
• sex linkage in cats

  • coat colour in cats
  • gene for coat colour is found on the non-homologous portion of the X chromosome
  • coat colour is determined by the codominant genes
  • tortoiseshell females are produced if a female is heterozygous for these alleles
  • in female cells during early embryonic development, one X chromosome is inactivated
  • this ensures that females dont have twice as many genes being expressed as males
  • a ginger patch is produced where the X chromosome carrying the black allele is inactivated and vice versa