Sex linked inheritance

• Chromosome

  • Normal somatic cells contain 23 pairs of chromosomes
  • 22 x autosomes
  • 1x sex
• Sex Determination

  • Dependent on which sex chromosomes received from parents
  • Once chromosome is received from each parent during fertilisation
• Punnet square

  • Therefore sex is determined by the chromosome inherited from the father
• Sex linked inheritance

  • Some genes are presents on X chromosome but not on Y (0) as it is so much shorter
  • Females inherit 2 alleles for each gene as they have 2 X chromosomes but males only inherit 1 allele of the gene on their one X chromosome
  • This means that if a male inherits a recessive genotype he will have the recessive phenotype.
• Example 1 - Red green Colourblindness

  • Caused by the recessive gene that causes deficiency in either red or green cone receptors in the eyes
  • The gene for the production of cone receptors is located on the X chromosome
• Colourblindness

  • The normal production allele = C as it is dominant
  • Abnormal cone production allele  = c as it is recessive
  • Due to the fact this gene is X-linked, it is also important to indicate whether the chromosome is X or Y
  • E.d. A man with colourblindness would be XcY0 (as he has the recessive allele on his X chromosome and no allele on his Y chromosome)
• Haemophilia

  • Normal clotting protein production allele = H as it is dominant
  • Abnormal protein production allele = h as it is recessive
  • Due to the fact that this gene is X-linked, it is also important to indicate whether the chromosome is X or Y
  • E.g. a man with Haemophilia would be XhY0 (as he has the recessive allele on his X chromosome and no allele on his Y chromosome)
• Example 2 - Haemophilia

  • Caused by the recessive gene that prevents clotting from occurring properly
  • The gene for clotting protein production is located on the X chromosome
• Example 3 - Rett Syndrome

  • Caused by a dominant gene that causes grey matter in the brain to work incorrectly
  • The gene for Rett syndrome is located on the X chromosome
• Rett Syndrome

  • Lets say that the allele for Rett Syndrome is represented by R as it is dominant
  • The allele for normal grey matter function is represented by r as it is recessive  
  • Due to the fact that this gene is X-linked it is also important to indicate whether the chromosome is X or Y
  • E.g. a woman with Rett syndrome could be XRXR or XRXr as she only needs 1 copy of gene to have a disorder