Sex determination

• sex chromosomes

  • Male: XY
  • female: XX
• why does the inheritance of a Y chromosome mean that an embryo develops into a male?

  testes development in an embryo is stimulated by a gene present on the Y chromosome
• what is a sex-linked characteristic?

  characteristic that is coded for by an allele found on a sex chromosome
• why are men more likely to show the phenotype for a recessive sex-linked trait than women?

  many genes are found on X chromosome that have no counterpart on the Y chromosome

  women (XX) have 2 alleles for each sex-linked gene whereas men (XY) often only have 1 allele

  only 1 recessive allele is required to produce the recessive phenotype in males
• why are the majority of genes found on the X chromosome rather than the Y chromosome?

  X chromosome is bigger than Y so more genes are carried on it