phenotypic ratios

• • Loci for different alleles = their position on a chromosome
  • Don’t forget alleles can be located on different chromosomes entirely
• • Linked genes are close together on the same chromosome; they have a similar locus
  • Unlinked genes are further apart
  • This means linked genes are likely to be inherited together
  • Whereas unlinked genes have a much more random inheritance
• unlinked genes have random inheritance because -

  • During meiosis the chromosomes line up randomly during metaphase
  • This means you have a random 50% chance of inheriting each unlinked gene
  • Or a 1:1:1:1 ratio in the offspring phenotype
• A sex-linked characteristic is controlled by a gene/allele that is located on the sex chromosomes, usually on the X chromosome.
• • There is far more genetic material on the X chromosome.
  • This means that there is no equivalent homologous portion of the Y chromosome (I.e. normally you have two alleles for hair colour, one from your mother and one from your father. If a trait is sex linked and is located on the X chromosome then there may only be one allele present for that trait)
  • The implication is that recessive characteristics can be expressed more easily on the X chromosome. E.g. colour blindness