Gene Linkage (HL) (HL IB Biology)

Gene loci are said to be linked if they are on the same chromosome

• Loci (singular: locus) refers to the specific linear positions on the chromosome that genes occupy
• If genes are on the sex chromosome, they are said to be sex-linked
  • Sex-linked genes have characteristics that generally only affect one gender of a species
  • These genes are usually on the X chromosome because the Y chromosome contains very few genes
  • In humans, colour-blindness and haemophilia are notable examples of genetic conditions that only affect males
• Linked genes located on the chromosomes 1-22, or any chromosome that is not a sex chromosome (called autosomes) are said to be examples of autosomal linkage
• The likelihood of genes being inherited together, or the extent to which they are linked, is measured in units called centimorgans, in honour of Thomas Hunt Morgan's work

Notation for linked genes

• A common way of denoting linked alleles is to link them with lines, representing the homologous chromosomes
  • E.g. in a cross between a double heterozygous individual where the dominant alleles G and R are linked and the recessive alleles g and r are linked, the notation would be:

Autosomal linkage

• Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis
• However, not all genes assort independently during meiosis
• Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination
• Linkage between genes affects how parental alleles are passed onto offspring through the gametes

Identifying autosomal linkage from phenotypic ratios

• In the following theoretical example, a dihybrid cross is used to predict the inheritance of two different characteristics in a species of newt
  • The genes are for tail length and scale colour
• The gene for tail length has two alleles:
  • Dominant allele T produces a normal length tail
  • Recessive allele t produces a shorter length tail
• The gene for scale colour has two alleles:
  • Dominant allele G produces green scales
  • Recessive allele g produces white scales

Without linkage

• The outcomes for this dihybrid cross if the genes are unlinked are as follows

Dihybrid Cross without Linkage Punnett Square Table

• Predicted ratio of phenotypes in offspring =
  • 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales
• Predicted ratio of genotypes in offspring =
  • 1 TtGg : 1 Ttgg : 1 ttGg : 1 ttgg

With linkage

• However, if the same dihybrid cross is carried out but this time the genes are linked, we get a different phenotypic ratio
  • There would be a 1 : 1 phenotypic ratio (1 normal tail, green scales : 1 short tail, white scales)
  • This change in the phenotypic ratio occurs because the genes are located on the same chromosome
  • The unexpected phenotypic ratio, therefore, shows us that the genes are linked
• The explanation for this new phenotypic ratio is given in the worked example below:

Dihybrid Cross with Linkage Punnett Square Table

• Predicted ratio of genotypes in offspring =
  • 1 (TG)(tg) : 1 (tg)(tg)
• Predicted ratio of phenotypes in offspring =
  • 1 normal tail, green scales : 1 short tail, white scales