6.2.6 Predicting Inheritance: Identifying Linkage

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
• When writing linked genotypes it can be easier to keep the linked alleles within a bracket
  • For example, an individual has the genotype FFGG. However, if there is linkage between the two genes, it would be written as (FG)(FG)

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

• A newt heterozygous for a normal tail and green scales is crossed with a newt that has a shorter tail and white scales

Parental phenotypes: normal tail, green scales x short tail, white scales

Parental genotypes:                  TtGg x ttgg

Parental gametes:           TG or Tg   or       tG or tg

Without linkage

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

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

Sex linkage

• Sex-linked genes are only present on one sex chromosome and not the other
• This means the sex of an individual affects what alleles they pass on to their offspring through their gametes
• The presence of sex linkage can be identified using pedigree diagrams and Punnett squares
  • When a gene is sex-linked the phenotypes are not spread evenly across the sexes
  • In the case of a gene that causes a sex-linked disease, one sex will be  disproportionately affected

• If the gene is on the X chromosome males (XY) will only have one copy of the gene, whereas females (XX) will have two
• Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
• Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
• A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
• If that offspring is a male, he will have the disease
• For sex-linked genes that occur on the X chromosome, there are three phenotypes for females: 'normal', 'carrier' and 'has the disease', whereas males have only two phenotypes: 'normal' or 'has the disease'
• The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:

Punnett square showing the inheritance of colourblindness, an X-linked condition

• In the cross above, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male
• Males are more likely to be affected by the condition

Monohybrid Cross with Sex-linkage Punnett Square Table

Predicted ratio of phenotypes in offspring = 1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting

Predicted ratio of genotypes in offspring = 1 X^FX^F : 1 X^FX^f : 1 X^FY : 1 X^fY