6.2.3 - Dihybrid Inheritance

• Investigations that examine the simultaneous inheritance of two characteristics are dihybrid cross.
• Mendel found that when true-breeding homozygous yellow and round are crossed with true-breeding green and wrinkled (dominant x recessive), all the F1 generation are dihybrid (heterozygous for each of the two genes) with phenotypes yellow and round.
• From the results of the dihybrid crosses, Mendel deduced that:

  • the alleles of the 2 genes are inherited independently of each other, so each gamete has 1 allele for each gene locus
  • during fertilisation, any one of an allele pair can combine with any one of another allele pair
• Dihybrid - The inheritance of 2 separate genes at 2 separate loci (often on different chromosomes)
• If we consider the two crosses for seed colour and seed shape as 2 independent monohybrid crosses, with the two sets of traits being inherited independently, we can predict the outcome of allowing the members of the F1 generation to self-fertilise.
• The chances of the traits for seed colour being inherited are not influenced by the chances of the seed shape being inherited.
• Assuming that seed colour and seed shape are two separate monogenic characteristics:

  • The characteristics of colour and shape would be predicted in a ratio of 3:1.
• When Mendel counted his pea plants in the F2 generation of a dihybrid cross, the obtained ratio was 9:3:3:1.
• When investigating dihybrid inheritance, Mendel, without knowing about genes or the process of meiosis, chose 2 characteristics which are on different chromosomes:

  • If 2 genes are on the same chromosome, the inheritance pattern is different.