6.2 - Patterns of Inheritance OCR A - A Level Biology

• Investigations that examine the simultaneous inheritance of 2 characteristics are dihybrid cross.
• Mendel found that when true breeding homozygous yellow and round (dominant) plants were crossed with true breeding homozygous yellow and round (recessive) plants, the F1 generation were all dihybrid with the phenotypes yellow and round (heterozygous).
• 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 2 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 chance 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 the 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.