OCR A Level Biology

Revision Notes

6.2.5 Predicting Inheritance: Dihybrid Crosses

Predicting Inheritance: Dihybrid Crosses

  • Monohybrid crosses look at how the alleles of one gene transfer across generations
  • Dihybrid crosses look at how the alleles of two genes transfer across generations
    • I.e. dihybrid crosses can be used to show the inheritance of two different characteristics in an individual
  • The genetic diagrams for both types of crosses are very similar
  • For dihybrid crosses, there are several more genotypes and phenotypes involved
  • When writing out the different genotypes, write the two alleles for one gene, followed immediately by the two alleles for the other gene. Do not mix up the alleles from the different genes
    • For example, if there was a gene with alleles Y and y and another gene with alleles G and g an example genotype for an individual would be YyGg

Worked Example

Worked example: Dihybrid genetic diagram

  • Horses have a single gene for coat colour that has two alleles:
    • B, a dominant allele produces a black coat
    • b, a recessive allele produces a chestnut coat
  • Horses also have single gene for eye colour
    • E, a dominant allele produces brown eyes
    • e, a recessive allele produces blue eyes
  • Each of these genes (consisting of a pair of alleles) are inherited independently of one another because the two genes are located on different non-homologous chromosomes
    • Such characteristics are said to be unlinked
  • In this example, a horse that is heterozygous for both genes has been crossed with a horse that is homozygous for one gene and heterozygous for the other

Parental phenotypes: black coat, brown eyes x chestnut coat, brown eyes

Parental genotypes:   BbEe                                      bbEe

Parental gametes:      BE or Be or bE or be             bE or be

Dihybrid Cross Punnett Square Table

Dihybrid Cross Punnett Square Table, downloadable AS & A Level Biology revision notes

  • Predicted ratio of phenotypes in offspring = 3 black coat, brown eyes : 3 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes
  • Predicted ratio of genotypes in offspring = 3 BbEE : 3 bbEE : 1 Bbee : 1 bbee

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