Dihybrid Inheritance – Inheritance Ep 3

Dihybrid inheritance looks at the possibilities of two characteristics being inherited together. Each characteristic is coded for by a separate gene. In this case it is much easier to use a Punnett square to work out the possible genotypes of the offspring as a genetic diagram would get very messy and difficult to read.

In the example below, we are looking at the inheritance of wing colour in beetles as we did in the monohybrid inheritance article. B is the dominant allele for blue wings, b is the recessive allele for white wings. However, this time we are looking at how this gene could be inherited with the gene for wing size. Y is the dominant allele for normal wings, y is the recessive allele for short wings. The Punnett square shows all the possible genotypes if the parents are heterozygous for both genes.

The phenotypic ratio is 9:3:3:1. This means that there is the highest chance (9 in 16) that the offspring will express both dominant characteristics, but only a 1 in 16 chance that the offspring will express both recessive characteristics. However, sometimes this phenotypic ratio is not found in reality. This hints that there could be linkage between the genes. We will focus on linkage in the next article.

For A-Level biology, it is helpful to know the expected phenotypic ratios for a monohybrid cross, a dihybrid cross, and crosses involving codominance. Sometimes it can save time in exams!

Summary

• Dihybrid inheritance looks at how two different genes can be inherited together.
• The expected phenotypic ratio of the offspring from a dihybrid cross of two heterozygous parents is 9:3:3:1.

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