Dihybrid crosses, linkage and epistasis

Last time

Before we start: last time, in Striated muscle and the sliding filament model, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Two genes at once

Mendel did not stop at one gene. He followed two features at the same time, seed shape and seed colour. The results gave a famous ratio of nine to three to three to one. But when real organisms break that ratio, the reason is nearly always one of two things: the genes sit together on one chromosome, or one gene interferes with another. Those are linkage and epistasis.

The dihybrid cross

A pure breeding red, round fruit plant R R S S is crossed with a yellow, oval one r r s s. All the F one are heterozygous, R r S s, and show both dominant features. When the F one are crossed with each other, each parent makes four kinds of gamete in equal numbers: R S, R s, r S and r s. Independent assortment makes this happen. The Punnett square has sixteen boxes. The phenotype ratio is nine dominant for both, three dominant for one, three dominant for the other, and one recessive for both.

A worked example

Suppose 1440 offspring are produced. The expected numbers are nine sixteenths, three sixteenths, three sixteenths and one sixteenth of that. One sixteenth is ninety. So the expected counts are 810, 270, 270 and 90. In the dihybrid test cross, R r S s crossed with r r s s, the ratio is one to one to one to one.

A past-paper example

October November 2024, Paper 42, question 4. A cross was carried out between a pure breeding red fruited tomato plant and a plant with yellow fruit, and a second gene is also followed. Work out the parental genotypes, the F one genotype, the gametes, and the F two ratio. A nine to three to three to one ratio shows independent assortment. Show the key and the working in the answer.