The dihybrid cross

To understand the dihybrid cross it is necessary to understand the process of meiosis. You must have a clear understanding of the stages of meiosis. In the first meiotic division, the homologous pairs of chromosomes have aligned at random before being separated from each other. The separation of the chromosomes gives rise to the independent assortment of the chromosomes. In the second meiotic division, the four cells that are formed develop into haploid gametes.

All genetic crosses show how characteristics are inherited through the generations:

• a monohybrid cross shows the inheritance of one alternative pair of characteristics through two generations. From parents to the F₁ generation and then from the F₁ generation to the F₂ generation.
• a dihybrid cross shows the inheritance of two alternative pairs of characteristics through two generations. In a dihybrid cross the genes which determine two pairs of different characteristics are located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.

The diagrams show the alleles for two genes on two different pairs of homologous chromosomes in gamete mother cells from two different pea plants (parents).

Crosses have a recognised format and the structures of chromosomes are not shown. Instead, the letters used to identify the alleles of the genes are used to represent the chromosome. Thus, when showing a cross, YYRR would represent two homologous pairs of chromosomes. On one of the homologous pairs both alleles would be for the dominant form of the gene for seed colour and in the other, both alleles would be for the dominant form of the gene for seed shape.

Example 1

Show the results of a cross between the following pea plants through to the second generation:

• A homozygous (true-breeding) pea plant with yellow round seeds crossed with a homozygous pea plant with green wrinkled seeds.
• The allele for yellow (Y) is dominant to the allele for green (y).
• The allele for round (R) is dominant to the allele for wrinkled (r).

First stage of parental cross

First stage of parental cross

This shows a cell with two homologous pairs of chromosomes at the start of meiosis. The chromosomes have already replicated. The dominant allele for the gene for seed colour is present on both members of one of the homologous pairs and the dominant allele for the gene for seed shape on the other pair.

The first task is to show the cross to the F₁ generation. In the format of the cross the phenotypes and genotypes of the parents must be shown together with the genotypes of the gametes that can be formed as a result of meiosis.

Summary

All the F₁ generation have the YyRr genotype because in meiosis the only gametes that can be formed will be YR in the yellow round seeded parent and yr in the green wrinkle seeded parent. All the F₁ generation have a yellow round phenotype as they have all inherited copies of these dominant alleles from the yellow round seeded parent.

Second stage of parental cross

Second stage of parental cross

Pea plants grown from the seeds of the F₁ generation are crossed. In the format of the cross the phenotypes and genotypes of the parents must be shown together with the genotypes of the gametes that can be formed as a result of meiosis.

Four types of gamete are produced for each parent. This is the result of random alignment of the homologous pairs of chromosomes and their separation during the process of meiosis.

Diagram 1: Original Maternal and Paternal chromosomes aligned on the same side	Diagram 2: Original Maternal and Paternal chromosomes not aligned

The diagrams show the two different ways in which the homologous pairs may align before being separated.

Cross between members of the F₁ generation

Diagram 1 - The homologous pairs have aligned in such a way that the chromosomes with the dominant alleles will be separated together at the next stage in meiosis and those with the recessive alleles will also be separated together.

Diagram 2 - The homologous pairs have aligned in such a way that a chromosome with a dominant allele of one of the genes will be separated together with a chromosome with the recessive allele for the other characteristic at the next stage in meiosis.

Each parent has produced the same four types of gamete. Fertilisation is a chance process, therefore, any one of the gametes from one parent can fuse with any one of the four types of gamete of the other parent. These possible combinations of gamete are best shown in a Punnett square. The possible gametes of each parent are set out along different axis and the possible combinations of gametes shown within the squares:

When the phenotypes of the 16 possible combinations are identified, it is found that the expected ratio in the F2 generation is:

The results from such crosses show the expected ratios of the four different phenotypes.

Another common dihybrid cross is between individuals heterozygous for both characteristics and individuals homozygous recessive for both characteristics. This cross is shown using the same characteristics as in the previous example.

When the phenotypes of the 4 possible combinations are identified, it is found that the expected ratio is:

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