Dihybrid Inheritance

What dihybrid inheritance studies

Dihybrid inheritance follows two characteristics at once, each controlled by a separate gene with two alleles. It builds directly on monohybrid inheritance, but tracks two genes and their alleles together instead of just one.

Independent assortment

Dihybrid inheritance depends on the law of independent assortment: if the two genes lie on different chromosomes, the alleles of one gene separate into gametes independently of the alleles of the other gene. This means a double heterozygote, such as TtYy, produces four different gamete types in equal proportion, TY, Ty, tY, and ty, rather than just two.

This independence arises during meiosis. When homologous chromosomes line up at the cell's equator, each pair orients at random and independently of the other pairs, so the maternal and paternal alleles of one gene are shuffled separately from those of a gene on a different chromosome.

That random arrangement mixes the alleles into new combinations in the gametes, and it is a main reason offspring differ from their parents and from one another. Independent assortment holds only when the two genes lie on different chromosomes; genes close together on the same chromosome tend to be inherited together.

The 9:3:3:1 ratio

When two double heterozygotes are crossed, for example TtYy x TtYy, the four gamete types from each parent combine to give sixteen possible offspring combinations. Grouped by phenotype, these fall into four classes in the ratio 9:3:3:1, nine showing both dominant characteristics, three showing the first dominant with the second recessive, three showing the first recessive with the second dominant, and one showing both recessive characteristics.

This ratio is the signature result of a dihybrid cross between two double heterozygotes, in the same way that 3:1 is the signature result for a monohybrid cross.

How this is examined

SPM questions typically describe two characteristics controlled by separate genes, ask you to determine the gamete types produced by a double heterozygote, and require you to state or work out the resulting phenotype ratio, usually via a 4 x 4 Punnett square.

Working out the gametes

The trick to a dihybrid cross is writing the gametes correctly. A double heterozygote such as RrYy carries one allele from each pair in every gamete, so it makes four combinations: take R or r, then pair it with Y or y.

This gives RY, Ry, rY and ry, each equally likely.

A quick check is that the number of different gametes equals 2 raised to the number of heterozygous gene pairs. RrYy has two heterozygous pairs, so 2 x 2 = 4 gamete types.

A plant that is RRYy has only one heterozygous pair, so it makes just two gamete types, RY and Ry.

Common mistakes to avoid

Worked exam-style question

Question. In pea plants, round seed shape (R) is dominant to wrinkled (r), and yellow seed colour (Y) is dominant to green (y). Two plants heterozygous for both genes (RrYy) are crossed.

(a) State the four gamete types each parent can make. (b) Using a Punnett square, state the phenotype ratio of the offspring.

(c) What is the chance that an offspring has wrinkled, green seeds?

Model answer. (a) Each RrYy parent makes four gamete types in equal proportion: RY, Ry, rY and ry. (b) The Punnett square below combines the gametes into sixteen boxes; grouping them by phenotype gives 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green.

(c) Only the rryy box is both wrinkled and green, 1 out of 16, so the chance is 1/16.

Practice question

Try it. Using the same pea plants (R round dominant to r wrinkled; Y yellow dominant to y green), a plant of genotype RrYy is crossed with a plant that is rryy. (a) State the gamete types of each parent.

(b) Predict the phenotype ratio of the offspring.

Exam tip

Key terms

• Gene, a length of DNA controlling one characteristic; a dihybrid cross follows two.
• Allele, an alternative form of a gene, such as R or r.
• Genotype, the alleles an organism carries, such as RrYy.
• Phenotype, the observable characteristic, such as round yellow seeds.
• Monohybrid inheritance, inheritance of a single gene, which gives the 3:1 ratio.

Source:SRC-DSKP-EN

Frequently asked questions

More for Inheritance