Carrying out a genetic cross

Monohybrid crosses

Genetic crosses of single gene combinations (monohybrid inheritance) can be examined and shown using a Punnett square. This shows the possible offspring combinations that can be produced, as well as allowing you to calculate the of these different combinations occurring.

Example one

The height of pea plants is controlled by a single gene which has two alleles – tall and short.

The tall allele is dominant and is shown as T.

The short allele is recessive and is shown as t.

Complete this Punnett square to show the possible allele combinations of the offspring produced when two pea plants are bred.

Table with the rows labelled as Male alleles (t,t) and the columns labelled as Female alleles (T,T)

The female parent plant is tall – TT.

The male parent plant is short – tt.

Table with the rows labelled as Male alleles (t,t) and the columns labelled as Female alleles (T,T). The cells of the table all contain Tt

If you add the combinations into each box, you see that all the possible offspring have the same allele combination – Tt. This means that all the offspring in the F1 (first generation) will be tall.

If were to occur, the pollen of one flower will be transferred to the stigma of the same flower. When this happens, the flower is said to be self-pollinating, meaning the F1 genotype is crossed with itself.

Table with the rows labelled as Male alleles (T,t) and the columns labelled as Female alleles (T,t). The cells of the table contain TT, Tt, Tt and tt

75 per cent of the offspring will be tall, because they contain the dominant allele. 25 per cent of the offspring will be short, because \(\frac{1}{4}\) of the genotypes are homozygous recessive, and will therefore show the recessive trait.

Example two

In this genetic cross, the female alleles are Dd and the male alleles are dd. This cross is known as a backcross.

Half of the possible offspring have the allele combination Dd, and the other half have the allele combination dd.

These examples are single gene combinations, but remember that most phenotypes are controlled by multiple genes.

How to use probability and simple ratios

You can express the outcome of a genetic cross using probability or ratios.

Example three

Table with the rows labelled as Male alleles (A,a) and the columns labelled as Female alleles (a,a). The cells of the table contain Aa, Aa, aa and aa

Probability

The probability of the offspring being Aa is \(\frac{1}{2}\) (or 50%). The probability of the offspring being aa is also \(\frac{1}{2}\) (or 50%).

Ratio

The ratio of Aa to aa is 1:1.

It is important to remember that during the process of fertilisation, the allele combinations created are a random process, and that is why probability is used, as nothing is guaranteed. Therefore, each of the four possible offspring combinations is as likely to happen during every fertilisation event.

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DNA and inheritance – WJECCarrying out a genetic cross