Carrying out a genetic cross

Monohybrid crosses

Genetic crosses of single gene combinations monohybrid inheritance can be shown and examined using Punnett squares. This shows which possible offspring combinations could be produced, and the probablity of these combinations occurring.

Worked example 1

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 small 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.

TT
t
t

The female is TT - tall

The male is tt - short

TT
tTtTt
tTtTt

If you add the combinations into each box you see that all the possible offspring have the same allele combination - Tt.

Note: you should always write the dominant allele first.

This means that all (100 per cent) of the offspring produced will be tall.

Worked example 2

In this genetic cross, the female alleles are Dd and the male alleles are dd.

Dd
dDddd
dDddd

Half (50 per cent) of the possible offspring have the same allele combination; Dd. The other 50 per cent have the dd combination.

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

Worked example 3

A male dog with brown fur and a female with black fur have puppies. The mother is homozygousdominant and the father is heterozygous. The allele for black fur is dominant and represented with the letter B. The allele for brown fur is recessive and represented with the letter b.

Mother's genotype: BB (she is homozygous dominant)

Father's genotype: Bb (he is heterozygous)

The mother can only pass on B alleles, but the father can pass on B or b alleles.

BB
BBBBB
bBbBb

All of the offspring inherit at least one B allele and so have black fur, however, 50 per cent of the offspring would be homozygous dominant and 50 per cent would be heterozygous.