How is genetic information inherited? - OCR 21st CenturyCarrying out a genetic cross
Our genes are inherited from our parents, and the different combinations of these genes make us unique. Genetic inheritance controls the characteristics of all living things.
Genetic crosses of single gene combinations (monohybrid inheritance) can be shown and examined using Punnett squareA mathematical model used to predict all the possible combinations of alleles at fertilisation.. This shows the possible offspring combinations could be produced, and the probabilityThe extent to which something is likely to be the case. of these combinations can be calculated.
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.
Draw out an empty Punnett square, like the one below, to help predict the possible gene combinations.
T
T
t
t
t
T
T
t
T
T
The female is TT - tall
The male is tt - short
The Punnett square below shows the possible allele combinations of the offspring produced when two homozygous pea plants, tall female and short male, are bred.
T
T
t
Tt
Tt
t
Tt
Tt
t
T
Tt
T
Tt
t
T
Tt
T
Tt
Adding the combinations into each box shows that all the possible offspring have the same allele combination - Tt.
This means that all the offspring produced will be tall.
Worked example 2
In this genetic cross, the mother is heterozygous for the wet earwax gene (Ee). The father is homozygous for the dry earwax allele (ee).
E
e
e
Ee
ee
e
Ee
ee
e
E
Ee
e
ee
e
E
Ee
e
ee
Half of the possible offspring will have wet earwax (Ee). The other half will have dry earwax (ee).
These examples are single gene combinations, but remember that most phenotypes are controlled by multiple genes.