# More examples of genetic crosses

More possible examples of single gene combinations:

Aa x AA where Aa = female alleles and AA = male alleles

Aa
AAAAa
AAAAa

Half of the possible offspring have the same allele combination; AA homozygous dominant and the other half have the Aa heterozygous combination.

calculations are: two out of four possible combinations create 50% chance for AA, and 50% chance for Aa alleles.

There is a ratio of 1:1

Ee x Ee where Ee = female alleles and Ee = male alleles

Ee
EEEEe
eEeee

There are three different offspring combinations. One is EE (homozygous dominant), two are Ee (heterozygous) and one is ee (homozygous recessive).

Probability calculations are: one out of four possible combinations create 25% chance for EE, two offspring create a 50% chance for the Ee alleles, and finally 25% chance for the ee alleles.

This can be shown simply as:

EE - 25%

Ee - 50%

ee - 25%

The probability values must add up to 100% (25 + 50 + 25 = 100)

The ratio can be summarised as 1: 2 : 1.

AllelePercentageRatio
EE251
Ee502
ee251

Step 1:

Female: Aa Male: Aa

Step 2: Aa x Aa

Step 3:

Aa
A
a

Step 4: Female: A and a

Male: A and a

Step 5:

Aa
A
a

Step 6:

Aa
AAAAa
aAaAa

Allele letter choice select a letter that has a clearly different lower case, for example: Aa, Bb, Dd.

Method:

• Work out the new possible genetic combinations inside the Punnett square
Question

Coat type in dogs is determined by a single gene with two alleles.

The straight coat allele is dominant (D) and the curly coat allele is recessive (d).

Two dogs with the alleles Dd breed. Two dogs with the alleles Dd breed.

Complete this Punnett square to show the allele combinations of the possible offspring produced.

Dd
D
d
Dd
DDDDd
dDddd
Question

Calculate the probability that an offspring will have the combination DD.

25%.

Question

Determine the ratio of straight hair offspring to curly hair offspring.

3 straight : 1 curly

Question

Higher question

Flower colour in pea plants is inherited genetically.

The white allele is recessive and the pink allele is dominant.

A plant that is homozygous recessive is bred with a plant that is heterozygous. If 48 seeds are produced, calculate an estimate for how many will have pink flowers.

aa
AAaAa
aaaaa

Offspring that are Aa are pink so there is a 50% probability of a plant being pink. 48 × (50 ÷ 100) = 24 plants.