More examples of genetic crosses - Higher

More possible examples of single gene combinations:

Example 1

Aa × AA

Where Aa = female genotype

AA = male genotype

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.

Probability 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.

Note that all the phenotypes are identical, and will show the dominant characteristic.

Example 2

Ee × Ee

Where Ee = female genotype

Ee = male genotype

Ee
EEEEe
eEeee

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

Note that the phenotypes of the EE and Ee offspring will be identical - they will look the same.

Probability calculations for example 2 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

Using the worked example to help you, work through the additional examples below.

Step 1:

Female: Aa

Male: Aa

Step 2: Aa × Aa

Step 3:

Female alleles: A and a

Male alleles: A and a

Step 4:

Aa
A
a

Step 5:

Aa
AAAAa
aAaaa

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

Method:

  • Draw your Punnett square.
  • Write the alleles into your Punnett square around the edges.
  • 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 Dd genotypes 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 tier 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, estimate 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.