Monohybrid inheritance and Punnett squares, dominant and recessive alleles, predicting ratios and probabilities, genetic variation and the causes of variation, and mutations.

What this dot point is asking

Edexcel wants you to use Punnett squares to predict the outcome of a monohybrid cross, work with dominant and recessive alleles, express results as ratios and probabilities, explain the genetic and environmental causes of variation, and describe mutations.

Dominant and recessive alleles

An organism with two identical alleles (for example $AA$ or $aa$) is homozygous; one with two different alleles ($Aa$) is heterozygous. A heterozygous organism shows the dominant characteristic but can still pass on the recessive allele.

Punnett squares

A Punnett square predicts the proportions of offspring genotypes from a cross. You write one parent's gametes along the top, the other's down the side, and fill in the combinations.

A predicted ratio is a probability, not a guarantee: a $3:1$ ratio does not mean exactly three of every four real offspring will be brown-eyed, especially in small numbers. The more offspring there are, the closer the actual results tend to be to the predicted ratio, which is why genetic crosses are easier to confirm in organisms that produce many offspring, such as fruit flies or pea plants.

Inherited disorders and sex determination

Some conditions are caused by faulty alleles passed on from parents. Cystic fibrosis is caused by a recessive allele, so a person must inherit two copies (one from each parent) to have the disorder; people with one copy are unaffected carriers. A Punnett square of two carriers ($Ff \times Ff$) predicts a $1$ in $4$ chance of an affected child. Sex in humans is also decided genetically: females have two X chromosomes ($XX$) and males have an X and a Y ($XY$). Because the father can pass on either an X or a Y, a Punnett square of $XX \times XY$ predicts a $1:1$ ratio of girls to boys.

Causes of variation

Genetic variation arises through sexual reproduction (mixing alleles from two parents) and through mutations.

Mutations

A mutation is a random change in the DNA base sequence. Mutations:

• Happen continuously and can be caused or sped up by radiation and some chemicals.
• Create new alleles, and so are the original source of all genetic variation.
• Are usually neutral, sometimes harmful, and occasionally beneficial. A beneficial mutation can spread through a population by natural selection.

Try this

Q1. A cross of $Aa \times Aa$ gives what ratio of dominant to recessive phenotypes? [1 mark]

• Cue. $3:1$.

Q2. Give one example of environmental variation. [1 mark]

• Cue. A scar, a sun tan, a tattoo, or a learned skill such as a language (any feature caused by surroundings).