Monohybrid and dihybrid inheritance, sex linkage, the sources of variation, natural selection and speciation.

What this dot point is asking

WJEC wants you to carry out and interpret monohybrid and dihybrid crosses, explain codominance and sex linkage, describe the sources of variation, and explain natural selection and speciation.

Monohybrid and dihybrid inheritance

When writing a cross, set out the parental phenotypes and genotypes, the gametes, the Punnett square of offspring genotypes, and finally the phenotype ratio. A test cross (crossing an unknown dominant with a homozygous recessive) reveals whether the dominant individual is homozygous or heterozygous.

Codominance and sex linkage

Variation, selection and speciation

Variation arises from mutation (the ultimate source of new alleles), the independent assortment and crossing over of meiosis, and random fertilisation. In natural selection, a selection pressure means individuals with favourable alleles survive and reproduce more, so the frequency of those alleles increases over generations. Speciation happens when two populations become reproductively isolated (often geographically) and accumulate genetic differences through selection until they can no longer interbreed to produce fertile offspring.

Examples in context

Example 1. Peppered moth industrial melanism. Before industrialisation the pale form of the peppered moth was camouflaged on lichen-covered trees; soot darkened the trees, so the dark allele became favourable and its frequency rose sharply. As pollution fell, the pale form recovered. This is the classic British example of natural selection changing allele frequency.

Example 2. ABO blood groups and codominance. The $\text{I}^A$ and $\text{I}^B$ alleles are codominant, so an $\text{I}^A\text{I}^B$ person has blood group AB and makes both antigens, while $\text{I}^O$ is recessive. This shows codominance and multiple alleles in one familiar human example examiners often use.

Try this

Q1. State the expected phenotype ratio from crossing two heterozygotes for a single gene. [1 mark]

• Cue. $3:1$ (dominant to recessive).

Q2. Explain why colour blindness is more common in males than females. [2 marks]

• Cue. The allele is recessive and X-linked; males have only one X, so a single recessive allele is expressed, whereas females need two.

Q3. A dihybrid cross $AaBb \times AaBb$ produces $160$ offspring. Calculate how many are expected to show both recessive traits. [2 marks]

• Cue. The double recessive fraction is $\frac{1}{16}$; $\frac{1}{16} \times 160 = 10$.