Apply patterns of inheritance including dihybrid, codominance, multiple alleles, sex linkage and polygenic inheritance

What this dot point is asking

SCSA wants you to recognise which pattern a cross follows, predict genotype and phenotype ratios using Punnett squares, and explain why some traits do not follow simple Mendelian dominance. A strong answer chooses the right symbols and links the pattern to the underlying allele behaviour.

Dihybrid inheritance

A dihybrid cross follows two genes at once. When an individual heterozygous for both genes is crossed with another (for example RrYy by RrYy), the offspring show a characteristic 9:3:3:1 phenotype ratio, provided the two genes assort independently. This ratio reflects independent assortment in meiosis, where the alleles of one gene separate independently of the other.

Codominance and incomplete dominance

These describe what happens when neither allele is fully dominant.

• Codominance: both alleles are expressed fully and separately in the heterozygote. In cattle, a red animal crossed with a white animal can give roan offspring showing both red and white hairs.
• Incomplete dominance: the heterozygote shows an intermediate, blended phenotype, such as a pink flower from red and white parents.

Multiple alleles: ABO blood groups

A gene can have more than two alleles in a population, even though any individual carries only two. The human ABO blood group is the classic example: the alleles are $I^A$, $I^B$ and $i$. The $I^A$ and $I^B$ alleles are codominant with each other, and both are dominant over $i$. This gives four blood groups (A, B, AB and O) from three alleles.

Sex linkage

Genes on the sex chromosomes, especially the X chromosome, show sex-linked inheritance. Because males are XY and have only one X, a single recessive allele on the X is expressed in males, while females (XX) need two copies. This is why X-linked recessive conditions such as red-green colour blindness and haemophilia are far more common in males.

Polygenic inheritance

Some traits are controlled by many genes acting together, each adding a small effect. The result is continuous variation, a smooth range rather than distinct categories. Human height and skin colour are polygenic, which is why they form a bell-shaped distribution in a population rather than falling into a few discrete classes.

Choosing the right pattern

The key skill is reading a problem and identifying the pattern: a 3:1 ratio suggests simple dominance, 9:3:3:1 suggests a dihybrid cross, three phenotypes from a cross of two heterozygotes suggests incomplete dominance or codominance, a trait far more common in males suggests sex linkage, and a continuous range suggests polygenic inheritance. Once the pattern is clear, the correct symbols and Punnett square follow.