VMBO Year 3 · Inheritance and evolution · Section 3.3

Genetic crosses

What is a genetic cross?

A cross produces offspring from two parents to investigate inheritance, often focusing on one characteristic. The parents are the P generation. Their offspring form F1, the first filial generation. Offspring produced by crossing F1 individuals form F2. Each step is a new generation.

Drawing a genetic cross

A crossing diagram, often including a Punnett square, shows:

• The parents' genotypes and phenotypes.
• Which alleles their sex cells can carry.
• The possible genotypes and phenotypes of their offspring.

For example, in a simplified coat-colour model, black is dominant to yellow. A black female AA crossed with a yellow male aa produces F1 offspring that are all Aa and black.

The F2 generation

Crossing two F1 individuals, Aa × Aa, gives possible genotypes AA, Aa and aa. Their expected ratio is 1 AA : 2 Aa : 1 aa. With complete dominance, the expected phenotype ratio is 3 black : 1 yellow. These are probabilities for each offspring, not a guarantee that every group of four contains exactly three black animals and one yellow animal.

Incomplete dominance

For a flower-colour gene with incomplete dominance, RR × WW produces pink RW offspring. Crossing these, RW × RW, gives expected genotype and phenotype ratios of 1 red : 2 pink : 1 white.

Ratios in different crosses

• Aa × aa: 1 Aa : 1 aa, giving a 1 : 1 phenotype ratio under complete dominance.
• Aa × Aa: 1 AA : 2 Aa : 1 aa, giving a 3 : 1 phenotype ratio under complete dominance.
• RW × WW: 1 RW : 1 WW, giving 1 pink : 1 white.
• RW × RW: 1 RR : 2 RW : 1 WW, giving 1 red : 2 pink : 1 white.

With larger numbers of offspring, observed proportions tend to approach these expected ratios.