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A genetic cross is a probability model, not a guarantee of exact offspring counts. Define alleles and parental genotypes before predicting gametes and phenotypes.
Video by Cognito. Foundation recap: the written lesson develops the A-level detail.
Viewing focus: Use genotype, phenotype and dominance precisely
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: A dominant allele is defined through its effect in:
A heterozygote. Dominance is not the same as frequency or advantage.
After watching 2: Aa×Aa gives a genotype ratio of:
1 AA : 2 Aa : 1 aa. List and combine the two gamete types.
What you'll be able to do
Genotype is an organism’s genetic constitution at the loci considered; phenotype reflects expression and interaction with the environment. A homozygote carries matching alleles at a locus and a heterozygote different alleles. Dominance concerns phenotype in a heterozygote, not an allele’s frequency, strength or desirability.
Define symbols before writing a cross and distinguish a genotype from an observed phenotype. For a dominant phenotype, either a homozygous or heterozygous genotype may be possible. A recessive phenotype in the simple model identifies a homozygous recessive genotype, but only under the assumptions stated for that trait.
Write parental genotypes, list the alleles in possible gametes, combine one gamete from each parent and translate offspring genotypes to phenotypes using the dominance rule. A heterozygote-by-heterozygote cross under complete dominance gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio.
Each fertilisation is a probability event. Four offspring do not have to consist of exactly three dominant and one recessive phenotype. Larger samples tend to give more stable proportions, but observed and expected frequencies can differ; an appropriate chi-squared analysis can test whether a departure is unusually large under the model.
For an X-linked locus, write allele symbols on the X chromosome and distinguish the Y where it lacks the corresponding allele. A typical XY individual is hemizygous at such a locus, so a recessive X-linked allele can be expressed with only one copy.
A father passes his X chromosome to daughters and his Y to sons in the simplified model, so direct father-to-son transmission is not the expected pattern for an X-linked allele. Pedigree conclusions should consider family size, missing genotypes and alternative explanations instead of relying on one affected individual.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.