Monohybrid inheritance

In the syllabus and the exam

Monohybrid inheritance is the pattern of inheritance for a single characteristic controlled by one gene with two alleles, and it is examined almost entirely through the construction and correct labelling of a genetic diagram (or Punnett square) tracing that characteristic from parents through an F1 and, where relevant, an F2 generation. A full-mark answer states the phenotypes and genotypes of the parents, the gametes each parent can produce, the possible genotype combinations of the offspring set out systematically (usually in a Punnett square), and the resulting genotype and phenotype ratios of the offspring, all using consistent, correctly cased letter symbols throughout. A very common error is losing marks for a technically correct final ratio built from an incomplete or incorrectly labelled diagram, since the process itself, not only the final answer, carries significant marks in Paper 2's structured questions. Candidates should also be able to work in both directions: predicting offspring ratios from known parental genotypes, and, in reverse, deducing an unknown parental genotype from a given offspring ratio, which is the more demanding higher-order version of this skill. Monohybrid inheritance should not be confused with dihybrid inheritance, which tracks two different genes at once and is taught as a separate, more complex content standard building on the same underlying logic. Candidates should also be able to identify a test cross correctly within a monohybrid inheritance scenario, recognising it as a cross specifically designed to reveal an unknown genotype rather than simply another routine cross between two known parents.

Example

Crossing a pure-breeding tall pea plant (TT) with a pure-breeding short pea plant (tt) produces an F1 generation that is entirely Tt (all tall), and self-pollinating this F1 generation gives an F2 generation with a 3:1 ratio of tall to short plants.

Related terms

GenotypePhenotypeAllele

Source:SRC-DSKP-EN