Lesson
From alleles to probabilities
A Punnett square is a model of chance under stated assumptions.
Learning goals
- Use a monohybrid cross to find expected genotypic and phenotypic ratios.
- State an assumption that would break the simple Mendelian model.
Explanation
An allele is a version of a gene. In a simple dominant–recessive pair, a heterozygote shows the dominant phenotype. A Punnett square lists equally likely gametes and the offspring combinations. The ratios are expectations for a large number of offspring, not a guarantee for a family of four.
Real inheritance is often messier: incomplete dominance, linked genes, multiple genes, environment. A pedigree can suggest a pattern—skipping generations hints at recessive—but a small family can fool you. Treat the chart as evidence, not as a verdict carved in stone.
Key terms
- Allele. A particular version of a gene.
- Heterozygote. An individual with two different alleles of a gene.
Common mistakes
- Treating a 3:1 ratio as a promise for every litter.
- Calling a trait “dominant” because it is common in the population.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A cell’s surface-area-to-volume ratio is important because it affects:
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
A trait is common in a population but hidden in heterozygotes. Is it necessarily dominant?
Answer. No. Frequency in a population is not the same as dominance in a genotype.
Dominance describes a heterozygous phenotype, not how many people have the allele.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Biology course page
Official eight-unit framework, science practices, and lab resources.
College Board · All rights reserved · accessed 2026-10-01
Open sourceReference only — do not copy
OpenStax Biology 2e
Biology reference with commercial-use restrictions on the current publisher page.
OpenStax · CC BY-NC-SA 4.0 with additional restrictions · accessed 2026-10-01
Link only unless permission is obtained.
Open source