Predict inheritance patterns with Punnett squares, and connect DNA, genes, and traits across generations — most of the difficulty here is tracking which allele combination produces which observable trait. Every question comes with a written walkthrough of the cross.
Genotype is the underlying allele combination (like Bb); phenotype is what's actually observed (like brown eyes) — two different genotypes, Bb and BB, can produce the identical phenotype if B is dominant.
By convention the dominant allele is written as the capital letter, but the problem still has to state which trait each letter represents — assuming the "first" trait mentioned is automatically dominant is a guess, not a rule.
A Punnett square gives probabilities for each offspring independently, not a guarantee that exactly 1 in 4 offspring in a small litter will show a specific trait — the ratio describes the expected proportion over many offspring.
Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.
In pea plants, purple flowers (P) are dominant over white flowers (p). If a plant with genotype Pp is crossed with a plant with genotype pp, what fraction of the offspring are expected to have purple flowers?
A — 1/2. Crossing Pp × pp produces offspring in a 1:1 ratio of Pp to pp. Since P is dominant, Pp offspring show purple flowers — that's 2 out of 4 boxes in the Punnett square, or 1/2. 3/4 (C) is the ratio you'd expect from a heterozygous × heterozygous cross (Pp × Pp), not this cross.
Two brown-eyed parents (Bb) have four children. Based on the Punnett square, what is the probability that any one child has blue eyes (bb)?
A — 1/4. Crossing Bb × Bb gives a genotype ratio of 1 BB : 2 Bb : 1 bb, so 1 out of 4 offspring is expected to be bb (blue eyes). 0, since both parents have brown eyes (D) ignores that both parents carry a hidden recessive b allele — a recessive trait can appear in offspring even when neither parent shows it, as long as both are carriers.
In guinea pigs, black fur (B) is dominant over white fur (b), and short hair (H) is dominant over long hair (h). A guinea pig with genotype BbHh is crossed with another BbHh guinea pig. What fraction of the offspring are expected to have white fur and long hair?
A — 1/16. Each trait segregates independently: the probability of white fur (bb) from Bb×Bb is 1/4, and the probability of long hair (hh) from Hh×Hh is also 1/4. Multiplying the two independent probabilities gives 1/4 × 1/4 = 1/16. 3/16 (C) comes from a common dihybrid Punnett square ratio, but doesn't match the specific bbhh combination this question asks for.
Genetics questions are almost entirely about setting up the cross correctly — once the Punnett square is built right, reading off the ratio is usually the easy part.
Draw out the grid and fill in every box, even for a question that seems to have an obvious answer. Untimed practice is where that habit prevents shortcut errors.
Move to timed sessions once monohybrid crosses are instant. Dihybrid crosses take longer since there's a bigger grid to fill.
Pair genetics with cell biology in a mock — DNA and cell division are the physical basis for everything a Punnett square predicts.
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