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Science · Biology

Genetics

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.

Practise genetics nowSee sample questions
3
Difficulty tiers
35s
Target pace
54%
Average first-attempt score

What's covered

  • Dominant and recessive alleles — recognize that a dominant allele masks a recessive one in a heterozygous individual, and that a recessive trait only appears when both alleles are recessive.
  • Genotype vs. phenotype — distinguish the genetic makeup (genotype, like Bb) from the observable trait it produces (phenotype, like brown eyes).
  • Punnett squares — cross two parents' alleles to predict the genotype and phenotype ratios of their offspring.
  • Monohybrid vs. dihybrid crosses — track one trait at a time in a monohybrid cross, or two traits simultaneously in a dihybrid cross, using a larger grid for the second.
  • DNA, genes, and traits — connect the chain from DNA sequence to gene to protein to observable trait, understanding that a gene is a segment of DNA that codes for a specific trait.

Where students lose marks

Confusing genotype with phenotype

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.

Mixing up which allele 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.

Misreading a Punnett square's ratios

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.

Three sample questions

Straight from the bank — one per difficulty tier. Reveal the answer to see the explanation you'd get in a real session.

Sample 01Foundation

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?

Sample 02Core

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)?

Sample 03Advanced

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?

How to practise this

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.

Build the full Punnett square before answering

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.

Then 35 seconds a question

Move to timed sessions once monohybrid crosses are instant. Dihybrid crosses take longer since there's a bigger grid to fill.

Fold into a mock

Pair genetics with cell biology in a mock — DNA and cell division are the physical basis for everything a Punnett square predicts.

Stop guessing the ratio.
Start building the square.

  • A cross-by-cross walkthrough on every item
  • Timed or untimed sessions, any length you like
  • Tracked separately — see your genetics accuracy and pace over time
Start practising free

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