Genetic inheritance — AQA GCSE Biology

SPEC 4.6.1.6 PAPER 2

Genetic Inheritance & Punnett Squares

The 10 spec terms, the notation, and the Punnett square method that appears on virtually every Paper 2. From alleles to ratios, the chain from genotype to phenotype.

Why This Matters

Punnett squares appear on almost every Paper 2. The June 2024 paper had a 4-mark polydactyly Punnett. The 2024 specimen paper had a 5-mark recessive Punnett. The 2023 paper had a Punnett for sex determination. Master this method and you bank 4–5 marks per paper.

10

Spec terms you must define

2

Classic ratios: 3:1 and 1:1

4-5

Marks for a Punnett question

What Examiners Reward

Mark schemes for Punnetts award marks for: parents' gametes correct (1) → offspring genotypes correct (1-2) → phenotypes identified correctly (1) → probability or ratio stated (1). Every step matters.

Learning Objectives

By the end of this lesson, you'll be able to:

Define the 10 spec genetic terms: gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype.

Use the capital / lowercase notation for dominant and recessive alleles.

Complete a Punnett square for a single-gene cross.

Interpret genotype ratios, phenotype ratios, and probabilities from a cross.

Extract and interpret information from family trees / pedigrees.

Explain that most characteristics are controlled by multiple genes — not just one.

Foundation vs HT

Foundation: complete a Punnett square diagram provided in the question. HT: construct the Punnett square yourself from the genotypes given, and use probability theory to make predictions. Both tiers test the same concept — just at different starting points.

Key Terms — The 10 Spec Definitions

Mark schemes are strict on wording. Learn these exact definitions:

TermDefinition

GameteA sex cell (sperm or egg). Has half the number of chromosomes.

ChromosomeA structure in the nucleus made of DNA . Contains many genes.

GeneA small section of DNA on a chromosome that codes for a specific protein .

AlleleA different version of the same gene (e.g. allele for brown eyes vs allele for blue eyes).

DominantAn allele that is always expressed , even if only one copy is present.

RecessiveAn allele that is only expressed if two copies are present (i.e. no dominant allele).

HomozygousHaving two of the same allele for a gene (e.g. RR or rr).

HeterozygousHaving two different alleles for a gene (e.g. Rr).

GenotypeThe combination of alleles an organism has (e.g. RR, Rr, or rr).

PhenotypeThe observable characteristic that the genotype produces.

Mark Scheme Trap (MS Jun 2024 P2 Q6.1)

For "what is a dominant allele?", the mark scheme accepts: "(an allele) that is always expressed" OR "(an allele) that is expressed even when the other / recessive allele is present". Use these exact phrasings.

Allele Notation Conventions

By convention, alleles are written using letters:

The Two Rules

Capital letter = the dominant allele (e.g. R)

Lowercase same letter = the recessive allele (e.g. r)

The 3 Possible Genotypes

RR = homozygous dominant

Rr = heterozygous

rr = homozygous recessive

Which Phenotype Shows?

RR or Rr → dominant trait shows (R is always expressed)

rr → recessive trait shows (no R present)

Choosing The Letter

Exam questions usually give you the symbols to use (e.g. "Use A = dominant allele, a = recessive allele"). If you ever have to choose your own, pick a letter that's clearly different in capital and lowercase form — R/r, B/b, T/t are good. Avoid C/c, S/s, X/x, O/o — they look the same in capital and lowercase, which causes marking confusion.

Genotype Determines Phenotype

The combination of alleles (genotype) determines the observable trait (phenotype). Here's the rule for a flower colour gene where R (red) is dominant over r (white):

The Key Insight

The heterozygous plant (Rr) shows the same phenotype as the homozygous dominant (RR) — both look red. The recessive allele only shows when there's no dominant allele to override it.

Single-Gene Inheritance Examples

The spec names two examples of characteristics controlled by a single gene:

Example 1: Fur Colour In Mice

One gene controls the colour. Different alleles produce different colours. A black-fur allele (B) may be dominant over a brown-fur allele (b).

BB → black fur

Bb → black fur

bb → brown fur

Example 2: Red-Green Colour Blindness

One gene determines whether someone can distinguish red and green. The colour-blindness allele is recessive — you need two copies to be affected.

Most people have the normal-vision allele — can see colour

Two recessive alleles → red-green colour blind

Spec Boundary

The spec explicitly mentions these two examples. The exam may give you any single-gene scenario — pea height, plant colour, mouse fur, a fictional disorder — but the method is always the same. Apply the rules to whatever scenario you're given.

The Punnett Square Method

A Punnett square predicts the genotypes and phenotypes of offspring from a cross. Here's the 5-step method:

The 5 Steps

Identify the parents' genotypes (e.g. Rr × Rr).

Work out the gametes each parent can produce. Each gamete carries ONE allele, not both. (Rr produces R or r.)

Set up a 2×2 grid. Put one parent's gametes along the top, the other's down the side.

Fill in the grid by combining each pair of gametes — these are the possible offspring genotypes.

Read off the ratios — genotype ratio AND phenotype ratio.

Common Error

Each gamete carries ONE allele, not both. The parent Rr produces R or r gametes — never "Rr" as a single gamete. Mixing this up is the #1 mistake in Punnett questions.

Worked Example 1: Rr × Rr (the classic 3:1)

Cross two heterozygous parents. This gives the famous 3:1 phenotype ratio:

Reading The Square

Out of 4 possible offspring: 1 RR, 2 Rr, 1 rr. Three of them (RR, Rr, Rr) show the dominant phenotype. One (rr) shows the recessive. That's the 3:1 phenotype ratio.

Worked Example 2: Dd × dd (the 1:1 dominant disorder)

If a dominant disorder is inherited (e.g. polydactyly), and an affected heterozygous parent crosses with an unaffected partner, you get a 1:1 ratio:

Two Ratios To Memorise

3:1 phenotype ratio → both parents are heterozygous (e.g. Rr × Rr)

1:1 phenotype ratio → one heterozygous parent, one homozygous recessive parent (e.g. Dd × dd)

Family Trees / Pedigrees

The spec also requires you to extract and interpret information from family trees. A family tree shows how a trait is inherited across multiple generations. The same Punnett square logic applies — just from given individuals.

The Symbols You Must Know

Square = male; Circle = female

Filled / shaded = has the trait (affected); Empty / unshaded = doesn't have the trait

Horizontal line between two = couple (parents)

Vertical line below = their offspring

How To Use With Punnett Squares

Family trees let you deduce the genotypes of family members (from their phenotypes and parents' genotypes). Once you know two parents' genotypes, you can draw a Punnett square to predict the probability of their next child being affected. Family trees are very common in inherited disorder questions (covered in Lesson 6.05).

Probability vs Actual Outcome

This is the most-tested concept in Punnett questions. A Punnett square tells you the probability — not what definitely happens.

The Key Idea

For two heterozygous parents (Rr × Rr), the probability of a recessive offspring is 1 in 4 (25%). But this doesn't mean exactly 1 of every 4 children will be recessive. Each child's genotype is determined independently — by chance.

Acceptable Phrasings (Mark Scheme)

"1 in 4"

"1/4" (a fraction)

"25%"

"0.25" (decimal probability)

"3:1 ratio"

Common Error

Writing "exactly 1 out of 4 children will be affected" is wrong — it's a probability, not a guarantee. Use language like "the probability is" or "on average".

Worked Mini-Example

Two heterozygous parents have four children. By chance, none of them could be homozygous recessive — or all of them could be. The 1-in-4 probability is for each individual child, calculated independently.

Most Characteristics Use Multiple Genes

The spec emphasises a reality check at the end: single-gene examples are useful for teaching Punnetts, but most real traits are controlled by many genes together.

Single-Gene vs Multi-Gene

Single-gene (single inheritance)Multi-gene (most traits)Fur colour in mice; red-green colour blindness; cystic fibrosis; polydactylyEye colour; height; skin colour; weightPredictable with a Punnett square (clean 3:1 or 1:1 ratios)Not predictable with a simple Punnett — continuous variation

Spec Phrase

"Most characteristics are a result of multiple genes interacting, rather than a single gene." Memorise this exact phrase — it scores in any question about why a Punnett square can't predict real-world traits like height.

Examiner Alerts

Four traps that cost marks on Paper 2.

Alert 1 — Genotype vs Phenotype (reversed)

Students often swap these terms. Genotype = the alleles (RR, Rr, rr). Phenotype = the observable trait (red flower, brown fur). Reversing them voids the answer.

Alert 2 — Homozygous vs Heterozygous (reversed)

Homozygous = SAME alleles (RR or rr). Heterozygous = DIFFERENT alleles (Rr). Memory tip: "homo" = same; "hetero" = different.

Alert 3 — Gametes carry ONE allele

A parent Rr produces gametes R or r — never "Rr" as a single gamete. Gametes have half the genetic material. This is the #1 mistake in Punnett squares.

Alert 4 — Ratio confusion

3:1 only applies to heterozygous × heterozygous (Rr × Rr). For Dd × dd, the ratio is 1:1. Always derive the ratio from your Punnett square — don't assume.

PEE Worked Example

Question: In pea plants, the allele for tall (T) is dominant over the allele for short (t). Two heterozygous tall pea plants are crossed. Use a Punnett square to predict the phenotype ratio of the offspring. 4 marks

P

Point 1 — Identify parents and gametes: Both parents are Tt (heterozygous tall). Each produces gametes T or t.

Point 2 — Punnett square:

Tt

TTTTt

tTttt

E

Evidence — Offspring genotypes: 1 TT, 2 Tt, 1 tt. The genotype ratio is 1 : 2 : 1.

E

Conclusion — Phenotype ratio: TT and Tt both show tall (T is dominant). tt shows short. So the ratio is 3 tall : 1 short.

Why This Scores 4/4

Four mark-scheme points: (1) parents' gametes identified as T and t; (2) Punnett square correctly filled (TT, Tt, Tt, tt); (3) genotype ratio stated (1:2:1); (4) phenotype ratio stated (3:1 tall:short). Every step earns a mark.

Practice Question

In mice, the allele for black fur (B) is dominant over the allele for brown fur (b).

(a) A black-furred mouse with genotype Bb is crossed with a brown-furred mouse. Draw a Punnett square to predict the offspring genotypes. 3 marks

(b) State the probability that an offspring from this cross will have brown fur. 1 mark

(c) Two of the offspring (both with black fur) have four pups. All four pups have black fur. Explain why this result is still consistent with the Punnett square predictions, even though it doesn't match the predicted 3:1 ratio. 2 marks

(a)

Brown-furred mouse genotype is bb (must be homozygous recessive to show brown).

Cross: Bb × bbBbbBbbbbBbbb Offspring: 2 Bb (black), 2 bb (brown) — 1:1 ratio.

Marks: (1) parents' gametes correct, (1) offspring genotypes correctly filled, (1) phenotypes identified.

(b)

1/2 or 0.5 or 50% (any of these accepted).

(c)

The two black-furred offspring are both Bb (heterozygous); a Bb × Bb cross predicts a 3:1 phenotype ratio (3 black : 1 brown) (1)

This is a probability, not a guarantee — each pup's genotype is determined independently, so with only 4 pups, all four could (by chance) inherit at least one B allele and show black fur (1)

Note: Parts (a) and (b) test the 1:1 ratio (heterozygous × homozygous recessive). Part (c) brings in the 3:1 ratio AND tests the probability concept. Total 6 marks.

Recap — 3 Takeaways

The Vocabulary

Allele = version of a gene. Genotype = allele combination (RR, Rr, rr). Phenotype = observable trait. Homozygous = same alleles. Heterozygous = different alleles. Dominant = always expressed; recessive = needs two copies.

The Punnett Method

Parents' genotypes → gametes (each carries ONE allele) → fill 2×2 grid → read genotypes → read phenotypes. 3:1 for Rr × Rr; 1:1 for Dd × dd.

Probability & Multi-Gene

Punnetts give probabilities, not guarantees. Small samples may not match the predicted ratio exactly. Most real traits are multi-gene — Punnetts only work for single-gene inheritance.

One-Sentence Method

Take both parents' genotypes → work out their gametes (each is ONE allele) → fill a 2×2 Punnett grid → read off the genotype ratio → group genotypes by phenotype (dominant vs recessive) to get the phenotype ratio.

NEXT: Lesson 6.05 — Inherited Disorders & Sex Determination (polydactyly, cystic fibrosis, XX/XY)