OCR GCSE Biology A: Single gene inheritance, genetic crosses, sex determination and the history of genetics (J247) – Revision Notes

These notes condense statements B5.1i to B5.1m (single gene inheritance, crosses, sex determination, multi-gene features and the history of genetics) of OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. For full explanations go to the study guide. Then test yourself with the practice questions, tick off topics on the printable checklist, find the rest of the course on the OCR GCSE Biology hub and use the diagnostics to find weak spots.

No statement here is in bold type, so it all applies to both tiers.

Key terms

Term	Meaning
Allele	A version of a gene
Genotype	The pair of alleles an individual has for a gene, e.g. Tt
Phenotype	The observable feature, e.g. tall
Homozygous	Same allele twice, as in TT or tt
Heterozygous	One of each allele, as in Tt
Dominant	Feature shows with one copy or two (capital letter)
Recessive	Feature shows only if both alleles are recessive (lower case)
Pure-breeding	Homozygous, so offspring always show the same feature
Punnett square	Grid showing all gamete combinations from a cross
Sex chromosomes	X and Y; XX is female, XY is male

Genotype to phenotype

Genotype	Description	Shows
TT	Homozygous dominant	Dominant feature
Tt	Heterozygous	Dominant feature
tt	Homozygous recessive	Recessive feature

• A dominant allele does not “dominate” or destroy the recessive allele. Dominance is a pattern in the phenotype.
• A recessive allele is a different version of the gene, not the absence of one. It is still present and can be passed on.
• A recessive feature can skip a generation because heterozygous parents hide it.
• Dominant does not mean common or stronger.

Method: Punnett square in five steps

1. Define the letters: which stands for the dominant allele, which for the recessive, and what feature each gives.
2. Write the parents’ genotypes.
3. Write one allele per gamete along the top and down the side.
4. Fill in the four boxes.
5. Read off genotypes and phenotypes, then give the ratio or probability asked for.

Probability: each box is 1/4, so 1 box = 25%, 2 boxes = 50%, 3 boxes = 75%.

The four crosses to know

Cross	Offspring genotypes	Phenotype ratio	Dominant : recessive
TT × tt	All Tt	All dominant	1 : 0
Tt × Tt	1 TT : 2 Tt : 1 tt	3 dominant : 1 recessive	3 : 1
Tt × tt	1 Tt : 1 tt	1 dominant : 1 recessive	1 : 1
TT × Tt	1 TT : 1 Tt	All dominant	1 : 0

Notice that two parents who both show the dominant feature can have a child with the recessive feature, but only if both are heterozygous. Two parents who both show a recessive feature can only have recessive-featured children.

Worked reminder: expected numbers

Two heterozygous plants are crossed. 200 seeds grow.

• Ratio 3 : 1 gives 200 × 3/4 = 150 dominant and 200 × 1/4 = 50 recessive.
• Real counts such as 143 and 57 are acceptable: fertilisation is random.

Worked reminder: unknown genotype

An animal with the dominant phenotype could be TT or Tt. Cross it with a recessive (tt).

• Any recessive offspring shows the animal is Tt.
• All dominant offspring suggests TT, but the more offspring there are, the more confident you can be. For a Tt parent, each offspring has a 1/2 chance of being dominant, so six dominant offspring in a row has probability (1/2)⁶ = 1/64.

Worked reminder: a dominant condition

A parent has a dominant disorder and is heterozygous (Hh). The other parent is hh.

• Each child: 1/2 chance of the disorder.
• Three children, none affected: (1/2)³ = 1/8.

Worked reminder: a recessive condition

Two unaffected parents have a child with a recessive disorder.

• Both parents must be carriers (Aa), because the child needs two copies of the recessive allele, one from each parent.
• Next child: 1/4 affected, 1/2 unaffected carrier, 1/4 unaffected non-carrier.
• Two children both affected: (1/4)² = 1/16.

Worked reminder: two generations

A pure-breeding tall plant (TT) is crossed with a pure-breeding dwarf plant (tt).

• First generation: every plant is Tt, so every plant is tall.
• Cross two of these plants (Tt × Tt) to get 128 second-generation plants: 128 × 3/4 = 96 tall and 128 × 1/4 = 32 dwarf.
• Of the 96 tall plants, one third are TT (32) and two thirds are Tt (64). They look identical, so only a further cross can tell them apart.

Writing the answer well

• For “explain” questions, link the genotype to the phenotype in the same sentence: “Both parents are Aa, so each passes on a with probability 1/2, so a child is aa with probability 1/4.”
• For “calculate the probability” questions, show the square or the fraction working, then the final value with its form (fraction, decimal or percentage).
• For “describe” questions on history, name the scientist, say what they did and say what it showed.

Probability rules

Rule	Example
One outcome from a Punnett square	Fraction of boxes
Independent events: multiply	Two Tt × Tt offspring both recessive: 1/4 × 1/4 = 1/16
Either of two orders: add	One boy and one girl: 1/4 + 1/4 = 1/2
Past results do not change future odds	Four girls does not change the chance of a boy

Sex determination (B5.1k)

	X (egg)	X (egg)
X (sperm)	XX	XX
Y (sperm)	XY	XY

• Females XX, males XY.
• All eggs carry X. Sperm carry X or Y in equal numbers.
• The sperm decides the sex.
• Ratio 1 female : 1 male. Probability of a boy = 1/2 for every pregnancy.
• Two children, both girls: 1/4. Three children, all boys: 1/8.

Multi-gene features (B5.1l)

• Single gene: features fall into clear categories and Punnett squares work.
• Most features (for example height and skin colour) are controlled by multiple genes, so the population shows a range of values.
• Many genes plus the environment mean one Punnett square cannot predict them.
• Genes and the environment both influence the phenotype.

Development of understanding (B5.1m)

When	Who or what	Idea or evidence
1860s	Mendel	Bred pea plants, counted offspring, found separate inherited units and 3 : 1 ratios
Around 1900	Scientists repeating his work	Mendel’s ratios recognised as important
1800s into early 1900s	Microscopy	Chromosomes seen; later linked to the units
1953	Double helix	Structure of DNA worked out
Around 2003	Genome sequencing	Human genome sequenced

Why Mendel’s work was not accepted at first: chromosomes and genes were not known, so there was no physical explanation, and his results were not widely read.

Working scientifically links:

• Theories develop as new technology gives new evidence.
• Peer review and communication let others check results.
• Genetic knowledge raises ethical issues, such as privacy of genetic information.

Must-know distinctions

• Genotype and phenotype. Genotype is the alleles, phenotype is the feature.
• Homozygous and heterozygous. Same alleles and different alleles.
• Ratio and probability. 3 : 1 and 3/4 are different forms of the same prediction.
• Prediction and result. Punnett squares give probabilities, not guarantees.
• Single gene and multi-gene. Clear categories and a range of values.

Quick self-test

1. What does heterozygous mean?
2. What is the genotype of an individual with the recessive phenotype?
3. Give the phenotype ratio from Aa × Aa when A is dominant.
4. A cross between Bb and bb produces 60 offspring. How many are expected to show the recessive feature?
5. Two parents who do not have a recessive disorder have an affected child. What are the parents’ genotypes?
6. State the probability that a baby is a girl.
7. A couple have four boys. What is the probability that the fifth child is a girl?
8. What is the probability that three children are all boys?
9. Why can a Punnett square not predict adult height?
10. Name the scientist who bred pea plants and counted the offspring.
11. Give one reason why his results were not accepted at first.
12. Why do real results often differ from predicted ratios?

Answers

1. Having two different alleles of a gene.
2. Homozygous recessive, for example aa.
3. 3 dominant : 1 recessive.
4. Half: 60 × 1/2 = 30.
5. Both heterozygous carriers (Aa and Aa).
6. 1/2 (50%).
7. 1/2: each birth is independent.
8. (1/2)³ = 1/8.
9. Height is controlled by many genes, plus the environment, so it varies across a range.
10. Gregor Mendel.
11. Chromosomes and genes were not yet known, so his units had no physical explanation (or: his work was not widely read).
12. Fertilisation is random, so small samples vary; larger samples come closer to the prediction.

Where marks are usually lost

• Not defining letters, then using them inconsistently.
• Writing two alleles in a gamete, or leaving the gamete circles or headings out of the square.
• Giving genotypes when the question asks for phenotypes.
• Describing the dominant allele as stronger, more common or “overpowering”.
• Writing 3 : 1 where a probability such as 3/4 is wanted, or a percentage where a fraction is wanted.
• Saying a parent “carries” the disorder without saying which allele.
• Claiming a sex outcome becomes more likely after a run of the other sex.
• Treating Mendel’s dates and findings as vague: say what he did and what he found.
• Using one gene to explain a feature such as height or skin colour.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018, published by OCR (Oxford Cambridge and RSA). Statements B5.1i to B5.1m.