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

This guide covers statements B5.1i to B5.1m of OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. They sit in module B5.1 Inheritance. No statement in this unit is shown in bold, so the content is for both tiers. Terms such as allele, genome and meiosis are taught in other units; here you use them to predict and explain results.

Next, use the revision notes to condense this guide and the practice questions to test yourself. Find the rest of the course on the OCR GCSE Biology hub, tick off topics on the printable checklist, and use the diagnostics to see where to start. For exam technique, see the exam preparation page.

What this unit covers

Statement	What you must be able to do
B5.1i	Explain single gene inheritance, using homozygous, heterozygous, dominant and recessive
B5.1j	Predict the results of single gene crosses with Punnett squares, giving ratios and probabilities
B5.1k	Describe sex determination in humans using a genetic cross
B5.1l	Recall that most features are the result of many genes, not one
B5.1m	Describe how our understanding of genetics developed, including the work of Mendel

The specification also lists the maths skills: use direct proportions and simple ratios in crosses, use probability to predict outcomes, and extract and interpret information from charts, graphs and tables.

Single gene inheritance (B5.1i)

A single gene feature is controlled by one gene, which has different versions called alleles. In body cells you have two copies of each gene, one from each parent, so your genetic make-up for that gene is a pair of alleles.

Two words describe the pair:

• Homozygous: the two alleles are the same (for example VV or vv).
• Heterozygous: the two alleles are different (for example Vv).

Two more words describe how an allele shows up:

• Dominant: the feature appears if at least one copy of the allele is present. It is written as a capital letter.
• Recessive: the feature appears only if both alleles are the recessive one. It is written as a lower-case letter.

So a dominant feature appears in homozygous dominant (VV) and heterozygous (Vv) individuals. A recessive feature appears only in vv.

Two more terms make answers precise. The genotype is the pair of alleles an individual has, such as Vv. The phenotype is the feature you can observe, such as long wings.

OCR warns about two misconceptions. The word “dominant” can mislead. The dominant allele does not suppress or overpower its partner; dominance is only a pattern seen in the phenotype. A recessive allele is also not just the absence of a dominant one: it is a different version of the gene that is still present and can be passed on. This is why a heterozygous parent can have a child who shows the recessive feature.

Dominant does not mean common or stronger. Some inherited conditions are caused by dominant alleles and some by recessive alleles, and the allele that causes a disorder can be either type.

Punnett squares (B5.1j)

A Punnett square shows every way two parents’ gametes can combine. Each parent contributes one allele per gamete.

Method:

1. Choose one letter for the gene. Make it a capital for the dominant allele and lower case for the recessive, and say what they stand for.
2. Write each parent’s genotype.
3. Write each parent’s possible gametes along the top and down the side.
4. Fill the boxes to give the offspring genotypes.
5. Convert to phenotypes, then to a ratio or probability.

Each box is equally likely, so one box out of four is a probability of 1/4, 0.25 or 25%. State the form the question asks for.

Worked example 1: two heterozygous parents

Fruit flies with normal wings (V) are dominant over vestigial, very short, wings (v). Two heterozygous flies (Vv) are crossed.

	V	v
V	VV	Vv
v	Vv	vv

Genotypes: 1 VV : 2 Vv : 1 vv. Phenotypes: three boxes have normal wings and one has vestigial wings, so the ratio is 3 normal : 1 vestigial. The probability of a vestigial-winged offspring is 1/4, or 25%.

If the flies produce 240 offspring, the expected numbers are 240 × 3/4 = 180 normal and 240 × 1/4 = 60 vestigial.

Worked example 2: heterozygous parent and homozygous recessive parent

Huntington’s disease is caused by a dominant allele (H). A man with the disease is heterozygous (Hh). His partner does not have the disease (hh).

	h	h
H	Hh	Hh
h	hh	hh

Two boxes are Hh (affected) and two are hh (unaffected). The ratio is 1 affected : 1 unaffected, so every child has an even chance (50%) of being affected.

Each child is an independent event, so earlier children do not change the odds for the next one. The chance that four children are all unaffected is (1/2)⁴ = 1/16.

Worked example 3: homozygous parents

A homozygous dominant parent (VV) crossed with a homozygous recessive parent (vv) can only give V from one parent and v from the other. All offspring are Vv and all show the dominant feature. If two of these offspring are then crossed (Vv × Vv), the result is example 1 again: 3 dominant : 1 recessive.

This is the classic pattern in a breeding programme: pure-breeding parents, a first generation that all show the dominant feature, and a second generation in which the recessive feature returns in about a quarter.

Worked example 4: working out an unknown genotype

A cow with the dominant polled (hornless) phenotype could be PP or Pp. You cannot tell from her appearance. Cross her with a horned animal (pp), which can only pass on p.

• If she is PP, every calf is Pp and polled.
• If she is Pp, half the calves are Pp (polled) and half are pp (horned).

One horned calf proves she is Pp. Five polled calves in a row make PP likely but do not prove it: if she were Pp, the chance of five polled calves is (1/2)⁵ = 1/32, which is small but not zero. More calves make the conclusion safer.

Ratios and real results

A ratio such as 3:1 is a prediction about probability, not a guarantee. Fertilisation is random, so real counts vary. With 16 offspring you might see 10 : 6, which is far from 12 : 4 but not surprising. With several hundred offspring the result usually sits much closer to 3:1. Tossing a coin ten times rarely gives exactly five heads, but a thousand tosses gets close to half. OCR suggests a coin or die as a way to explore this.

Sex determination in humans (B5.1k)

Humans have 23 pairs of chromosomes. One pair is the sex chromosomes. Females have two X chromosomes (XX). Males have an X and a Y (XY).

Every egg carries an X. Sperm are of two kinds: half carry X and half carry Y. Whichever sperm reaches the egg first fixes the baby’s sex.

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

Two boxes are XX (girl) and two are XY (boy). The ratio is 1 female : 1 male, so each baby has a 50% chance of being a boy. Each fertilisation is independent, so a family with three daughters still has a 50% chance of a boy next time.

For a family of three children, the probability that all are girls is (1/2)³ = 1/8 (12.5%). The probability of one boy and one girl in two children is 2 × (1/4) = 1/2, because there are two orders: boy then girl, or girl then boy.

Most features involve many genes (B5.1l)

Some features, including a few inherited disorders, are controlled by a single gene and fall into clear categories that Punnett squares predict well. Most phenotypic features are the result of multiple genes, not a single gene. Height and skin colour are examples. Many genes each add a small effect, so the population shows a smooth range of phenotypes, not two or three clear types.

This is why you cannot predict height from one Punnett square. The number of possible allele combinations grows quickly: if each gene had just two alleles, two genes give 3² = 9 genotypes and three genes give 3³ = 27. OCR also states that characteristics are influenced by the genome and its interaction with the environment, so both genes and surroundings affect many features.

The development of our understanding of genetics (B5.1m)

The specification names the work of Mendel. This unit also links to the working scientifically ideas that scientific methods and theories develop over time as new technology gives new evidence, that ethical issues arise from scientific developments, and that peer review and communication matter.

Mendel (mid-1800s). Gregor Mendel, a monk, bred pea plants. He chose plants that grow quickly, produce many offspring and have clear either-or features, and he started with pure-breeding lines. He controlled which plants crossed, followed one feature at a time, counted large numbers of offspring and looked for ratios. He concluded that features are passed on as separate units that come in pairs, that offspring receive one from each parent, and that some units hide the effect of others in the first generation but return in the second, at roughly 3:1.

Why his work was ignored. He published in the 1860s, but the ideas were not widely accepted until around 1900. Reasons include that chromosomes and cell division were not yet understood, so his “units” had no physical explanation, and that his work was not widely read. This links to peer review: ideas need to be shared, checked and repeated before they are accepted.

Later work. Better microscopes allowed scientists to see chromosomes. In the early 1900s it was proposed that Mendel’s units are carried on chromosomes. In 1953 the double helix structure of DNA was worked out, which explained how information can be stored and copied. The human genome was later sequenced, around 2003. New technology kept supplying new evidence, and explanations changed to fit.

Modern knowledge raises ethical issues, such as who should have access to a person’s genetic information and how it should be used. Be ready to give a balanced view.

Common errors

• Writing Aa for a homozygous parent. Homozygous means two identical alleles.
• Saying the dominant allele is “stronger” or “more common”.
• Forgetting to define your letters before using them.
• Putting two alleles in one gamete. A gamete carries one.
• Giving the ratio when asked for a probability. 3:1 is not 3/4.
• Saying a family of girls makes the next child more likely to be a boy.

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.