OCR GCSE Biology A: Gateway Science (J247)

B5.1 Inheritance

OCR J247 B5.1 is the topic that tests whether you can move fluently between five linked ideas — gene, allele, genotype, phenotype, and Punnett square — and use them to predict the outcomes of a monohybrid cross. The OCR mark schemes are built around very specific phrasings (heterozygous parents both Rr, recessive allele only expressed when two copies are present, kitten's genotype is ff, puppy with genotype bb will be chocolate coloured) and this page works through them all. We cover DNA → chromosomes → genes → alleles, dominant vs recessive, two worked Punnett squares, sex inheritance, family pedigrees, and a brief look at cystic fibrosis and polydactyly.

Why this matters

Children look like their parents but not exactly like them. Why? The answer is the gene — a section of DNA, sitting on a chromosome, that codes for a specific protein and is passed from parent to offspring in the gametes. Each gene comes in alternative versions called alleles, and the combination of alleles you inherit (your genotype) determines the observable traits you end up with (your phenotype). The story of how we know this stretches from Gregor Mendel's pea-plant breeding experiments in the 1860s, through the discovery of chromosomes around 1900, to Watson and Crick's double-helix model of DNA in 1953, to the Human Genome Project completed in the early 2000s. Mendel realised that inheritance involved discrete units (not blending), that some versions of a unit could mask others (dominant over recessive), and that each parent contributes one copy of each unit to the offspring. He had no idea what the units were physically — chromosomes and DNA only came later. Modern genetics combines all of this. We now know that humans have 23 pairs of chromosomes; that 22 of those pairs are autosomes and the 23rd pair is the sex chromosomes (XX female, XY male); and that around 20 000 protein-coding genes are spread across them. For OCR B5.1 you do not need the molecular detail — you need to be able to read a genetic cross, set up a Punnett square, predict ratios, interpret a family pedigree, and apply the language of allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive to brand-new contexts (cats, dogs, peas, sweet peas, hamsters).

How to learn this topic

Build on what you already know

  • B2 / KS3: cells contain a nucleus, which contains chromosomes made of DNA.
  • B5: meiosis halves the chromosome number; each gamete carries one chromosome from each pair, and therefore one allele of every gene.
  • Basic probability — 1 in 4, 1 in 2, expressing chance as a fraction, decimal or percentage.
  • KS3: characteristics can be inherited from parents and can also be affected by the environment.
  1. Build the vocabulary first: DNA → chromosome → gene → allele.
  2. Distinguish genotype (alleles you carry) from phenotype (what you see).
  3. Introduce dominant vs recessive using capital and lower-case letters (R vs r).
  4. Set up a Punnett square step by step for Rr × Rr, then for Rr × rr.
  5. Translate the grid into a genotype ratio (1 : 2 : 1) and a phenotype ratio (3 : 1) and into a probability (1 in 4).
  6. Apply the same method to sex inheritance (XX × XY) to give 1 : 1 female : male.
  7. Read a three-generation family pedigree and work out individual genotypes.
  8. Connect to two inherited disorders — cystic fibrosis (recessive) and polydactyly (dominant).

Key terms

gene
A short section of DNA on a chromosome that codes for a specific protein and is the unit of inheritance passed from parents to offspring. (OCR wants 'section of DNA' AND 'codes for a (specific) protein'. Two distinct marking points in one sentence.)
allele
A version of a gene. Each gene typically has two or more alleles, and an individual carries two alleles for every gene (one from each parent). (Define as 'a different form / version of a gene'. Do NOT call an allele a gene — they are not the same thing.)
chromosome
A long, coiled molecule of DNA that carries many genes. Human body cells contain 23 pairs of chromosomes (46 total). (Marking phrase: 'chromosomes are made of DNA' and 'contain genes'.)
genotype
The combination of alleles an individual carries for a gene, e.g. RR, Rr or rr. (Mark schemes accept written allele pairs (e.g. 'Bb') as a genotype answer.)
phenotype
The observable characteristic of an individual produced by their genotype (and sometimes the environment), e.g. round seeds, short fur, brown eyes. (Distinguish from genotype carefully — 'phenotype' is what you SEE; 'genotype' is what you HAVE.)
homozygous
Having two identical alleles for a gene (e.g. RR or rr). 'Homo' = same. Homozygous dominant = RR; homozygous recessive = rr. (Used in OCR rubric — 'kitten's genotype is ff' = homozygous recessive.)
heterozygous
Having two different alleles for a gene (e.g. Rr). 'Hetero' = different. A heterozygous individual carries one dominant and one recessive allele. (OCR rubric uses 'both parents heterozygous (Bb)' / 'both parents are Rr' as direct marking points.)
dominant allele
An allele that is expressed even when only one copy is present. Always written with a capital letter (e.g. R, B, F). (Marking phrase: 'dominant allele always expressed' or 'expressed even when one copy is present'.)
recessive allele
An allele that is only expressed when two copies are present (i.e. in a homozygous individual). Always written with a lower-case letter (e.g. r, b, f). (OCR mark schemes hammer one phrase: 'recessive allele only expressed when two copies are present'.)
Punnett square
A 2 × 2 (or larger) grid used to predict the probability of offspring genotypes and phenotypes from a genetic cross. Parental gametes go along the top and side; offspring genotypes fill the cells. (OCR wants clear labelling of parental gametes and correct genotype combination inside each cell.)
carrier
An individual who is heterozygous for a recessive allele. They do not show the recessive phenotype themselves but can pass the allele on to their offspring. (Critical for pedigree questions — unaffected parents of an affected child must be carriers.)
cystic fibrosis
A recessive inherited disorder of cell membranes that causes thick, sticky mucus to build up in the lungs and digestive system. Sufferers are homozygous recessive (ff). (OCR uses cystic fibrosis as the standard recessive example.)
polydactyly
An inherited condition in which an individual is born with extra fingers or toes. Caused by a dominant allele, so the trait usually appears in every generation. (OCR uses polydactyly as the standard dominant example — important contrast with cystic fibrosis.)
monohybrid cross
A genetic cross that follows the inheritance of a single gene with two alleles, e.g. Rr × Rr.

Notes

From DNA to alleles — the vocabulary chain

Every cell in your body (except your gametes) contains a nucleus, and inside the nucleus are chromosomes. Chromosomes are long, coiled molecules of DNA. Stretches of DNA along a chromosome are called genes, and each gene codes for a specific protein. Build this chain into a single sentence and you have one of OCR's favourite marking points:

> Chromosomes contain genes; genes are sections of DNA that code for specific proteins.

Genes come in different versions. These alternative forms of the same gene are called alleles.

  • The gene for fur length in cats is one gene. The two versions of it — long-fur and short-fur — are two alleles of that gene.
  • The gene for seed shape in peas is one gene. The two versions — round-seed and wrinkled-seed — are two alleles of it.

Mark-scheme phrasing: gene = section of DNA that codes for a protein; allele = a version of a gene.

Genotype vs phenotype

For every gene, you carry two alleles — one inherited from your mother (via the egg) and one inherited from your father (via the sperm). The two alleles together are your genotype for that gene. What you actually look like — the observable characteristic — is your phenotype.

  • Genotype = the combination of alleles you carry (e.g. Rr, BB, ff).
  • Phenotype = the observable characteristic that the alleles produce (round seeds, black coat, short fur).

These are not the same thing. Two animals with different genotypes can have the same phenotype (this is the classic Rr-vs-RR trap, see below).

Homozygous vs heterozygous

The two alleles you carry can be the same or different.

  • If both alleles are the same (e.g. RR or rr) you are homozygous for that gene. RR is homozygous dominant; rr is homozygous recessive.
  • If the two alleles are different (e.g. Rr) you are heterozygous for that gene.

Dominant vs recessive

When the two alleles in a heterozygous individual disagree, one usually wins. The version that is expressed even when only one copy is present is the dominant allele and is written with a CAPITAL letter (R, B, F). The version that is only expressed when two copies are present — i.e. only when the individual is homozygous for it — is the recessive allele and is written with a lower-case letter (r, b, f).

  • Dominant allele: expressed in both RR and Rr genotypes — needs only one copy.
  • Recessive allele: expressed only in rr genotype — needs two copies.

This is why a heterozygous Rr individual has the same phenotype as a homozygous RR individual: the dominant R masks the r.

Worked Punnett square 1 — Rr × Rr

Two round-seeded pea plants, both heterozygous, are crossed.

Step 1 — Write down the parental genotypes: Rr × Rr.

Step 2 — Work out the gametes. Each parent produces gametes carrying ONE allele (because of meiosis). Parent 1 makes gametes carrying R or r. Parent 2 also makes gametes carrying R or r.

Step 3 — Set up a 2 × 2 grid. Parent 1's gametes go down the side; parent 2's go across the top.

| | R | r |

|-------|-------|-------|

| R | RR | Rr |

| r | Rr | rr |

Step 4 — Read off the offspring.

  • Genotype ratio: 1 RR : 2 Rr : 1 rr.
  • Phenotype ratio: 3 round : 1 wrinkled (because RR and Rr both look round).
  • Probability of a wrinkled offspring: 1 in 4 = 25%.

The rr offspring is the only wrinkled one — the recessive allele is only expressed when two copies are present.

Worked Punnett square 2 — Rr × rr

A heterozygous parent is crossed with a homozygous recessive partner. (This kind of cross is sometimes called a test cross because it reveals whether the dominant-phenotype parent is RR or Rr.)

| | r | r |

|-------|-------|-------|

| R | Rr | Rr |

| r | rr | rr |

  • Genotype ratio: 1 Rr : 1 rr.
  • Phenotype ratio: 1 round : 1 wrinkled.
  • Probability of a wrinkled offspring at every fertilisation: 1 in 2 = 50%.

Half the offspring show the recessive phenotype. If the unknown parent had been RR, no wrinkled offspring would ever appear — so a single wrinkled offspring in a test cross is enough to prove the unknown parent is Rr.

Sex inheritance — XX × XY

Humans have 23 pairs of chromosomes. Twenty-two of those pairs are the same in females and males. The 23rd pair is the sex chromosomes:

  • Females are XX (two X chromosomes).
  • Males are XY (one X and one shorter Y).

The mother is XX, so every egg she makes contains an X chromosome. The father is XY, so half his sperm carry X and half carry Y. The Punnett square:

| | X | Y |

|-------|-------|-------|

| X | XX | XY |

| X | XX | XY |

  • Ratio: 2 XX : 2 XY = 1 female : 1 male.
  • Probability of a girl (or a boy) at every fertilisation: 1 in 2 = 50%.

The father's sperm determines the sex of the child: X-bearing sperm → female, Y-bearing sperm → male.

Family pedigrees

A pedigree is a family tree drawn with a standard set of symbols, used to follow an inherited trait across generations.

  • Square = male.
  • Circle = female.
  • Filled shape = affected (shows the trait, e.g. has the disorder).
  • Empty shape = unaffected (does not show the trait).
  • Half-filled shape = carrier (heterozygous, doesn't show the recessive trait but can pass the allele on).
  • Horizontal line between two shapes = partners.
  • Vertical line down from a partner pair = their children.
  • Generations are usually labelled with Roman numerals (I at the top, II below, III below that).

To work out genotypes from a pedigree, follow the rules of the trait. If the trait is recessive, an affected individual must be homozygous (e.g. ff). Any unaffected parent of an affected child must be a carrier (Ff). If the trait is dominant, every affected individual must carry at least one dominant allele — and at least one of their parents must also be affected (unless a new mutation has occurred).

Two inherited disorders

Cystic fibrosis is a recessive disorder of cell membranes that causes thick, sticky mucus to build up in the lungs and digestive system. An affected person is homozygous (ff). Two unaffected carrier parents (Ff × Ff) have a 1 in 4 chance at each pregnancy of having an affected child — the same Rr × Rr cross we worked through above, just relabelled.

Polydactyly (having extra fingers or toes) is caused by a dominant allele. An affected person has at least one dominant allele (Dd or DD). Because the allele is dominant, the trait usually appears in every generation — there are no unaffected carriers, because anyone carrying the allele shows the trait.

Reading the language of probability

OCR examiners accept probability written as a fraction, a ratio, a decimal or a percentage — as long as the answer matches the cross. Useful templates:

  • 3 : 1 phenotype ratio in Rr × Rr cross.
  • 1 : 2 : 1 genotype ratio in Rr × Rr cross.
  • 1 in 4 / 25% / 0.25 chance of homozygous recessive offspring in Rr × Rr.
  • 1 in 2 / 50% / 0.5 chance of any sex at every fertilisation.

A Punnett square gives an expected ratio — actual numbers in a small family vary by chance. A couple with three boys in a row have not 'used up' their probability of a daughter — every pregnancy is still 50:50.

Exam tips

  • Always WRITE the parental genotypes first, then the gametes, then the Punnett square. Working out the gametes in your head and going straight to the grid loses easy marks for the gamete row.
  • Use a CAPITAL letter for the dominant allele and a LOWER-CASE letter for the recessive allele — and use the letter the question gives you. If the question uses B/b for coat colour, don't switch to R/r.
  • When a Punnett square gives ratios like 3 : 1 or 1 : 2 : 1, say 'EXPECTED ratio' or 'probability' — never claim a small family must contain exactly those proportions.
  • Watch for the RR-vs-Rr trap: a round-seeded parent could be EITHER RR or Rr. If the question doesn't tell you, you cannot assume. A test cross with rr reveals which.
  • On the sex-inheritance question, finish with the line 'the FATHER'S gamete determines the sex of the offspring' — it's a frequent extra mark.
  • If asked for a 'probability', give your answer as a fraction, a percentage and (if useful) a ratio. '1 in 4 / 25% / 1 : 3' is bullet-proof.
  • On pedigree questions, label genotypes onto the diagram as you go. Affected individuals come first (their genotype is forced by the trait), then unaffected parents of affected children (must be carriers, Ff).

Mark-scheme phrasing

Common misconceptions

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Worked example

Question:

Answer:

Frequently asked questions

What is the difference between a gene and an allele?

A gene is a section of DNA that codes for a specific protein and therefore controls a particular characteristic (for example, the gene for fur length in cats, or the gene for seed shape in peas). An allele is a particular version of that gene — for the fur-length gene there is a long-fur allele and a short-fur allele. Every individual carries TWO alleles of every gene, one inherited from each parent. Mark schemes punish students who use the words interchangeably: define gene as 'a section of DNA that codes for a protein' and allele as 'a different form / version of a gene'.

Why do RR and Rr both look the same?

Because R is the dominant allele. A dominant allele is expressed even when only one copy is present, so RR (two copies of the dominant) and Rr (one copy of the dominant and one copy of the recessive) both produce the same phenotype. The recessive allele r is only expressed when two copies are present — that is, in the homozygous recessive genotype rr. So RR and Rr have the same phenotype but different genotypes — this is the heart of most OCR Punnett-square questions.

How do I set up a Punnett square step by step?

Step 1: write down the parental genotypes (e.g. Rr × Rr). Step 2: write the gametes each parent can produce — each gamete carries ONE allele because of meiosis. Step 3: draw a 2 × 2 grid. Put parent 1's gametes down the left-hand side (one per row) and parent 2's gametes across the top (one per column). Step 4: fill in each cell by combining the row allele and the column allele — these are the possible offspring genotypes. Step 5: read off the genotype ratio (e.g. 1 RR : 2 Rr : 1 rr), translate to phenotypes using which allele is dominant, and give probabilities as 1 in 4, 25%, etc.

How do XX and XY decide whether a baby is a boy or a girl?

Humans have 23 pairs of chromosomes. The 23rd pair are the sex chromosomes: XX in females and XY in males. The mother (XX) always passes an X chromosome in her eggs — every egg has an X. The father (XY) passes either an X or a Y in his sperm — half of his sperm carry X, the other half carry Y. If an X-sperm fertilises the egg, the offspring is XX (female). If a Y-sperm fertilises the egg, the offspring is XY (male). The probabilities are 50% each at every fertilisation, giving an expected 1 : 1 ratio of girls to boys. The father's gamete is what determines the sex of the child.

What does it mean to be a 'carrier' of a genetic disorder?

A carrier is someone who is heterozygous for a recessive allele — they carry one copy of the recessive disease allele and one copy of the dominant 'healthy' allele. Because the disease allele is recessive, the carrier does not show the disease themselves. But they can pass the recessive allele to their children. If two carriers have a child together (e.g. Ff × Ff for cystic fibrosis), there is a 1 in 4 chance at every pregnancy of producing an affected child (ff). On pedigrees, carriers are usually drawn as half-shaded shapes.

Why is cystic fibrosis recessive but polydactyly dominant?

Cystic fibrosis is caused by a recessive allele in the gene that makes a salt-transporting protein in cell membranes. People with one healthy copy can still make enough working protein to be unaffected, so the disease allele only causes symptoms when both copies are faulty (genotype ff). Polydactyly is caused by a dominant allele that promotes the growth of extra fingers or toes during development — a single copy is enough to produce the trait, so anyone carrying the allele shows it (genotype Dd or DD). The contrast is a useful OCR test: recessive disorders can 'skip' generations (carriers transmit them silently), whereas dominant disorders normally appear in every generation.