OCR GCSE Biology: Genetics, Inheritance and Evolution Guide (B5)
OCR GCSE Biology: Genetics, Inheritance and Evolution Guide (B5)
Topic B5 — Genes, inheritance and selection is one of the most rewarding parts of the OCR Gateway Science A GCSE Biology specification (J247). It takes you from the structure of a single DNA molecule all the way up to the slow reshaping of whole species by natural selection, and it ties together ideas you have met across the course. Students often worry about genetics because it involves probability and unfamiliar vocabulary, but B5 is in fact one of the most predictable topics to revise. The question types repeat, the genetic cross has a fixed method, and once you can build a Punnett square cleanly you can answer almost any inheritance question the examiner sets.
This guide walks through every major idea in B5 at GCSE depth: reproduction and meiosis, DNA and the genome, monohybrid inheritance with worked Punnett squares, inherited disorders and family trees, sex determination, variation and mutation, evolution by natural selection, evidence for evolution, classification, and selective breeding and genetic engineering. Higher-tier-only material is flagged with [H] throughout. For structured practice alongside this guide, work through the LearningBro OCR GCSE Biology: Genes, Inheritance and Selection course, which covers every section below with exam-style questions in the OCR format.
How B5 Fits the J247 Specification
OCR Gateway Science A GCSE Biology (J247) is assessed by two papers. Paper 1 covers topics B1–B3, and Paper 2 covers topics B4–B6, so B5 sits firmly on Paper 2. Each paper is worth 90 marks, lasts 1 hour 45 minutes, and counts for 50% of the qualification. The same content is examined on Foundation and Higher tiers, but Higher papers reach further into the more demanding ideas — three-domain classification, the detail of evolutionary evidence, and some of the genetic-engineering applications — which is why the [H] flags below matter for targeting your revision.
Genetics questions frequently use the OCR command words precisely, so read them with care. "Describe" asks you to say what happens; "Explain" asks you to give reasons or a mechanism; "Complete the Punnett square" asks for the offspring genotypes in the grid; and "Calculate the probability" or "Give the ratio" asks you to turn that grid into a number. Knowing exactly what each word wants will stop you writing a mechanism when a single ratio was all that was needed.
Reproduction: Sexual, Asexual and Meiosis
Organisms pass on genetic information in two fundamentally different ways, and you must be able to compare them.
Sexual versus Asexual Reproduction
Sexual reproduction involves two parents and the fusion of gametes (sex cells). Because the offspring receive a mixture of alleles from each parent, they are genetically different from one another and from their parents. This variation is a major advantage: if the environment changes, some offspring may carry combinations of alleles that help them survive.
Asexual reproduction involves only one parent and no fusion of gametes. The offspring are clones — genetically identical to the parent. It is fast and needs only one organism, which suits stable conditions; the disadvantage is a total lack of variation, so an entire population can be wiped out by the same disease or environmental change. Many organisms use both strategies — strawberry plants send out runners (asexual) but also flower (sexual).
Mitosis versus Meiosis
These two types of cell division do very different jobs, and a clear comparison earns marks reliably.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One | Two |
| Daughter cells produced | Two | Four |
| Chromosome number | Same as parent (diploid) | Halved (haploid) |
| Genetically | Identical to parent | All genetically different |
| Used for | Growth, repair, asexual reproduction | Making gametes |
Mitosis produces two genetically identical diploid daughter cells and is used for growth, repair and asexual reproduction. Meiosis produces the gametes. It involves two rounds of division, so a single parent cell gives rise to four genetically different haploid gametes, each with half the chromosome number. In humans the body cells contain 46 chromosomes, so each gamete contains 23; at fertilisation two gametes fuse and the full diploid number of 46 is restored. Meiosis generates variation by shuffling the chromosomes randomly as they separate.
DNA, Genes and the Genome
DNA (deoxyribonucleic acid) is the molecule that stores the genetic instructions of every living organism. Its shape is a double helix — two strands twisted around each other and joined by pairs of bases.
There are four bases, and they pair according to strict, complementary rules:
- A (adenine) always pairs with T (thymine)
- C (cytosine) always pairs with G (guanine)
This complementary base pairing is what allows DNA to be copied accurately, because each strand acts as a template for a new partner strand. You should be able to write the complementary sequence to a given strand: the strand A–T–G–C pairs with T–A–C–G.
Some key vocabulary you must use precisely:
- A gene is a short section of DNA that codes for a particular sequence of amino acids, which folds into a specific protein.
- An allele is a different version of the same gene.
- A chromosome is a long, coiled molecule of DNA carrying many genes.
- The genome is the entire genetic material of an organism. The Human Genome Project mapped all human genes, which has helped scientists search for genes linked to disease, develop new medicines, and trace human migration and evolution.
Genetic Terms You Must Know
Before working any cross, lock down this vocabulary — most inheritance marks are lost on imprecise terminology, not on the genetics itself.
- A dominant allele is expressed whenever it is present; it is written as a capital letter (B).
- A recessive allele is only expressed when two copies are present; it is written as the matching lower-case letter (b).
- Homozygous means both alleles are the same (BB or bb).
- Heterozygous means the two alleles are different (Bb).
- The genotype is the combination of alleles an organism carries (e.g. Bb).
- The phenotype is the characteristic that genotype produces (e.g. brown eyes).
A useful consequence: a recessive characteristic only ever shows in a homozygous recessive (bb) individual, whereas a dominant characteristic shows in both BB and Bb.
Monohybrid Inheritance with Punnett Squares
A monohybrid cross follows a single gene with two alleles from parents to offspring. The tool for this is the Punnett square, a simple grid that pairs each gamete from one parent with each gamete from the other. Build it the same way every time and the ratios fall out cleanly.
Worked cross 1: Bb × Bb (two heterozygous parents)
Suppose B is the dominant allele for brown fur and b is the recessive allele for white fur. Both parents are heterozygous, Bb. Each parent can pass on either a B or a b, so we put one parent's possible gametes across the top and the other's down the side.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Reading the four boxes, the offspring genotypes are 1 BB : 2 Bb : 1 bb. Now translate that into phenotypes. BB and Bb both show brown fur (because B is dominant), and only bb shows white fur, so the phenotype ratio is:
3 brown:1 white
This is the classic 3 : 1 ratio from crossing two heterozygotes. You can express the same result in three equivalent ways, and the exam may ask for any of them:
- as a ratio: 3 : 1
- as a fraction: 43 brown and 41 white
- as a percentage: 75% brown and 25% white
A vital point of language: each offspring has a 41 probability of being white. The ratio describes the expected proportions over many offspring, not a guarantee — four offspring could easily all be brown.
Worked cross 2: Bb × bb (heterozygous × homozygous recessive)
Now cross a heterozygous brown parent (Bb) with a white parent (bb). The Bb parent can pass on B or b; the bb parent can only ever pass on b.
| b | b | |
|---|---|---|
| B | Bb | Bb |
| b | bb | bb |
The offspring genotypes are 2 Bb : 2 bb, which simplifies to 1 Bb : 1 bb. Translating to phenotypes, Bb is brown and bb is white, so the phenotype ratio is:
1 brown:1 white
Equivalently, each offspring has a 21 (that is, 50%) chance of being brown and a 21 chance of being white. A cross of a heterozygote with a homozygous recessive always gives this 1 : 1 ratio, which is why such a "test cross" is a neat way to reveal whether a dominant-looking individual is BB or Bb.
A reliable method for any cross
- Choose clear letters — capital for dominant, lower-case for recessive — and write each parent's genotype.
- Work out the possible gametes from each parent (each gamete carries one allele).
- Draw the grid and fill every box by combining the row and column allele.
- Count the genotypes, then convert to phenotypes using the dominance rule.
- Express the answer in the form the question asks: ratio, fraction or percentage.
Inherited Disorders and Family Trees
Some conditions are caused by faulty alleles passed from parents to children, and B5 expects you to handle two standard examples.
- Polydactyly (having extra fingers or toes) is caused by a dominant allele. Because the allele is dominant, a child can inherit the condition from just one affected parent who carries a single copy.
- Cystic fibrosis is a disorder of cell membranes caused by a recessive allele. A child is only affected if they inherit the faulty allele from both parents, so two unaffected carriers (each Ff, where f is the recessive allele) can have an affected child.
Work the cystic fibrosis carrier cross to see the risk. With both parents heterozygous carriers, Ff × Ff:
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
The offspring are 1 FF : 2 Ff : 1 ff. Only ff has cystic fibrosis, so there is a 41 (that is, 25%) chance each child is affected, a 21 chance of being an unaffected carrier, and a 41 chance of carrying no faulty allele at all. This is the same underlying 3 : 1 unaffected-to-affected pattern as worked cross 1.
A family tree (pedigree) chart shows inheritance across generations. By convention, squares are males and circles are females, and a shaded shape shows an affected individual. To read one, look for an affected child with two unaffected parents — that pattern reveals a recessive condition carried silently by the parents. Dominance and the genetic cross then let you work out unknown genotypes and the probability that a future child is affected.
Sex Determination
In humans, sex is decided by one pair of chromosomes, the sex chromosomes. Females are XX and males are XY; the other 22 pairs are the same in both sexes. Because the mother is XX she can only ever pass on an X, while the father is XY and passes on either an X or a Y. The sex of the child therefore depends on the father's gamete.
| X (from mother) | X (from mother) | |
|---|---|---|
| X (from father) | XX | XX |
| Y (from father) | XY | XY |
The offspring are 2 XX : 2 XY, a 1 : 1 ratio of female to male — so each child has a 50% chance of being female and a 50% chance of being male.
Variation and Mutation
Variation is the differences between individuals of the same species. It has two sources:
- Genetic variation comes from the alleles you inherit, and arises through meiosis, the random fusion of gametes, and mutation.
- Environmental variation comes from your surroundings and lifestyle — a plant grown in shade, or scars and language, are environmental.
Most characteristics are influenced by both genes and environment together, for example human height.
A mutation is a random change in the DNA base sequence. Mutations happen continuously and most have no effect on the phenotype, because they fall in non-coding DNA or do not change the protein. Occasionally a mutation alters a protein enough to change a characteristic; very rarely it produces a beneficial new allele. New alleles created by mutation are the ultimate source of all genetic variation, and they provide the raw material on which natural selection acts. The rate of mutation can be increased by factors such as ionising radiation and some chemicals.
Evolution by Natural Selection
Evolution is the slow change in the inherited characteristics of a population over many generations, through the process of natural selection first set out by Charles Darwin. The logic runs in clear steps, and examiners reward seeing them laid out in order:
- Individuals in a species show variation because of differences in their alleles.
- Organisms produce more offspring than the environment can support, so they compete for resources such as food, mates and shelter.
- Individuals with characteristics best suited to the environment are more likely to survive and reproduce — "survival of the fittest".
- The alleles for those advantageous characteristics are passed on to the next generation, so they become more common over time.
- Over many generations the species gradually changes — it evolves. If two populations become so different that they can no longer interbreed to produce fertile offspring, a new species has formed (speciation).
Antibiotic-resistant bacteria: evolution we can watch
The clearest modern evidence for natural selection is the rise of antibiotic-resistant bacteria such as MRSA. Bacteria reproduce extremely quickly, and random mutations occasionally produce a bacterium that happens to be resistant to an antibiotic. When the antibiotic is used, it kills the non-resistant bacteria but the resistant one survives and reproduces, passing its resistance allele to its many descendants until the whole population is resistant. This is natural selection compressed into a timescale we can observe, which is why doctors are urged to prescribe antibiotics only when necessary and patients to complete the full course.
Evidence for Evolution and Classification
Fossils
A fossil is the preserved remains, or traces, of an organism from many thousands or millions of years ago. Fossils form when hard parts are replaced by minerals, when an organism is preserved where decay cannot happen, or as traces such as footprints. By arranging fossils in the rock layers in which they are found, scientists can see how organisms have changed over time, which is powerful evidence for evolution. The fossil record is incomplete, however, because many early organisms were soft-bodied and left few fossils, and others have been destroyed by geological activity. Resistant bacteria and the fossil record together give both fast and slow lines of evidence.
Classification: Linnaean and three-domain
Living things are sorted into groups in a system devised by Carl Linnaeus. The Linnaean system arranges organisms into a nested hierarchy: Kingdom, Phylum, Class, Order, Family, Genus, Species. Within it, each organism is given a two-part Latin name in the binomial system — the genus first (capitalised) then the species, as in Homo sapiens or Panthera leo.
As microscopes and then DNA analysis revealed more about the internal and molecular detail of organisms, the classification system was revised. The modern three-domain system [H], proposed by Carl Woese, divides all life into three domains based on differences in cellular and molecular biology: Archaea, Bacteria, and Eukaryota (organisms whose cells have a nucleus — animals, plants, fungi and protists). This change is a good example of how scientific models are improved as new evidence and technology become available.
Selective Breeding and Genetic Engineering
Humans have long manipulated the genetics of other species, and B5 asks you to weigh the benefits and risks even-handedly.
Selective Breeding
Selective breeding (artificial selection) is choosing organisms with desired characteristics and breeding them together over many generations so those characteristics become more common. It has given us crops with high yields or disease resistance, cattle that produce more milk, and dogs bred for temperament. The main risk is reduced genetic variation: repeatedly breeding from closely related individuals (inbreeding) makes a population vulnerable to a single new disease and can let harmful recessive alleles accumulate, sometimes causing inherited health problems.
Genetic Engineering
Genetic engineering transfers a gene from one organism into another so that the second organism shows the desired characteristic. A gene for a useful protein is cut out, inserted into the DNA of the target organism, and the modified organism then produces that protein. Real applications include bacteria engineered to make human insulin for treating diabetes, and crops engineered to resist disease or insect attack or to carry more nutrients.
A balanced view weighs the benefits against the concerns:
| Potential benefits | Potential risks and concerns |
|---|---|
| Larger yields and more reliable food supply | Possible unforeseen effects on health, still being studied |
| Crops with added nutrients to combat deficiency | Reduced biodiversity if one modified variety dominates |
| Bacteria producing medicines such as insulin | Concern that inserted genes could spread to wild plants |
| Disease- and pest-resistant crops reduce pesticide use | Ethical questions about modifying living organisms |
The exam may ask you to discuss genetic engineering, which means presenting both sides rather than simply approving or condemning it. A strong answer states a benefit, states a risk, and reaches a reasoned conclusion.
Common Mistakes in B5
The same slips recur every year. Knowing them is half the battle.
- Mixing up the letters. Always use a capital for the dominant allele and the same letter in lower case for the recessive one. Using B and r for one gene loses clarity and marks.
- Confusing genotype and phenotype. The genotype is the alleles (Bb); the phenotype is the characteristic (brown). Read which one the question wants.
- Forgetting that gametes carry one allele each. A Bb parent's gametes are B or b — never Bb. This is the single most common Punnett-square error.
- Treating a ratio as a guarantee. A 3 : 1 ratio means a 41 probability per offspring, not that exactly one in four children will definitely be affected.
- Saying organisms evolve "in order to" survive. Natural selection has no purpose or foresight; advantageous variations arise randomly and are then selected, not produced on demand.
- Confusing meiosis and mitosis. Meiosis makes four genetically different haploid gametes; mitosis makes two identical diploid cells. Spelling them apart matters.
Exam Technique for B5 on J247
- Show the Punnett square. Even when only a ratio is asked for, the grid earns method marks and protects you against a slip.
- Match the answer form to the command word. "Give the ratio" wants 3 : 1; "calculate the probability" wants 41 or 25%. Do not give a paragraph when a number is wanted.
- Lay natural selection out in steps. Variation → competition → survival of the fittest → inheritance of alleles → change over generations. Examiners credit the sequence.
- Use precise vocabulary. Allele, genotype, phenotype, homozygous, heterozygous, dominant, recessive — the right word in the right place is where the marks are.
- Give balanced answers on ethics. For genetic engineering and selective breeding, state benefits and risks before concluding.
Prepare with LearningBro
The LearningBro OCR GCSE Biology: Genes, Inheritance and Selection course covers every part of B5 — reproduction and meiosis, DNA and the genome, genetic crosses, inherited disorders, evolution, classification and genetic technologies — with worked examples and practice questions that match the OCR J247 format, plus immediate feedback on your answers.
For broader preparation across the whole specification and both papers, the OCR GCSE Biology Exam Prep course walks you through the paper structure, command words and answering technique. And for the wider picture of the entire subject, start with our OCR GCSE Biology complete revision guide.
Genetics rewards practice above almost any other topic. The more crosses you draw and the more selection arguments you write out in full, the more automatic the methods become — and B5 turns from a worry into one of the most reliable places on the paper to score.
Good luck with your revision.