What you'll be able to do by the end
- Use the vocabulary of genetics precisely — gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous
- Explain the difference between sexual and asexual reproduction, and evaluate each
- Describe meiosis and explain why it produces genetic variation
- Complete and interpret a Punnett square
- Predict a monohybrid cross and express it as a ratio, fraction and percentage
- Explain how sex is determined in humans
- Describe inherited disorders — polydactyly and cystic fibrosis — and predict risk
- Discuss the ethical issues around embryo screening
- Explain the structure of DNA and what a gene does
Teach this after Natural Selection — that unit relied on "alleles are passed to offspring" without the mechanism. This is the mechanism.
Part 1 — The vocabulary
Get this right first. Almost every lost mark in genetics is a vocabulary error, not a reasoning error.
| Term | Meaning |
|---|---|
| Gene | A short section of DNA that codes for a protein |
| Allele | A different version of the same gene |
| Genome | The entire genetic material of an organism |
| Chromosome | A long molecule of DNA, carrying many genes |
| Genotype | The alleles an organism has — written as letters, e.g. Bb |
| Phenotype | The characteristic you can observe — e.g. brown eyes |
| Dominant | Shows in the phenotype even with one copy. Capital letter |
| Recessive | Only shows if two copies are present. Lower case |
| Homozygous | Two identical alleles — BB or bb |
| Heterozygous | Two different alleles — Bb |
Genotype is the letters. Phenotype is what you see. A hedgehog with genotype Aa and one with AA can have exactly the same phenotype — "standard grey." Same phenotype, different genotype — that's the whole reason genetics is interesting, and what "recessive alleles hiding in a population" means.
There is a gene for coat colour. There are several alleles of that gene — one for dark, one for pale, and so on. "The gene for brown eyes" is loose; it should be "the allele for brown eyes." Exam boards increasingly penalise this.
Part 2 — Sexual and asexual reproduction
| Sexual | Asexual | |
|---|---|---|
| Parents | Two | One |
| Cells | Gametes fuse at fertilisation | Mitosis only |
| Offspring | Genetically different | Genetically identical — clones |
| Cell division | Meiosis to make gametes | Mitosis |
| Variation | Yes | No (except by mutation) |
Sexual — advantages: produces variation; variation gives a survival advantage if the environment changes; natural selection can act, so the species can adapt; selective breeding is possible. Disadvantages: needs two parents; slower; more energy.
Asexual — advantages: only one parent; much faster; less energy; many identical offspring when conditions are favourable. Disadvantages: no variation, so the whole population is vulnerable to one disease or change; cannot adapt to a changing environment.
Malarial parasites reproduce asexually in the human host, sexually in the mosquito. Fungi use spores (asexual) when conditions are good, sexual reproduction when they deteriorate. Many plants use seeds (sexual) and runners (asexual). The pattern: asexual when conditions are good — make many copies fast; sexual when conditions are difficult or changing — make variation and hope some survive.
Part 3 — Meiosis
Meiosis makes gametes — sperm and egg in animals, pollen and egg in plants.
- The cell copies its genetic information — two copies of each chromosome
- The cell divides twice, producing four gametes
- Each gamete has half the number of chromosomes — a single set
- All four gametes are genetically different from each other
In humans, body cells have 46 chromosomes (23 pairs); gametes have 23. At fertilisation a sperm (23) fuses with an egg (23) to make a zygote with 46 — restored. If gametes had the full 46, every generation would double the number; halving in meiosis and doubling at fertilisation keep it constant.
| Mitosis | Meiosis | |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Making gametes |
| Divisions | One | Two |
| Cells produced | 2 | 4 |
| Chromosome number | Same as parent | Half of parent |
| Genetically | Identical | All different |
Mitosis makes more of the same. Meiosis makes mixed-up halves. Two divisions, four cells, half the chromosomes, all different.
Part 4 — Genetic crosses
A Punnett square predicts the possible genotypes of offspring from two parents.
Worked example — hedgehog coat colour
African pygmy hedgehogs come in many colour morphs. Here, standard grey (A) is dominant and albino (a) is recessive. Two heterozygous grey hedgehogs are bred: Aa × Aa. Each parent can pass on A or a:
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotype ratio: 1 AA : 2 Aa : 1 aa. Phenotype ratio: 3 grey : 1 albino. Probability of albino: 1 in 4 = 25% = ¼.
"One in four of the offspring will be albino." No — there is a 1 in 4 probability for each offspring. A litter of four could easily contain no albinos, or three. Write "a 25% chance" or "a probability of ¼." Never "one in four will be."
The three crosses you must know
Cross 1 — AA × aa: all offspring Aa — all show the dominant phenotype, all carry the recessive allele.
Cross 2 — Aa × Aa: 3 : 1 phenotype ratio; 25% chance of the recessive phenotype.
Cross 3 — Aa × aa: 1 : 1 phenotype ratio; 50% chance of the recessive phenotype.
An organism showing the dominant phenotype could be AA or Aa — you can't tell by looking. Breed it with a homozygous recessive (aa): if any offspring show the recessive phenotype, the parent must be Aa; if all offspring show the dominant phenotype, it's probably AA. This is exactly what animal breeders do to find out whether an animal carries a recessive allele — including one for a disorder.
Part 5 — Sex determination
Humans have 23 pairs of chromosomes. 22 pairs control characteristics; the 23rd pair determines sex. Female: XX. Male: XY.
| X | X | |
|---|---|---|
| X | XX | XX |
| Y | XY | XY |
Result: 2 XX : 2 XY — a 50% chance of each sex. The mother can only pass on X; the father passes X or Y, so the father's gamete determines the sex of the child.
Sex determination differs across species. In birds, the female has the differing pair. In many reptiles — including bearded dragons and some tortoises — sex is determined by incubation temperature, not chromosomes. This has a real conservation consequence: rising temperatures are skewing sex ratios in wild turtle populations, with some beaches producing almost entirely female hatchlings.
Part 6 — Inherited disorders
Polydactyly — caused by a dominant allele
Extra fingers or toes. Because the allele (D) is dominant, only one copy is needed. A parent with polydactyly can pass it on even if the other parent is unaffected — Dd × dd:
| d | d | |
|---|---|---|
| D | Dd | Dd |
| d | dd | dd |
50% chance of a child with polydactyly. Key point: a dominant disorder cannot skip a generation — if a child has it, at least one parent has it.
Cystic fibrosis — caused by a recessive allele
A disorder of cell membranes causing thick, sticky mucus in the lungs and digestive system. The allele (f) is recessive, so a person needs two copies to be affected. Carriers are heterozygous (Ff) — they have the allele but no symptoms. Two carriers, Ff × Ff:
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
FF unaffected, not a carrier (25%) · Ff unaffected carrier (50%) · ff has cystic fibrosis (25%). A recessive disorder can skip generations — two unaffected carriers can have an affected child; 50% of children are carriers; neither parent has the disorder, which is why it can appear unexpectedly.
If two unaffected parents have an affected child, the allele must be recessive — the parents are both carriers. If an affected child has two unaffected parents, it cannot be dominant. That single observation answers half of all pedigree questions.
Embryo screening
In IVF, embryos can be tested for disorder alleles before implantation; embryos without the disorder allele are implanted.
For: prevents suffering; reduces cost to the health service; laws prevent it going too far; parents already make choices about having children. Against: it implies people with genetic disorders are less valuable; it may lead to selecting non-medical characteristics; unused embryos are destroyed; it is expensive and unequally available.
This is an evaluate question and it will be levels-marked. To reach the top band you must give both sides and reach a judgement. There is no "right" opinion — the marks are for the quality of reasoning, not the conclusion.
Part 7 — DNA and the genome
DNA is a polymer made of two strands forming a double helix. Each strand is made of repeating units called nucleotides, each containing a sugar, a phosphate group, and one of four bases: A, C, G, T. Complementary base pairing: A pairs with T, and C pairs with G — always. So one strand determines the other, which is how DNA copies itself accurately.
What a gene does
A gene is a section of DNA that codes for a particular sequence of amino acids, which fold to make a specific protein. Three bases code for one amino acid. The order of bases determines the order of amino acids, which determines the protein's shape, which determines what it does.
The genome, and why sequencing it matters
The genome is the entire genetic material of an organism; the human genome was sequenced in 2003. It's useful to: 1. search for genes linked to disease; 2. understand and treat inherited disorders; 3. trace human migration patterns.
The same technology classifies organisms into the three domains, identifies which species a piece of meat came from, and shows that archaea are as distinct from bacteria as we are. Genome sequencing links this unit to classification, evolution, disease and forensics — one technique underpinning several topics.
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 47 marks.
Question 1
In African pygmy hedgehogs, the allele for standard grey coat (A) is dominant to the allele for albino coat (a).
(Total 9 marks)
Question 2
Cystic fibrosis is caused by a recessive allele (f).
(Total 11 marks)
Question 3
(Total 6 marks)
Question 4
(Total 6 marks)
Question 5
(Total 9 marks)
Question 6 — extended response
Embryos produced by IVF can be screened for alleles that cause inherited disorders; embryos carrying those alleles may not be implanted. Evaluate the use of embryo screening. (6)
(Total 6 marks)
TOTAL FOR PAPER: 47 MARKS
Mark scheme
Question 1
| (a) 2. Standard grey | 1 |
| A is dominant, so only one copy is needed for the characteristic to show | 1 |
| (b) 4. Parental genotypes correctly identified as Aa × Aa | 1 |
| Gametes correctly shown as A and a for each parent | 1 |
| Punnett square correctly completed: AA, Aa, Aa, aa | 1 |
| States 25% (accept ¼, 1 in 4, 0.25) | 1 |
Do not credit "one in four offspring will be albino" — the answer must be expressed as a probability.
| (c) 3. Cross the grey hedgehog with an albino (aa) | 1 |
| If any offspring are albino, the grey parent must be Aa / a carrier | 1 |
| If all offspring are grey, the grey parent is probably AA | 1 |
Credit reference to needing many offspring — a small litter could produce no albinos by chance even from an Aa parent.
Question 2
| (a) 2. A carrier is heterozygous — one copy of the recessive allele (Ff) | 1 |
| No disorder themselves (one dominant allele is enough for normal function), but they can pass the allele on | 1 |
| (b) 4. Both parents are carriers, genotype Ff | 1 |
| Correct Punnett square: FF, Ff, Ff, ff | 1 |
| Each parent passes on the f allele | 1 |
| The child receives two recessive alleles (ff), so has cystic fibrosis | 1 |
| (c) 2. 25% (or ¼) | 1 |
| Each pregnancy is independent — the probability is not affected by the first child | 1 |
The second mark is the discriminator — many candidates assume the odds change.
| (d) 3. Cystic fibrosis is recessive, so carriers show no symptoms and the allele can pass silently through generations | 1 |
| It only appears when two carriers have a child and both pass on the recessive allele | 1 |
| Polydactyly is dominant, so anyone with the allele shows the condition — it cannot be hidden, so cannot skip a generation | 1 |
Question 3
(a) 2 — any two: mitosis 2 cells / meiosis 4; mitosis identical / meiosis different; mitosis maintains chromosome number / meiosis halves it; mitosis one division / meiosis two.
| (b) 2. At fertilisation two gametes fuse | 1 |
| If gametes had the full number, the chromosome number would double each generation | 1 |
| (c) 2. Sexual: produces variation, so some offspring may survive if conditions change or a disease appears | 1 |
| Asexual: faster / needs no partner / many identical offspring quickly when conditions are good | 1 |
Question 4
(a) 1 — Male XY, female XX. Both required.
| (b) 3. Parental genotypes: XX × XY | 1 |
| Correct Punnett square showing XX, XX, XY, XY | 1 |
| 2 out of 4 are XX, so a 50% chance of female | 1 |
| (c) 2. Rising temperature would produce a skewed sex ratio — mostly one sex | 1 |
| Fewer breeding pairs / difficulty finding mates, so the population could decline or become locally extinct | 1 |
Question 5
(a) 4 — one each: a polymer / two strands; twisted into a double helix; repeating nucleotides (sugar, phosphate, base); four bases A/C/G/T with A–T and C–G pairing.
| (b) 3. A sequence of three bases codes for one amino acid | 1 |
| The order of bases determines the order of amino acids in the protein | 1 |
| The amino acid order determines the protein's shape and therefore its function | 1 |
(c) 2 — any two: search for genes linked to disease; understand/treat inherited disorders; trace human migration.
Question 6 — extended response (6 marks, levels-marked)
Level 3 (5–6): balanced evaluation of both sides with explanation, reaching a justified conclusion. Level 2 (3–4): both sides with limited explanation, or one side thoroughly with brief acknowledgement of the other. Level 1 (1–2): relevant points, largely one-sided or listed.
Indicative — for: prevents a child being born with a serious disorder; reduces long-term cost; informed parental choice; regulation prevents misuse. Against: implies people with disorders are less valuable; risk of extending to non-medical traits; unused embryos destroyed; expensive/unequal; the disorder may vary in severity. Conclusion: any justified position — both are equally creditable; marks are for reasoning, not the view taken.
Common mistakes in this topic
"One in four will be albino." It's a 25% probability for each offspring, not a guaranteed outcome.
Assuming the odds change after an affected child. Each pregnancy is independent. Two affected children in a row is unlucky, not impossible.
Using "gene" where "allele" is meant. There's a gene for coat colour and several alleles of it.
Confusing genotype and phenotype. Genotype is the letters; phenotype is what you see.
Lower case for a dominant allele. Dominant is always a capital, recessive the same letter in lower case. B and r for the same gene is meaningless.
Forgetting to label the Punnett square. Gametes must be written on both edges; an unlabelled grid may not score.
One-sided answers to "evaluate." Both sides plus a judgement, every time.
Teaching notes
Vocabulary first, and drill it. Genotype/phenotype, gene/allele, homozygous/heterozygous. Students who muddle these lose marks on questions they understand. A five-minute recall starter every lesson for a fortnight beats any single explanation.
Punnett squares should be taught as a mechanical procedure. Parents' genotypes → gametes on the edges → fill the boxes → count. Students who improvise get lost; students with a fixed routine get full marks.
The probability point in Question 2(c) is the discriminator. Ask whether a couple who've had one affected child are more or less likely to have another — most say less. It's 25% every time, and the discussion sticks.
Hedgehog colour morphs work better than pea plants. Students who've seen a real albino hedgehog engage with aa in a way they don't with Mendel's peas — and colour morphs are a live topic among people who keep exotic pets.
Question 6 needs practising as a levels-marked question. Students trained on point-marked questions write a list and expect a mark each. Show them the level descriptors and mark two sample answers together.
Handle inherited disorders with care. Some students will have a relevant family history. Present them as conditions people live with, not tragedies to be prevented — particularly in the embryo-screening discussion, where a badly framed lesson can be genuinely hurtful.