What does Unit 4 of WJEC A-Level Biology cover?

Unit 4, Variation, Inheritance and Applications, is the final A2 unit. It covers sexual reproduction in humans and plants, inheritance and variation (genetic crosses, sex linkage, natural selection and speciation), applications of reproduction and genetics (genetic engineering, PCR, gene therapy, cloning and the Human Genome Project), and the musculoskeletal system.

How is Unit 4 examined in WJEC A-Level Biology?

Unit 4 is an A2 written paper of structured questions and extended responses, and it contributes to the synoptic paper. Expect genetic-cross problems with Punnett squares, explanations of the menstrual cycle and natural selection, an account of inserting a gene into a bacterium, and the sliding filament theory of muscle contraction. Calculation and ratio work are common.

How do I get genetic crosses right under exam pressure?

Always define your allele symbols first, then write the parental genotypes, draw a Punnett square, and read off the phenotype ratio. A monohybrid cross of two heterozygotes gives 3 to 1; a dihybrid gives 9 to 3 to 3 to 1. For sex linkage, show the alleles on the X chromosome. State ratios as expected, then explain why real results may differ.

What is the most predictable extended-answer in this unit?

Genetic engineering. Be able to describe cutting out a gene with a restriction enzyme to leave sticky ends, joining it into a plasmid vector with DNA ligase, transforming a host bacterium, and using a marker gene to identify success. The point that the same restriction enzyme is used for the gene and the plasmid is regularly tested.

How should I revise muscle contraction?

Learn the sarcomere structure (actin, myosin, Z lines), then the sliding filament theory as a cycle: calcium ions expose binding sites on actin, myosin heads form cross-bridges and pull (the power stroke), then ATP detaches and re-cocks the head. Stress that the filaments do not shorten; they slide, increasing overlap to shorten the sarcomere.

WalesBiology

WJEC A-Level Biology Unit 4 Variation, Inheritance and Applications: a deep dive on reproduction, genetics, biotechnology and the musculoskeletal system

A deep-dive WJEC A-Level Biology guide to Unit 4, Variation, Inheritance and Applications. Covers sexual reproduction in humans and plants, inheritance and variation, applications of reproduction and genetics, and the musculoskeletal system, with the exam patterns WJEC repeats.

Generated by Claude Opus 4.820 min readWJECUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Unit 4 actually demands
Sexual reproduction in humans and plants
Inheritance and variation
Applications of reproduction and genetics
The musculoskeletal system
How Unit 4 is examined
Check your knowledge

What Unit 4 actually demands

Variation, Inheritance and Applications brings the course to a close by following genetic information from reproduction, through inheritance and evolution, into the biotechnology that applies it, and finishing with how muscles produce movement. Examiners want fluent genetic-cross work, accurate accounts of multi-step processes, and the ability to apply principles to unfamiliar and ethical contexts.

This guide walks through all four clusters of the unit, then sets out the exam patterns WJEC repeats. Each cluster has a matching dot-point page with practice questions; this overview ties them together.

Sexual reproduction in humans and plants

Gametes are made by meiosis, so they are haploid; spermatogenesis makes many small motile sperm and oogenesis makes a few large ova. The menstrual cycle is controlled by four hormones: FSH stimulates a follicle, which secretes oestrogen to thicken the uterus lining and trigger an LH surge causing ovulation; the corpus luteum then secretes progesterone to maintain the lining. Fertilisation is the fusion of haploid gamete nuclei. In flowering plants, pollination is followed by pollen tube growth and double fertilisation, forming seeds and fruit.

Inheritance and variation

Genetic crosses follow predictable ratios: a monohybrid cross of heterozygotes gives 3 to 1, a dihybrid gives 9 to 3 to 3 to 1. Codominance means both alleles are fully expressed (as in blood groups), and sex-linked genes on the X chromosome make recessive conditions commoner in males. Variation comes from mutation, meiosis and random fertilisation. Natural selection acts on this variation, changing allele frequencies as a selection pressure favours advantageous alleles, and speciation occurs when populations become reproductively isolated and diverge.

Applications of reproduction and genetics

Genetic engineering transfers a gene: a restriction enzyme cuts it out leaving sticky ends, DNA ligase joins it into a plasmid vector to make recombinant DNA, and a host bacterium expresses it to make a protein such as insulin, with a marker gene identifying success. PCR amplifies DNA in repeated heating cycles using primers and a heat-stable polymerase, and gene probes locate specific sequences. Gene therapy inserts a working allele to treat disease, cloning makes genetically identical organisms, and the Human Genome Project mapped human genes, raising ethical questions.

The musculoskeletal system

Skeletal muscle is made of myofibrils divided into sarcomeres of overlapping actin and myosin. The sliding filament theory explains contraction: calcium ions released on stimulation expose binding sites on actin, myosin heads form cross-bridges and pull the actin inward (the power stroke), then ATP detaches and re-cocks each head to repeat the cycle, shortening the sarcomere. The skeleton supports the body and gives anchorage, and antagonistic muscle pairs move joints in both directions because muscles can only pull.

How Unit 4 is examined

A typical WJEC profile for this unit:

Genetic crosses. Monohybrid, dihybrid and sex-linked problems with Punnett squares and expected ratios.
Process accounts. The menstrual cycle, inserting a gene into a bacterium, PCR, and the sliding filament theory.
Application and ethics. Evaluating gene therapy, cloning and the use of the Human Genome Project.
Extended answers. How natural selection changes allele frequency, how recombinant DNA is made, and the roles of calcium and ATP in contraction are all predictable.

Check your knowledge

A mix of recall and application questions covering the whole of Unit 4. Attempt them under timed conditions, then check against the solutions.

Explain the roles of oestrogen and progesterone in the menstrual cycle. (4 marks)
A monohybrid cross of two heterozygotes is carried out. State the expected phenotype ratio and explain it. (2 marks)
Explain why colour blindness is more common in males than females. (2 marks)
Explain how natural selection changes the allele frequency of a population over time. (4 marks)
Describe how a gene can be inserted into a bacterium to make a human protein. (4 marks)
Explain what the polymerase chain reaction does and outline how it works. (3 marks)
Describe the roles of calcium ions and ATP in muscle contraction. (4 marks)
Explain why skeletal muscles must work in antagonistic pairs. (2 marks)

Solutions

Q1: Oestrogen, from the developing follicle, repairs and thickens the uterus lining and, at high levels, triggers the LH surge that causes ovulation. Progesterone, from the corpus luteum, maintains the thickened lining ready for implantation. Both inhibit FSH and LH by negative feedback.
Q2: The expected ratio is 3 to 1 (dominant to recessive). Crossing Aa with Aa gives genotypes AA, Aa, Aa and aa, so three show the dominant phenotype for every one showing the recessive.
Q3: The allele is recessive and carried on the X chromosome. Males have only one X, so a single recessive allele is expressed, whereas females need two copies to be affected.
Q4: Mutation and meiosis create variation. A selection pressure means individuals with a favourable allele are more likely to survive and reproduce. They pass on the allele, so its frequency increases over generations while less favourable alleles become rarer.
Q5: A restriction enzyme cuts out the gene, leaving sticky ends. A plasmid is cut with the same enzyme so its ends match, and DNA ligase joins the gene into the plasmid to form recombinant DNA. A bacterium takes up the plasmid (transformation), and transformed cells are identified with a marker gene and cultured to express the gene.
Q6: PCR amplifies DNA. The DNA is heated to separate the strands, primers bind to the target sequence, and a heat-stable DNA polymerase builds new strands. Repeated cycles double the DNA each time, making millions of copies.
Q7: Calcium ions are released and bind troponin, moving tropomyosin to expose the myosin binding sites on actin. Myosin heads form cross-bridges and flex to pull the actin inward (the power stroke). ATP then binds to detach the heads, and its hydrolysis re-cocks them for the next cycle.
Q8: Muscles can only pull (contract), not push. One muscle moves the joint one way and its antagonistic partner moves it back, so a pair is needed to move the joint in both directions.