QCE Biology Unit 4 Heredity and Continuity of Life: deep-dive 2026 guide
Deep-dive on QCE Biology Unit 4. DNA structure and replication, gene expression, Mendelian and non-Mendelian inheritance with Punnett squares, mutations and variation, and natural selection and speciation, with worked genetics problems and a Check your knowledge section.
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How Unit 4 fits into QCE Biology
Unit 4, Heredity and continuity of life, is the final unit and completes the externally assessed content. Topic 1 covers DNA, genes and the continuity of life (molecular genetics and inheritance); Topic 2 covers continuity of life on Earth (mutations, variation, natural selection and evolution). The unit draws on the nucleus and chromosomes from Unit 1 and the idea of selection introduced through antibiotic resistance in Unit 2. Genetics calculation problems and extended responses on natural selection are reliable marks every year, so this guide pairs the theory with worked crosses.
DNA structure and replication
DNA (deoxyribonucleic acid) is the molecule of heredity. It is a double helix of two antiparallel strands. Each strand has a sugar-phosphate backbone (deoxyribose linked by phosphodiester bonds) with nitrogenous bases projecting inward.
The four bases pair by complementary base pairing: adenine pairs with thymine (two hydrogen bonds) and cytosine pairs with guanine (three hydrogen bonds). Because the strands are complementary, each can act as a template for the other.
DNA replication is semiconservative: each new molecule keeps one original strand and one newly made strand.
- Helicase unwinds and separates the two strands at the replication fork.
- DNA polymerase adds free nucleotides to each template strand following the base-pairing rules.
- Two identical double helices result, each with one parental and one new strand.
Semiconservative replication ensures each daughter cell receives an accurate copy of the genetic information.
Gene expression
A gene is expressed when its DNA sequence is used to build a protein, in two stages.
- Transcription (in the nucleus): RNA polymerase reads the gene's template strand and builds a complementary single strand of messenger RNA (mRNA). RNA uses uracil in place of thymine.
- Translation (at the ribosome): the mRNA is read in triplets called codons. Transfer RNA (tRNA) molecules with matching anticodons bring the correct amino acids, which are joined into a polypeptide. A start codon begins and a stop codon ends translation.
The sequence of codons therefore determines the sequence of amino acids, which determines the protein's structure and function. This is the central dogma: DNA to RNA to protein.
Mendelian inheritance
Mendel's two laws describe single-gene inheritance.
- Law of segregation: each individual has two alleles for each gene, and the two separate during meiosis so each gamete carries only one.
- Law of independent assortment: alleles of different genes assort into gametes independently, provided the genes are on different chromosomes.
Key vocabulary: a gene codes for a trait; an allele is an alternative form of a gene; the genotype is the alleles carried; the phenotype is the observable trait; homozygous means two identical alleles; heterozygous means two different alleles; a dominant allele is expressed in the heterozygote (capital letter) while a recessive allele shows only in the homozygote (lowercase).
A monohybrid cross of two heterozygotes (Tt x Tt) gives a 1 TT : 2 Tt : 1 tt genotype ratio and a 3 : 1 phenotype ratio.
A dihybrid cross of two double heterozygotes (TtYy x TtYy) gives the classic 9:3:3:1 phenotype ratio, because the dihybrid ratio is the product of two independent monohybrid ratios (three quarters times three quarters for each combination).
A test cross reveals whether an individual showing the dominant phenotype is homozygous (TT) or heterozygous (Tt): cross it with a homozygous recessive (tt). If all offspring are dominant, the unknown is most likely TT; a roughly 1:1 ratio of dominant to recessive shows it is Tt. A single recessive offspring proves the unknown carries a recessive allele.
Non-Mendelian inheritance
Not all traits follow simple dominant-recessive Mendelian patterns.
- Incomplete dominance: the heterozygote shows an intermediate phenotype (red flower crossed with white flower gives pink). A cross of two pink heterozygotes gives a 1 red : 2 pink : 1 white phenotype ratio (genotype and phenotype ratios are the same).
- Codominance: both alleles are fully expressed in the heterozygote (the AB blood group, where both A and B antigens appear).
- Multiple alleles: more than two alleles exist in the population for a gene, though any individual carries only two (the ABO blood group has alleles I-A, I-B and i).
- Sex linkage: a gene on a sex chromosome (usually the X) shows different inheritance patterns in males and females (red-green colour blindness and haemophilia are X-linked recessive, more common in males because they have only one X).
- Polygenic inheritance: a trait controlled by many genes shows continuous variation (height, skin colour).
Pedigree analysis uses family-tree diagrams to deduce inheritance patterns and genotypes, distinguishing autosomal from sex-linked and dominant from recessive traits by how the trait passes between generations and sexes.
Mutations and genetic variation
A mutation is a change in the DNA sequence and the ultimate source of new alleles.
- Gene (point) mutations change a single gene's base sequence by substitution, insertion or deletion. A substitution can be silent (no amino acid change), missense (one amino acid changed) or nonsense (a premature stop codon). Insertions and deletions can cause a frameshift, altering every codon downstream.
- Chromosomal mutations change chromosome structure (deletion, duplication, inversion, translocation) or number (non-disjunction producing an abnormal count, such as trisomy 21).
Only mutations in gametes (germline mutations) are heritable; somatic mutations affect only the individual. Mutations, together with meiosis (crossing over, independent assortment) and sexual reproduction (random fertilisation), generate the genetic variation on which natural selection acts.
Natural selection and evolution
Charles Darwin's theory of evolution by natural selection rests on a logical sequence.
- Variation: individuals in a population vary in their inherited characteristics.
- Heritability: much of that variation is heritable (passed to offspring).
- Overproduction and competition: more offspring are produced than the environment can support, so individuals compete for limited resources.
- Differential survival and reproduction: individuals with variations better suited to the environment survive and reproduce more (have greater fitness).
- Change in allele frequency: favourable alleles become more common over generations.
This change in allele frequencies over time is evolution. Selection pressures include predation, competition, disease, climate and food availability. Antibiotic resistance in bacteria and pesticide resistance in insects are clear, fast examples.
Speciation is the formation of new species. In allopatric speciation a physical barrier divides a population; the isolated groups accumulate different mutations and experience different selection until they can no longer interbreed (reproductive isolation), forming separate species. Lines of evidence for evolution include the fossil record, comparative anatomy (homologous structures), comparative embryology, biogeography and molecular evidence (DNA and protein sequence similarity, which also allows molecular clocks and phylogenetic trees).
Check your knowledge
A mix of recall, genetics-calculation and exam-style application questions covering Unit 4 subject matter. Answer all under timed conditions (about 1 minute per mark), then check against the solutions block.
- Describe the structure of DNA, including the strands, the backbone, and the base-pairing rules. (4 marks)
- Explain why DNA replication is described as semiconservative, and name two enzymes involved. (3 marks)
- Outline the two stages of gene expression, identifying where each occurs and what is produced. (4 marks)
- In peas, round seed (R) is dominant to wrinkled (r). Cross two heterozygous plants (Rr x Rr). (a) Construct the Punnett square. (b) State the genotype and phenotype ratios. (c) State the law of segregation. (4 marks)
- A red-flowered snapdragon (incomplete dominance) is crossed with a white-flowered one, producing all pink offspring. Predict the offspring ratios when two of these pink plants are crossed, and explain why this is non-Mendelian. (3 marks)
- A farmer has a black bull (black B dominant to red b) of unknown genotype. (a) Describe a test cross to determine the genotype. (b) State the offspring outcome for each possible bull genotype. (c) Explain why a homozygous recessive partner is used. (4 marks)
- Distinguish between a gene mutation and a chromosomal mutation, giving one example of each, and explain why only some mutations are heritable. (4 marks)
- A population of beetles lives on tree bark and is preyed on by birds. A new pesticide-resistant green variant arises by mutation just as the area is sprayed. (a) Using the steps of natural selection, explain how the resistant variant could become common. (b) Explain why this is evolution. (c) Define allopatric speciation. (6 marks)