Genes, Inheritance and Selection: Synthesis and Exam Skills

You have now worked through the whole of Topic B5 of your OCR Gateway Combined Science course — reproduction and meiosis, DNA and the genome, monohybrid inheritance and sex determination, inherited disorders and family trees, variation and mutation, evolution by natural selection and its evidence, and selective breeding and genetic engineering. This final lesson pulls it all together. It shows how the topic forms a single through-line from DNA to evolution, drills the Punnett-square technique and the ratio and probability calculations that earn reliable marks, recaps the key vocabulary, and warns you about the misconceptions that catch students out. Treat it as a revision and exam-technique session rather than new content.

By the end of this lesson you should be able to trace the connections across B5, perform every B5 genetic calculation confidently, use the key terms precisely, and avoid the most common B5 errors.

This lesson draws together all three Assessment Objectives: AO1 (secure recall of B5 vocabulary), AO2 (applying genetic-cross calculations under exam conditions) and AO3 (interpreting data and evaluating ethical arguments in extended answers).

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How the B5 Topics Fit Together

The real strength of B5 is that it is not a list of separate facts but one connected story, running from the molecule of DNA all the way up to the evolution of species.

flowchart TD
  A["DNA / genes<br/>(code for proteins)"] --> B["Mutation<br/>(random change → new alleles)"]
  B --> C["Variation<br/>(differences within a species)"]
  C --> D["Inheritance<br/>(alleles passed to offspring)"]
  C --> E["Natural selection<br/>(best-suited survive and reproduce)"]
  E --> F["Evolution<br/>(species change over generations)"]
  D -.->|"same mechanism<br/>of inheritance"| E

Read the chain as a story: DNA carries genes that code for proteins; a mutation is a random change to the DNA that creates a new allele; new alleles are the source of variation within a species; this variation is inherited by offspring through the alleles in gametes; and when some variants are better suited to the environment, natural selection means they survive and reproduce more, so over many generations the species evolves. The same idea — alleles passed from parents to offspring — underlies both the inheritance you predict with Punnett squares and the selection that drives evolution. Being able to tell this story, moving between the molecular scale and the whole-population scale, is exactly what lifts an answer into the top band.

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The Through-Line in One Table

Stage	Key idea	Earlier lesson
DNA / genes	A gene is a section of DNA that codes for a protein	DNA, genes and the genome
Mutation	A random change to DNA produces a new allele	Variation and mutation
Variation	Differences within a species (genetic, environmental or both)	Variation and mutation
Inheritance	Alleles pass from parents to offspring; predicted with Punnett squares	Monohybrid inheritance; inherited disorders
Natural selection	The best-suited survive, reproduce and pass on their alleles	Evolution by natural selection
Evolution	Over many generations the species changes; evidence from fossils	Evolution by natural selection

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Mastering the B5 Calculations

The calculations in B5 are all built on the Punnett square and on turning its results into ratios, fractions, percentages and probabilities. Here is the technique again, with fresh worked examples so you can check your method.

1. The Punnett-square method (recap)

1. Write the genotypes of both parents.
2. Work out the gametes each can make (one allele each).
3. Draw the 2 × 2 grid; put one parent's gametes on top, the other's down the side.
4. Fill each box by combining its row and column allele.
5. Read off the genotypes and phenotypes and write the ratio.

2. A heterozygous cross — the 3 : 1 ratio

Worked example: Two pea plants heterozygous for height (Tt) are crossed. Tall (T) is dominant to short (t). Predict the offspring ratio.

	T	t
T	TT	Tt
t	Tt	tt

The boxes are TT, Tt, Tt, tt. Three have at least one T (tall); one is tt (short).

Answer: the expected phenotype ratio is 3 tall : 1 short — that is $\frac{3}{4}$43​ ($75\%$75%) tall and $\frac{1}{4}$41​ ($25\%$25%) short.

3. A test cross — the 1 : 1 ratio

Worked example: A heterozygous tall pea plant (Tt) is crossed with a short plant (tt). Predict the offspring ratio.

	T	t
t	Tt	tt
t	Tt	tt

The boxes are Tt, tt, Tt, tt — two tall (Tt) and two short (tt).

Answer: the expected ratio is 1 tall : 1 short ($50\%$50% each).

4. From ratio to number of offspring

Worked example: In the 3 : 1 cross above, if the plant produces 80 seeds, how many would you expect to grow into short plants?

Short is $\frac{1}{4}$41​ of the offspring, so:

$\frac{1}{4} \times 80 = 20$41​×80=20

Answer: about 20 short plants. (Say "about" or "expect" — it is a prediction, not a guarantee.)

Exam Tip: A Punnett square gives a probability, not a certainty. Convert a ratio to a percentage through the fraction (3 : 1 → $\frac{3}{4}$43​ and $\frac{1}{4}$41​ → $75\%$75% and $25\%$25%), and to a number of offspring by multiplying the fraction by the total. A common misconception is that a 3 : 1 ratio guarantees exactly 3 of every 4 offspring; it does not — the numbers only get close to 3 : 1 over many offspring.

5. Working backwards and reading family trees

Two further skills appear regularly and are worth rehearsing together, because they use the same logic. The first is deducing parents' genotypes from their offspring: a recessive offspring (e.g. bb or ff) must have received a recessive allele from each parent, so both parents must carry that recessive allele. If the parents themselves show the dominant characteristic, they must each be heterozygous. The second is reading a family (pedigree) tree: if two unaffected parents have an affected child, the allele is recessive and both parents are carriers; for a recessive disorder, every affected person is homozygous recessive. Spotting that one relationship usually unlocks the rest of a pedigree question, and lets you draw a Punnett square to predict the chance the next child is affected.

Exam Tip: The single most powerful deduction across these questions is that a recessive offspring reveals a hidden recessive allele in each parent. Whether you are working out parental genotypes or analysing a family tree, look first for an individual showing the recessive characteristic — it tells you the most.

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Reading an Evolutionary Tree

Alongside genetic crosses and family trees, B5 sometimes asks you to interpret an evolutionary (phylogenetic) tree — a diagram showing how species are thought to be related through evolution. The points where branches split represent common ancestors: the more recently two species share a branching point, the more closely related they are.

flowchart TD
  A["Common ancestor"] --> B["Ancestor of species 1 and 2"]
  A --> C["Species 3"]
  B --> D["Species 1"]
  B --> E["Species 2"]

In a tree like this, species 1 and 2 are more closely related to each other (they share a more recent common ancestor) than either is to species 3, which branched off earlier. Scientists build these trees using evidence from fossils, from the structures of living organisms, and increasingly from comparing their DNA — the same kind of evidence that led to modern revisions of classification.

Exam Tip: On an evolutionary tree, "closely related" means sharing a recent common ancestor — a recent branching point. A common misconception is to judge relatedness by how close two species appear on the page; instead, follow the branches back to where they last met. This mirrors the classification idea that species in the same small group (like a genus) are closely related.

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The B5 Vocabulary to Know Cold

Use this as a final recall list. Cover the right-hand column and test yourself.