OCR GCSE Biology A: Genome, alleles, variants and mutations (J247) – Revision Notes

These notes condense statements B5.1a to B5.1e of OCR GCSE (9-1) Gateway Science Biology A (J247), specification Version 4.0 (August 2026), for first assessment in 2018. B5.1e is in bold in the specification and is tested only in the Higher Tier papers. For full explanations read the study guide, then test yourself with the practice questions.

Related pages: the OCR GCSE Biology hub, the printable checklist, DNA structure, protein synthesis and enzyme action and the free diagnostics.

Definitions to learn word for word (B5.1a)

Term	Definition
Gamete	A sex cell that fuses with another gamete at fertilisation
Chromosome	A long, coiled molecule of DNA in the nucleus carrying many genes
Gene	A section of DNA that codes for a protein
Allele (variant)	A different version of the same gene
Genotype	The alleles an organism has for a gene
Phenotype	The observable characteristic produced
Homozygous	Two identical alleles of a gene
Heterozygous	Two different alleles of a gene
Dominant	An allele whose characteristic shows with one copy present
Recessive	An allele whose characteristic shows only with two copies

Use capital and lower-case letters as a pair, such as T and t. Genotype is letters; phenotype is words.

Must-know distinctions

• Genotype or phenotype: Gg is a genotype; green is a phenotype.
• Homozygous or heterozygous: GG and gg are homozygous; Gg is heterozygous.
• Dominant does not mean common or stronger. It describes which characteristic shows when both alleles are present. Dominant alleles do not block recessive ones.
• Gene or allele: a gene is the instruction; an allele is one version of it.
• Allele or variant: variant is the wider word. It includes differences in non-coding DNA.
• Genome or gene: the genome is all of the genetic material; genes are only part of it.

Method: working out a phenotype from a genotype

1. Identify the dominant allele (the capital letter).
2. If at least one dominant allele is present, the dominant phenotype shows.
3. If both alleles are recessive, the recessive phenotype shows.
4. For a dominant phenotype, say the genotype could be homozygous or heterozygous. For a recessive phenotype, the genotype is certain.

The genome (B5.1b)

• The genome is all the genetic material held by an organism.
• Order of size: genome, then chromosome, then gene, then DNA base sequence.
• Includes non-coding DNA, which is most of the human genome.
• Human genome: nuclear DNA plus mitochondrial DNA. Bacterial genome: main chromosome plus plasmids.

Worked reminder. A genome has 1 000 000 000 base pairs, and 2% is coding. Coding DNA = 1 000 000 000 × 0.02 = 20 000 000 base pairs. The rest, 980 000 000, is non-coding.

Genome and environment (B5.1c)

Feature	Discontinuous	Continuous
Values	Separate categories	Any value in a range
Examples	Eye colour, ABO blood group	Height, weight
Cause	Mostly genes, one or a few	Many genes plus environment
Graph	Bar chart with gaps	Histogram or bell-shaped curve

• Phenotype = genome + environment working together.
• Identical genomes (clones, identical twins) that differ must differ because of the environment.
• Examples of environmental effects: nutrition on height, light on plant growth, soil acidity on hydrangea flower colour.

Method: reading variation data

1. Check the total frequency adds up.
2. Find the modal class: the tallest bar or largest frequency.
3. Describe the shape: bell-shaped with no gaps is continuous; separate bars is discontinuous.
4. Percentage in a class = class frequency ÷ total × 100.
5. For clones in different conditions, compare the means and attribute the difference to the environment. Mention the small sample if there is one.

Worked reminder. Ten cuttings from one plant: five in warm conditions have a mean height of 30 cm, five in cool conditions have a mean of 24 cm. Difference = 6 cm; 6 ÷ 30 × 100 = 20% lower in the cool conditions. Same genome, so the cause is the environment.

Mutations (B5.1d)

• A mutation is a change in the DNA base sequence.
• All variants first arose as mutations.
• Most have no effect on phenotype, some influence it, a very few determine it.
• Random events. More likely with ionising radiation, ultraviolet light and some chemicals.
• Not all are harmful.

Why a mutation may have no effect: it falls in non-coding DNA, or it changes a base without changing the amino acid the codon specifies.

How variants change phenotype (B5.1e, Higher Tier only)

Where	What changes	Result
Coding DNA	Order of amino acids	Different protein shape, so different activity
Coding DNA of an enzyme	Active site shape	Substrate fits less well, so slower or no reaction
Non-coding DNA near a gene	How the gene is expressed	Transcription to mRNA is switched on or off differently, so more or less protein, or at a different time

Chain to write in full: base change, then amino acid sequence, then protein folding, then shape (active site), then activity, then phenotype.

For non-coding variants: protein sequence is unchanged; the amount or timing of the protein changes. A variant can stop a protein attaching to DNA that normally stops transcription, so the gene stays on. You do not need the terms promoter or transcription factor.

Examples: phenylketonuria (enzyme with lower activity), lactase persistence (a variant in non-coding DNA near the gene keeps it active in adults).

Worked reminder. Normal mRNA codons AUG GAA CCU UAA code Met-Glu-Pro then stop. If GAA changes to GAG, Glu is still coded and the protein is unchanged. If GAA changes to UAA, a stop codon appears and the protein is only Met.

Method: answering a data question on a variant enzyme (Higher Tier only)

1. Compare the two sets of figures at the same conditions, using numbers.
2. Work out the variant’s activity as a percentage of the normal enzyme’s: variant ÷ normal × 100.
3. Name the cause as a different amino acid sequence, giving a different protein shape.
4. Link the shape to the active site and say what happens to substrate binding.
5. State the effect on the reaction rate and, if asked, on the phenotype.

Worked reminder. Normal enzyme makes product at 3.0 units per minute; the variant makes 0.6 units per minute. 0.6 ÷ 3.0 × 100 = 20%. The variant works at one fifth of the normal rate, which fits an altered active site that binds the substrate poorly.

Three levels of effect at a glance

Level	Meaning	Example idea
No effect	Phenotype is the same	Change in non-coding DNA, or a codon that still gives the same amino acid
Influences phenotype	Phenotype is altered a little	One amino acid swapped, enzyme works more slowly
Determines phenotype	The characteristic is set by the variant	Enzyme made so short that it cannot work

Quick self-test

1. Define a gene.
2. A plant has genotype Rr and red petals, with R dominant. State whether its genotype is homozygous or heterozygous, and give the genotype of a plant with white (recessive) petals.
3. What is the genome?
4. Is blood group A, B, AB or O discontinuous or continuous variation?
5. Two clones differ in mass because one received more water. What does this show?
6. In a set of 40 seedlings, 14 are in the modal class. What percentage is that?
7. State what a mutation is.
8. Fill in: most mutations have ___ effect on phenotype, some ___ it and a very few ___ it.
9. (Higher Tier only) How can a mutation in the gene for an enzyme stop the enzyme working?
10. (Higher Tier only) A variant lies in non-coding DNA beside a gene, and the protein’s amino acid sequence is normal. How can it still change the phenotype?

Answers

1. A section of DNA that codes for a protein.
2. Heterozygous; rr (homozygous recessive).
3. The entire genetic material of an organism.
4. Discontinuous: it falls into separate categories.
5. The environment (water) influences phenotype, because the genomes are identical.
6. 14 ÷ 40 × 100 = 35%.
7. A change in the base sequence of DNA.
8. No; influence; determine.
9. The base change alters the amino acid sequence, so the protein folds into a different shape. If the active site changes, the substrate no longer fits, so the reaction slows or stops.
10. It alters how the gene is expressed, such as whether transcription to mRNA happens. More or less protein, or protein at a different time, changes the phenotype.

Where marks are usually lost

• Giving a loose definition of gene, such as “a piece of chromosome” or “what you inherit”, rather than a section of DNA coding for a protein.
• Using genotype and phenotype the wrong way round.
• Claiming a dominant phenotype tells you the genotype exactly.
• Writing that the dominant allele “takes over” or “kills” the recessive one.
• Saying the genome is “the genes”.
• Describing a graph without naming the shape, the modal class or the gaps.
• Calculating a percentage difference using the wrong starting value.
• Writing that all mutations are harmful, or that all of them change the phenotype.
• Writing “the mutation damages the enzyme” without the chain from amino acid sequence to shape to active site.
• Mixing up coding and non-coding effects: non-coding variants change expression, not the amino acid sequence.

Official syllabus

OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), for first assessment in 2018, published by OCR: module B5.1, statements B5.1a to B5.1e.