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

This guide is written for OCR GCSE (9-1) Gateway Science Biology A (J247), using the specification Version 4.0 (August 2026), for first assessment in 2018. It covers the first five statements of module B5.1, “Inheritance”: B5.1a to B5.1e. Statement B5.1e is in bold in the specification and is tested only in the Higher Tier papers, so it is labelled “Higher Tier only” below. Everything else is for both tiers.

Use it with the revision notes and the practice questions. For DNA structure and protein synthesis see DNA structure, protein synthesis and enzyme action. The OCR GCSE Biology hub, the printable checklist and the free diagnostics help you plan what to revise. The other statements of B5.1 (reproduction, meiosis, crosses, sex determination and the history of genetics) are not covered here.

What this unit covers

Statement	What you must be able to do	Tier
B5.1a	Explain gamete, chromosome, gene, allele/variant, dominant, recessive, homozygous, heterozygous, genotype, phenotype	Both
B5.1b	Describe the genome as the entire genetic material of an organism	Both
B5.1c	Describe how the genome and its interaction with the environment influence phenotype; use examples of discontinuous and continuous variation	Both
B5.1d	Recall that every variant starts as a mutation, and that most mutations leave the phenotype unchanged, some influence it and very few determine it	Both
B5.1e	Describe how variants may influence phenotype in coding DNA (altering protein activity, including enzyme active sites) and in non-coding DNA (altering how genes are expressed, including stopping transcription of mRNA)	Higher Tier only

The maths outcomes for this module include reading charts, graphs and tables, and using simple ratios. Both appear in the worked examples.

The key terms (B5.1a)

Learn each term as a precise definition. Vague answers such as “genes are what you inherit” gain nothing.

• Gamete: a sex cell (sperm or egg cell in humans) that fuses with another gamete at fertilisation.
• Chromosome: a long, coiled molecule of DNA in the nucleus, carrying many genes. Human body cells have 23 pairs.
• Gene: a section of DNA that codes for a protein.
• Allele (variant): one of the different versions of the same gene.
• Genotype: the alleles an organism has for a gene, written as letters such as Gg.
• Phenotype: the observable characteristic that results, such as shell colour.
• Homozygous: two identical alleles of a gene (GG or gg).
• Heterozygous: two different alleles of a gene (Gg).
• Dominant: an allele whose characteristic shows when only one copy is present.
• Recessive: an allele whose characteristic shows only when two copies are present and no dominant allele is.

The specification warns about two ideas that are wrong. A dominant allele does not “dominate” or switch off the recessive allele. Dominance is a pattern seen in the phenotype, not a different way of being inherited. A recessive allele is also not a missing allele: it is a version of the gene that is present.

The words allele and variant overlap. An allele is a version of a gene. “Variant” is wider, because a difference in DNA sequence can sit inside a gene or in non-coding DNA between genes. In this guide, allele is used for gene versions and variant for any DNA difference.

Dominant does not mean common. Huntington’s disease is caused by a dominant allele, yet it is rare.

Worked example 1: genotype to phenotype

In an invented beetle species, shell colour is controlled by one gene. The allele G (green) is dominant to g (yellow).

Genotype	Description	Phenotype
GG	homozygous dominant	green
Gg	heterozygous	green
gg	homozygous recessive	yellow

A green beetle could be GG or Gg, because one copy of G is enough to give green. You cannot tell which without more information. A yellow beetle must be gg: if it had even one G it would be green. That is why a recessive characteristic tells you the genotype exactly, and a dominant one does not.

The genome (B5.1b)

The genome is all of the genetic material in an organism. It is all of the DNA, not just the genes. In a human it includes the nuclear chromosomes and the small amount of DNA in the mitochondria. In a bacterium it includes the main chromosome and any plasmids.

Keep the levels in order. The genome contains the chromosomes, and a chromosome contains many genes. Genes make up only part of the genome; much of the rest does not code for protein. That non-coding DNA matters again in B5.1e.

Worked example 2: how much of a genome codes for protein?

Data supplied: a genome has 2 400 000 000 base pairs, and 1.2% of it is coding DNA.

1. Coding DNA = 2 400 000 000 × 0.012 = 28 800 000 base pairs.
2. Non-coding DNA = 2 400 000 000 − 28 800 000 = 2 371 200 000 base pairs.

So 98.8% of this genome does not code for protein. Writing “the genome is only genes” would be wrong.

Genome plus environment gives the phenotype (B5.1c)

An organism’s phenotype is shaped by two things: its genome and the environment it develops in. Neither works alone.

• Genome only: ABO blood group is set by genes and does not change with lifestyle.
• Environment only: a scar or a sun tan is not in the genome.
• Both: the height of a person depends on inherited genes and on nutrition and health in childhood. The flower colour of a hydrangea depends on its genes and on the acidity of the soil.

Clones make a clear test. Organisms with identical genomes show only the effect of environment.

Worked example 3: separating genes from environment

Six cuttings were taken from one parent plant, so all six have the same genome. Three were grown in full light and three in shade. Heights after six weeks (cm):

• Full light: 24, 26, 25
• Shade: 15, 17, 16

1. Mean in light = (24 + 26 + 25) ÷ 3 = 75 ÷ 3 = 25 cm.
2. Mean in shade = (15 + 17 + 16) ÷ 3 = 48 ÷ 3 = 16 cm.
3. Difference = 25 − 16 = 9 cm.
4. Percentage lower in shade = 9 ÷ 25 × 100 = 36%.

The genomes are identical, so the 9 cm difference is caused by the environment (light). Three cuttings per group is a small sample, so the conclusion is supported but not proved.

Discontinuous and continuous variation

	Discontinuous	Continuous
Values	Fall into separate categories	Any value in a range
Examples	Eye colour, ABO blood group	Height, mass
Usual influence	One or a few genes, little effect from environment	Many genes and the environment together
Graph	Bar chart, bars apart	Histogram or smooth bell-shaped curve

Eye colour is the specification’s example of discontinuous variation. Height and weight are its examples of continuous variation.

Worked example 4: reading a table

Lengths of 50 leaves from one bush (cm):

Length class (cm)	8-9	10-11	12-13	14-15	16-17	18-19
Number of leaves	2	7	17	15	7	2

1. The total is 2 + 7 + 17 + 15 + 7 + 2 = 50, which matches.
2. The modal class is 12-13 cm, with 17 leaves.
3. Percentage in the modal class = 17 ÷ 50 × 100 = 34%.
4. Most values lie near the middle, a few are at the extremes and there are no gaps. This is the bell shape of continuous variation.

A bar chart with gaps would be the wrong graph for these continuous data.

Mutations and variants (B5.1d)

A mutation is a change in the base sequence of DNA. Every variant that exists first arose as a mutation. Mutations happen at random, and the rate rises with ionising radiation, ultraviolet light and some chemicals.

You must recall three levels of effect:

1. Most mutations have no effect on the phenotype.
2. Some mutations influence the phenotype.
3. A very few mutations determine the phenotype.

There are reasons why so many do nothing. Many mutations fall in non-coding DNA. Others change a base in a gene but leave the protein the same, as you will see in B5.1e.

Worked example 5: sorting effects from data

Data supplied (invented for practice): 200 new variants were studied. 170 had no effect on the phenotype, 26 influenced it and 4 determined it.

• No effect: 170 ÷ 200 × 100 = 85%.
• Influenced: 26 ÷ 200 × 100 = 13%.
• Determined: 4 ÷ 200 × 100 = 2%.

The percentages total 100%, which supports “most have no effect”. Mutations are not all harmful.

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

A variant can act in two places.

Variants in coding DNA

Coding DNA holds the instructions for a protein. A change in the base sequence can change the order of amino acids. The polypeptide then folds differently, so the protein has a different shape and a different activity.

• For an enzyme, a change in shape may alter the active site. The substrate then fits less well or not at all, and the reaction slows or stops.
• The same logic applies to other proteins, such as transport proteins or receptors. A changed shape changes how well they work.

An example is phenylketonuria. Variants in the gene for the enzyme that breaks down the amino acid phenylalanine give an enzyme with much lower activity, so phenylalanine builds up.

Worked example 6: from base change to protein

Data supplied: mRNA codons AUG = Met, UUU = Phe, GAA = Glu, GAG = Glu, GUA = Val, AAG = Lys, GGU = Gly, UAA = stop.

Normal mRNA: AUG UUU GAA AAG GGU UAA. This gives Met-Phe-Glu-Lys-Gly, five amino acids.

• Variant A changes the third codon to GAG. GAG also codes for Glu, so the protein is unchanged. The mutation is real but has no effect on the phenotype.
• Variant B changes the third codon to GUA. Glu becomes Val, so one amino acid changes. The shape may change slightly. If this is near the active site, activity may fall.
• Variant C changes the third codon to UAA. This is a stop codon, so the protein ends after Met-Phe and is very short. It is unlikely to work.

Variants A to C show all three levels in B5.1d: none, some, and determining.

Variants in non-coding DNA

Not all DNA codes for protein. Some non-coding DNA sits near a gene and helps control gene expression, which means whether the gene is transcribed into mRNA and so whether its protein is made. A protein can attach to this DNA and, for some genes, stop transcription. A variant in the non-coding DNA may stop that attachment, so a gene that should be switched off stays on. Another variant may do the opposite. The protein’s amino acid sequence is unchanged; the amount produced, or when it is produced, changes.

A real example is lactase, the enzyme that digests lactose. In most mammals the gene is switched off after weaning. In some people a variant in non-coding DNA near the gene keeps it active into adulthood, so they can still digest milk sugar. The lactase protein itself is normal. You do not need the terms promoter or transcription factor.

Common errors

• Writing “a gene is a piece of chromosome” instead of “a section of DNA that codes for a protein”.
• Saying the dominant allele “overpowers” or “blocks” the recessive one.
• Saying a green beetle must be GG. It could be Gg.
• Treating genome as another word for genes.
• Saying continuous variation is caused by the environment alone, or discontinuous by genes alone with no qualification.
• Saying all mutations are harmful.
• Answering “the enzyme is damaged” without linking the base change to amino acid order, shape and active site.
• Confusing a non-coding variant with a coding one: the protein sequence is unchanged, but the expression of the gene is altered.

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.