Inheritance and DNA: KS3 Biology Guide to Genes, Chromosomes and Variation

Inheritance is the passing of genetic information from parents to offspring. That information is stored in DNA — a long molecule found in the nucleus of almost every cell — organised into genes on chromosomes. Understanding how traits are passed on, and why offspring are similar to but not identical to their parents, is core KS3 biology, typically taught in Year 9.

What is DNA?

DNA (deoxyribonucleic acid) is the chemical that carries the genetic instructions for building, running, and reproducing all living organisms. It is shaped as a double helix — two strands twisted around each other like a twisted ladder.

DNA is made of units called nucleotides, each containing one of four bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these bases forms a code. Specific sequences of bases make up genes — the instructions for making proteins, which in turn carry out all the functions in a cell.

Chromosomes and genes

Term | Definition
Chromosome | A long thread of coiled DNA found in the cell nucleus. Human body cells contain 46 chromosomes in 23 pairs.
Gene | A section of DNA on a chromosome that codes for a particular protein, and thereby determines a specific characteristic.
Allele | A version of a gene. Most genes have at least two alleles — e.g. a gene for eye colour may have a "brown" allele and a "blue" allele.
Genome | The complete set of genetic information in an organism — all its chromosomes.

How many chromosomes do humans have?

Human body cells (somatic cells) contain 46 chromosomes arranged in 23 pairs. One chromosome from each pair came from the mother (via the egg) and one from the father (via the sperm). Sex cells (eggs and sperm) contain only 23 chromosomes — one from each pair — so that when they fuse at fertilisation, the resulting cell has 46.

The 23rd pair determines biological sex: females typically have two X chromosomes (XX), males typically have one X and one Y chromosome (XY).

Dominant and recessive alleles

Because you have two copies of each chromosome (one from each parent), you also have two alleles of every gene.

A dominant allele is expressed (shows in the phenotype) whenever it is present, even if only one copy is inherited.

A recessive allele is only expressed when two copies are present — when no dominant allele is there to mask it.

Notation

Geneticists use letter notation:

A capital letter (e.g. B) represents the dominant allele.

A lower-case letter (e.g. b) represents the recessive allele.

Genotype | Name | Phenotype (for B dominant over b)
BB | Homozygous dominant | Shows dominant trait
Bb | Heterozygous | Shows dominant trait (B masks b)
bb | Homozygous recessive | Shows recessive trait

Worked example — tongue rolling

The ability to roll the tongue is controlled by a single gene. The tongue-rolling allele (R) is dominant over the non-rolling allele (r).

A person with genotype RR or Rr can roll their tongue.

A person with genotype rr cannot roll their tongue.

If two heterozygous parents (both Rr) have children, the possible genotypes of their offspring can be worked out using a Punnett square:

| R | r
R | RR | Rr
r | Rr | rr

Offspring genotypes: RR (25%), Rr (50%), rr (25%).

Offspring phenotypes: 75% can roll tongue (RR + Rr), 25% cannot (rr).

Ratio: 3 tongue-rollers : 1 non-tongue-roller

Variation: inherited and environmental

Variation is the differences between individuals of the same species. It can be:

Inherited variation

Caused by differences in genes. Features determined mainly by genes include:

Blood group (A, B, AB, O)

Eye colour

Skin colour (partly)

Genetic conditions such as cystic fibrosis

Environmental variation

Caused by the conditions in which an organism lives. Features influenced mainly by the environment include:

Body mass (diet and exercise)

Language spoken

Scars

Continuous vs discontinuous variation

Type | Description | Examples
Continuous | A range of values with no distinct categories; usually polygenic (controlled by many genes). | Height, body mass, skin colour
Discontinuous | Distinct categories with no intermediate forms; usually controlled by one gene. | Blood group, tongue rolling, attached/free earlobes

A histogram shows continuous variation (a smooth distribution); a bar chart shows discontinuous variation (separate categories).

How does inheritance work?

During sexual reproduction, each parent contributes one allele of each gene to the offspring — the allele carried in their egg or sperm. The offspring's genetic combination is therefore unique (unless the parents are identical twins).

The process:

Meiosis — a type of cell division that produces sex cells (gametes), each with half the normal chromosome number (23 in humans).

Fertilisation — egg and sperm fuse to form a zygote with 46 chromosomes (23 pairs, one from each parent).

Mitosis — the zygote divides repeatedly to form all the cells of the new organism.

The random combination of alleles from two parents produces variation within the offspring.

How this fits the KS3 national curriculum

The Department for Education's KS3 science programme of study requires pupils to understand "the variation between species and between individuals of the same species means some organisms compete more successfully," and to learn "the hereditary information stored in DNA" and "why individuals are different from each other." BBC Bitesize KS3 Biology covers DNA, chromosomes, genes, and inheritance through Punnett squares as part of the Year 9 genetics topic.

Common mistakes

Mistake 1 — Confusing "dominant" with "common." A dominant allele is not necessarily the most common in a population. It simply means it is expressed over the recessive allele.

Mistake 2 — Thinking an organism with one dominant allele (Bb) shows a "blended" trait. Dominance is not blending. A heterozygous person with one brown-eye allele and one blue-eye allele has brown eyes — not a mix. The dominant allele fully masks the recessive one.

Mistake 3 — Forgetting that sex cells have 23 chromosomes (not 46). Body cells have 46; sex cells (gametes) have 23. This halving, achieved by meiosis, ensures fertilisation restores the normal count.

Frequently asked questions

What is the difference between a gene and an allele?

A gene is a section of DNA that codes for a specific characteristic — for example, the gene that determines tongue rolling. An allele is a particular version of that gene. The tongue-rolling gene exists in two alleles: the rolling allele (R) and the non-rolling allele (r). Every person has two alleles of each gene — one inherited from each parent. Different alleles of the same gene produce different versions of the characteristic.

Why do children resemble their parents but are not identical to them?

Each parent contributes one allele of each gene via their sex cells. Because the allele contributed is chosen at random during meiosis, no two non-identical siblings receive the same combination. Furthermore, environmental factors during development shape the organism further. The result is that children share approximately 50 % of their DNA with each parent, giving family resemblance, but the unique combination means they are not identical.

What is the difference between genotype and phenotype?

Genotype is the genetic makeup of an organism — the specific alleles it carries (e.g. Bb). Phenotype is the observable characteristic — what you can see, measure, or detect (e.g. "can roll tongue"). The same phenotype can arise from different genotypes: both BB and Bb produce the tongue-rolling phenotype, even though their genotypes differ.

What is the difference between continuous and discontinuous variation?

Continuous variation shows a smooth range of values with no natural break points — human height, for example, ranges continuously from about 140 cm to over 200 cm in adults. Discontinuous variation falls into distinct categories with no in-between values — blood groups are either A, B, AB, or O, with no intermediate groups. Continuous variation is usually controlled by many genes (polygenic) plus the environment; discontinuous variation is usually controlled by one or a few genes.

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