Key Stage 4 science · Biology – Evolution, inheritance and variation

GCSE genetics: the genome and inheritance for parents

GCSE genetics explains how instructions in DNA are passed from parents to children and how those instructions, together with the environment, produce the features we can see. Students learn what the genome is, how to predict the outcome of a single-gene cross, how sex is determined and why most human features depend on many genes, and they consider how reading whole genomes is starting to change medicine.

The genome and inheritance, explained

The genome is all of the genetic material in an organism. In humans that is the DNA in the nucleus of a cell, arranged into chromosomes, plus a small amount in the mitochondria. A gene is a short section of DNA that codes for a particular protein. Different versions of the same gene are called alleles. Students are expected to move between these levels: a chromosome contains many genes, a gene has different alleles, and the alleles you carry for a gene are your genotype.

The phenotype is the feature that actually shows, such as eye colour, blood group or height. It depends on the genotype and also on the environment. Two plants with identical genes can end up different heights if one has more light and water, and identical twins can differ in weight or skin tone because of their lives. Most features, including height and skin colour, are controlled by many genes working together, which is why children look like a blend of family members rather than a copy of one parent. Only a minority of features, such as some inherited disorders, come down to a single gene.

For a single-gene feature, each parent passes on one allele in each gamete (egg or sperm), so a child receives two. A dominant allele shows in the phenotype even if there is only one copy. A recessive allele only shows when two copies are present. People with two identical alleles are homozygous, and people with two different alleles are heterozygous. Students draw genetic diagrams or Punnett squares to show the possible gametes and the possible children, and they express the outcomes as ratios, fractions or percentages. These show probabilities, not guarantees.

Sex is determined by one pair of chromosomes. Females have two X chromosomes and males have an X and a Y. Every egg carries an X, while half of the sperm carry an X and half carry a Y, so there is a one in two chance of each sex at every pregnancy. Finally, students consider genomics, the study of whole genomes. Sequencing can help doctors identify the genes behind an inherited disease, predict who is at greater risk, choose medicines that suit a patient’s genes and track how infections spread. It also raises questions about privacy, insurance and what people would want to know, and students are expected to discuss both sides.

Words your child will hear

Genome: All the genetic material of an organism, the complete set of its DNA.
Allele: One version of a gene. Brown and blue eye colour come from different alleles, for example.
Genotype: The combination of alleles an organism has for a gene, such as Ff.
Phenotype: The characteristic that can be seen or measured, which results from the genotype and the environment.
Dominant: An allele that shows in the phenotype even when only one copy is present.
Recessive: An allele that only shows in the phenotype when two copies are present.
Heterozygous: Having two different alleles of a gene. Homozygous means having two the same.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about the genome as the entire genetic material of an organism
Students should be taught about how the genome, and its interaction with the environment, influence the development of the phenotype of an organism
Students should be taught about the potential impact of genomics on medicine
Students should be taught that most phenotypic features are the result of multiple, rather than single, genes
Students should be taught about single gene inheritance and single gene crosses with dominant and recessive phenotypes
Students should be taught about sex determination in humans

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Cystic fibrosis is caused by a recessive allele, f. The dominant allele is F. Two parents are both carriers, meaning each has the genotype Ff and does not have the disorder. What is the chance that their child will have cystic fibrosis?

List the possible gametes. Each parent is Ff, so each can pass on either F or f, with an equal chance of each.
Draw a Punnett square with one parent’s gametes along the top and the other’s down the side, then fill in each box by combining the letters.
```
       F     f
  F   FF    Ff
  f   Ff    ff
```
Read off the genotypes: one FF, two Ff and one ff. That is a 1 : 2 : 1 ratio.
Work out the phenotypes. FF and Ff both have at least one dominant allele, so these children do not have the disorder (two of them are carriers). Only ff has two recessive alleles, so only that child has cystic fibrosis.
Turn it into a probability: one box in four is ff. This applies to every child the couple has, because each pregnancy is a separate chance.

There is a 1 in 4 (25%) chance that each child will have cystic fibrosis, and a 3 in 4 (75%) chance that each child will not.

Common mistakes and how to help

Thinking dominant means stronger, more common or better

In everyday English “dominant” sounds powerful, so students assume dominant alleles are widespread. Dominance only describes whether an allele shows when paired with a different one.

What helps: Remind your teenager that some dominant conditions, such as Huntington’s disease, are rare, while a harmful recessive allele like the one in the example can be carried by many healthy people without anyone knowing.

Believing that a 1 in 4 chance means one in every four children will be affected

Ratios from Punnett squares feel like a guaranteed pattern. In reality each child is an independent chance, like a coin toss, so a family could have four unaffected children or two affected ones.

What helps: Use a coin: flip two coins and ask what mixture of heads and tails is likely. Flip repeatedly and notice that the average evens out over many flips but not in any one family.

Mixing up gene, allele and chromosome

The words are often used loosely in the news, as in “the gene for blue eyes”. At GCSE the exam answer must separate a gene (the instruction), an allele (a version of it) and a chromosome (a structure carrying many genes).

What helps: Try a book analogy: the genome is the library, chromosomes are the volumes, genes are the chapters and alleles are different editions of a chapter. Ask your child to improve the analogy.

Thinking the mother’s egg can decide the baby’s sex, or that features depend only on genes

Sex determination is often explained in one sentence, so details are lost: every egg carries an X, so it is whether the sperm carries an X or a Y that decides. Equally, “gene for” headlines make phenotypes sound fixed, ignoring the environment and the many genes involved in most features.

What helps: Ask your child to draw the XX × XY cross and explain who gives the X or Y, and why that makes the chances even. Then name a feature, such as height, and list genes and environmental factors that affect it.

How to help at home

Draw your own family traits. Choose a simple trait, such as attached or free earlobes, and record who has it in your family. Ask whether the pattern fits a single gene, and notice how hard it is to tell. Many everyday traits are more complicated than they look in textbooks.
Practise crosses with coins. Label two coins with an allele each side, such as F on heads and f on tails. Flip both for many pretend children and tally the genotypes. The results will be close to the 1 : 2 : 1 ratio but never exactly the same every time.
Talk through the ethics. Ask whether people should be able to find out their risk of a disease before symptoms appear, and who should be allowed to see that information. There are no right answers, and thinking about both sides helps with extended writing.
Explain it back. Ask your child to teach you how to work out a cross, using a made-up plant with tall and short alleles. If they can explain each step in plain English, they understand it far better than if they only follow a method.

Builds on

Key Stage 3: DNA, genes and variation
Key Stage 3: Natural selection and biodiversity