Edexcel GCSE Combined Science CB3 Genetics: a complete overview of DNA, inheritance, genetic crosses and variation

What CB3 actually demands

Genetics explains how characteristics are passed from parents to offspring and why individuals differ. The examiners reward precise use of the genetic vocabulary (gene, allele, dominant, recessive, genotype, phenotype), confident Punnett-square work, and a clear understanding of where variation comes from.

This guide walks through the two halves of the topic and ties together the matching dot-point pages, each with its own practice questions.

DNA, genes and reproduction

DNA is the molecule that carries the genetic code, coiled into a double helix. A gene is a section of DNA coding for a protein; a chromosome is a whole DNA molecule (humans have 23 pairs); the genome is all of an organism's genetic material.

Sexual reproduction uses two parents and gametes made by meiosis, producing genetically varied offspring. Asexual reproduction uses one parent and mitosis, producing identical clones. Meiosis makes four genetically different, haploid gametes; fertilisation restores the diploid number.

Genetic crosses

A dominant allele (capital letter) is shown with one copy; a recessive allele (lower case) needs two. A Punnett square predicts the offspring of a cross:

• $Aa \times Aa$ gives a 3:1 ratio of dominant to recessive phenotypes.
• $Aa \times aa$ gives a 1:1 ratio.
• $AA \times aa$ gives all heterozygous ($Aa$) offspring.

A predicted ratio is a probability: the chance of a recessive offspring from $Aa \times Aa$ is $\frac{1}{4}$, but real small samples vary.

Variation and mutation

Variation has genetic causes (different alleles), environmental causes (surroundings), and combinations of both. Genetic variation comes from sexual reproduction and from mutations, which are random changes to the DNA base sequence. Mutations create new alleles and are the source of all genetic variation; most are neutral, some harmful, a few beneficial.

How CB3 is examined

• Vocabulary. Defining and using gene, allele, dominant, recessive, genotype, phenotype, homozygous and heterozygous.
• Crosses. Completing Punnett squares and stating ratios or probabilities.
• Reasoning. Explaining why meiosis halves the chromosome number and why fertilisation restores it.
• Variation. Distinguishing genetic from environmental causes and describing mutations.

Check your knowledge

A mix of recall and genetic-cross questions covering CB3. Attempt them under timed conditions, then check against the solutions.

1. Define an allele. (1 mark)
2. State the number of parents in asexual reproduction. (1 mark)
3. How many gametes does meiosis produce from one cell? (1 mark)
4. A cross of $Bb \times Bb$ gives what ratio of dominant to recessive phenotypes? (1 mark)
5. State the probability of a recessive offspring from $Aa \times Aa$. (1 mark)
6. Give one example of environmental variation. (1 mark)
7. Define a mutation. (2 marks)
8. State whether a $Tt$ tall plant is homozygous or heterozygous. (1 mark)