Reproduction — AQA GCSE Combined Science: Trilogy
Sexual and asexual reproduction, meiosis, DNA and the genome, genetic crosses, inherited disorders and sex determination.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Sexual reproduction is the joining (fusion) of male and female gametes, such as sperm and egg cells in animals. It mixes genetic information, so the offspring show variation. Asexual reproduction involves only one parent, with no fusion of gametes and no mixing of genetic information. Only mitosis is involved, so the offspring are genetically identical to the parent (clones).
Meiosis is the type of cell division in the reproductive organs that forms gametes. The cell copies its genetic information, then divides twice to form four gametes, each with a single set of chromosomes, and all genetically different. Human gametes have 23 chromosomes instead of 46. Fertilisation restores the full number, and the new cell then divides by mitosis.
DNA is the genetic material in the nucleus. It is a polymer made of two strands forming a double helix, and it is contained in chromosomes. A gene is a small section of DNA that codes for a particular sequence of amino acids, to make a specific protein. The genome is the entire genetic material of an organism.
Genetic inheritance is how alleles, the different forms of a gene, are passed from parents to offspring. A dominant allele is always expressed, even if only one copy is present, but a recessive allele is only expressed if two copies are present. The genotype is the alleles present and the phenotype is how the characteristic is expressed.
Inherited disorders are caused by the inheritance of certain alleles. Polydactyly, having extra fingers or toes, is caused by a dominant allele, so one copy is enough to have it. Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so two carrier parents have a 1 in 4 chance of a child with the disorder.
Sex in humans is determined by one of the 23 pairs of chromosomes, the sex chromosomes. Females are XX and males are XY. All eggs contain an X chromosome, while half the sperm contain an X and half contain a Y, so each child has a 50% chance of being a boy or a girl.
Exam questions
7 exam-style questions (23 marks), each with its mark scheme.
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21 cards: flip them, mark what you knew, and practise the rest.
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The whole of inheritance, variation and evolution on one page, so you can see where this subtopic fits.
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Quick check questions
Why are the offspring of asexual reproduction genetically identical to the parent?
Show answer
There is only one parent and no fusion of gametes, and only mitosis is involved.
How does meiosis halve the number of chromosomes?
Show answer
The cell divides twice to form four gametes, each with a single set of chromosomes.
Define the genome of an organism.
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The entire genetic material of that organism.
What is the difference between genotype and phenotype?
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Genotype is the alleles present; phenotype is how the characteristic is expressed.
What are the sex chromosomes of a human female and male?
Show answer
Female XX, male XY.
Frequently asked questions
Why is meiosis important for sexual reproduction?
Meiosis is important because it halves the number of chromosomes in gametes, so that fertilisation restores the full number. Human gametes have 23 chromosomes, and when an egg and a sperm join, the new cell has 46. Meiosis also makes all the gametes genetically different, which leads to variation in the offspring.
What is a genotype and phenotype?
A genotype is the alleles present for a characteristic, such as Bb, and a phenotype is how the characteristic is expressed, such as black fur. The alleles in the genotype operate at a molecular level to develop the characteristics seen in the phenotype. A black mouse could be BB or Bb, but a brown mouse must be bb.
What is a Punnett square used for in genetics?
A Punnett square is used to show the possible combinations of alleles from the gametes of two parents. Each box is equally likely, so it predicts the probability and ratio of the offspring's genotypes and phenotypes. For example, crossing two Bb mice gives a 3 : 1 ratio of black to brown, so a 25% chance of a brown mouse.
How is the sex of a baby determined?
The sex of a baby is determined by the sperm that fertilises the egg. All eggs contain an X chromosome, but half the sperm carry an X and half carry a Y. An X sperm gives a girl (XX) and a Y sperm gives a boy (XY), so each child has a 50% chance of either.
How is cystic fibrosis inherited?
Cystic fibrosis is inherited through a recessive allele, so a child must inherit two copies to have the disorder. Parents who each have one copy (Ff) do not have cystic fibrosis themselves but are carriers. Two carrier parents have a 1 in 4 (25%) chance of having a child with cystic fibrosis each time they have a child.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026