5.1.3Genes, Inheritance and Selection

Sexual Reproduction — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions

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📚 OCR Gateway GCSE Combined Science A J250 - Biology Hub / Topical Revision

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Part 1 of 7Gametes and fertilisation

In sexual reproduction two specialised sex cells, or gametes, fuse in fertilisation.

In animals the gametes are sperm and egg cells; in flowering plants they are the male gamete in pollen and the female gamete (egg cell) inside the ovule.

Haploid

Each gamete is haploid: it contains one set of chromosomes (23 in humans).

Zygote

When they fuse the resulting zygote is diploid, with 46 chromosomes, and has a mixture of genetic information from both parents.

The zygote then divides by mitosis to form an embryo.

Part 2 of 7Meiosis

Gametes are made by a type of cell division called meiosis in the reproductive organs (testes and ovaries in humans). Meiosis halves the number of chromosomes, which is essential, because without it the chromosome number would double in every generation.

The genetic material is copied before division.

The cell divides, and the chromosomes are shuffled so that each new cell receives one chromosome from each pair; pairs are separated.

The cell divides a second time.

Four haploid cells result, and each is genetically different from the others and from the parent cell.

Compare this with mitosis, which produces two genetically identical diploid cells for growth and repair.

	Mitosis	Meiosis
Where	All body cells	Reproductive organs only
Divisions	One	Two
Cells made	2	4
Chromosome number	Stays diploid	Halved to haploid
Genetic identity	Identical to parent	Genetically different

Part 3 of 7Why offspring show variation

Sexual reproduction produces variation for three reasons. First, independent assortment during meiosis: each gamete gets a random mix of the maternal and paternal chromosomes. Second, random fertilisation: any of the many sperm may fuse with any egg. Third, mutations may occur. As a result, offspring are not identical to either parent or to each other, except for identical twins.

The advantage of variation is that, in a changing environment or when a new disease appears, some individuals will have features that help them survive. The disadvantages are that two parents are needed, so it takes time and energy to find a mate, and fewer offspring are produced than by asexual methods.

Part 4 of 7Sex determination in humans

Of the 23 pairs of human chromosomes, one pair are the sex chromosomes.

Females have two X chromosomes (XX) and males have one X and one Y (XY).

The Y chromosome carries a gene that triggers the development of testes.

All eggs carry an X chromosome, whereas half of the sperm carry X and half carry Y.

The sex of the child depends on the sperm: X gives a girl and Y a boy.

A genetic cross shows a 1:1 ratio, or a 50% chance of either sex each time.

Parent gametes

mother X, X;
- father X, Y.
Offspring: XX, XX, XY, XY.

Part 5 of 7Sexual reproduction in plants and hormones in humans

In flowering plants, pollen is transferred from an anther to a stigma (pollination); the pollen tube grows down the style, and the male gamete fuses with the egg cell in the ovule.

The fertilised ovule becomes a seed.

Cross-pollination between different plants produces greater variation than self-pollination.

In humans, puberty is triggered by sex hormones: testosterone in males from the testes and oestrogen in females from the ovaries,

causing secondary sexual characteristics and the production of gametes.

These topics link to the hormones of the menstrual cycle covered elsewhere in the course.

Part 6 of 7Sexual reproduction in the exam: linking ideas

Questions often ask you to combine meiosis with inheritance.

If a parent has genotype Aa, meiosis separates the alleles so that half of the gametes carry A and half carry a.

That is the physical reason for a 1:1 gamete ratio in the Punnett square.

If a body cell has 46 chromosomes, you should expect a sperm to have 23, the zygote 46 and a cell from the embryo after five mitotic divisions also 46.

Errors in meiosis, in which a pair fails to separate, can give gametes with an extra or missing chromosome;
- Down syndrome, with three copies of chromosome 21, is an example.

Questions may also ask why a species that reproduces sexually can adapt more quickly than one that does not,

which relies on the variation described above.

Part 7 of 7Exam-style questions

Explain why gametes are produced by meiosis rather than mitosis. [3]3 marks
A couple have three daughters. What is the probability that their fourth child is a boy? Explain your answer. [2]2 marks
Give two ways in which sexual reproduction produces genetic variation in offspring. [2]2 marks

Key terms

Term	Meaning
Gamete	A sex cell, such as sperm or egg, with half the normal chromosome number.
Fertilisation	The fusion of a male and a female gamete.
Zygote	The cell formed by fertilisation.
Meiosis	Cell division producing four genetically different haploid gametes.
Mitosis	Cell division producing two genetically identical diploid cells.
Sex chromosomes	The pair of chromosomes that determine sex: XX female, XY male.
Independent assortment	Random distribution of chromosomes into gametes during meiosis.
Pollination	The transfer of pollen from an anther to a stigma.

Avoid these mistakes

Saying meiosis makes identical cells. The four cells differ genetically.

Believing that having had several children of one sex makes the next more likely to be the other.

Stating that the egg determines the sex of the child. Eggs always carry X; the sperm determines it.

Mixing up the number of cellsmitosis gives two, meiosis gives four.

Forgetting that the zygote is diploid and that gametes are haploid.

Last updated: 20 October 2026