OCR GCSE Biology A: Sexual and asexual reproduction, haploid and diploid cells and meiosis (J247)

This guide covers statements B5.1f, B5.1g and B5.1h (part of “B5.1 Inheritance”) of the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. None of these statements is printed in bold, so none is marked as Higher Tier only. Use it with the revision notes and the practice questions. Course links: the OCR GCSE Biology hub, the printable checklist and the free diagnostics. Mitosis is explained in Transport across membranes, mitosis and stem cells, and the wider course in Cell Level Systems.

What this unit covers

Statement	You must be able to
B5.1f	Explain some advantages and disadvantages of asexual and sexual reproduction in a range of organisms, including the number of live offspring per birth, how quickly organisms can reproduce, and the need for variation in a population caused by environmental pressures
B5.1g	Explain the terms haploid and diploid
B5.1h	Explain the role of meiotic cell division in halving the chromosome number to form gametes, including that this keeps cells diploid when gametes combine and that it is a source of genetic variation

The specification’s mathematical outcomes for this topic include direct proportions and simple ratios, probability, and extracting information from charts, graphs and tables. All three appear below. Gametes, genes, alleles and the genome are covered in other units, so this guide uses those words only where needed.

Two ways to reproduce (B5.1f)

Asexual reproduction involves one parent. The offspring are produced by mitosis, so they have the same genetic information as the parent. They are clones. No gametes and no fertilisation are involved.

Sexual reproduction involves the fusion of two gametes, one from each parent, in fertilisation. The fertilised egg, the zygote, then divides by mitosis and grows into a new individual. Each parent contributes half of the offspring’s genetic information, so the offspring differ from both parents and from each other.

Organisms and examples

Organism	Way it reproduces	Notes
Bacteria	Asexually by binary fission	A cell copies its DNA and splits in two. Under good conditions some species can divide about every 20 minutes.
Strawberry plant	Asexually by runners, and sexually by flowers and seeds	A runner is a stem that grows along the ground and forms a new plant at its tip.
Potato plant	Asexually from tubers, and sexually by flowers	A tuber that is planted grows into a plant genetically identical to the parent.
Hydra and many fungi	Asexually (budding, spores)	One parent is enough.
Humans	Sexually only	Usually one baby per birth.
Mice	Sexually only	Several young per litter.
Many flowering plants	Sexually by pollination, fertilisation and seeds	Some also reproduce asexually.

Some organisms, such as strawberries and many fungi, can use both methods. That lets them use each one when it suits them best.

Advantages and disadvantages

	Asexual reproduction	Sexual reproduction
Advantages	Only one parent is needed, so there is no need to find a mate. Reproduction can be fast, so numbers rise quickly. Offspring are identical to a parent that is already well suited to its habitat.	Offspring show variation. If the environment changes, or a new disease appears, some individuals are likely to survive and reproduce. Variation is the raw material for natural selection.
Disadvantages	Little or no variation. If conditions change, or a pathogen appears that can infect one individual, it can infect them all.	Two parents (or two gametes) are needed, so finding a mate takes time and energy. It is usually slower, and often produces fewer offspring at a time.

Number of live offspring per birth. A mammal such as a human or an elephant usually has one offspring at a time, and the young need a long period of care. A mouse has a litter of several young. A bacterium that divides every 20 minutes can produce vast numbers of offspring in a day without any parental care. Fewer offspring per birth means a slower rise in numbers, which is a disadvantage when conditions are good and an organism could otherwise spread quickly. Many offspring, however, are only useful if they can survive.

Speed of reproduction. Asexual reproduction needs no mate, gametes or fertilisation, so it is usually quicker. A single bacterium on a food source can become a very large population before sexual organisms have found a partner.

Variation and environmental pressures. Environmental pressures include disease, predators, competition for food, and changes in temperature or water supply. A population of clones is fine while conditions suit it. If a new pressure appears, no clone may have a feature that helps it survive, so the whole population is at risk. A population with variation is more likely to contain some individuals that survive. A real example is the potato, which is grown from tubers and so is propagated as a clone. When a potato blight pathogen spread through Ireland in the 1840s, the crop failed on a vast scale.

Worked example 1: comparing growth

A bacterium divides by binary fission every 30 minutes. How many bacteria can one cell give rise to in 4 hours, if no cell dies?

1. Number of divisions: 240 min ÷ 30 min = 8.
2. Each division doubles the number, so the number after 8 divisions is 2⁸.
3. 2⁸ = 256.

The answer is 256 bacteria. A human couple would not have 256 children in 4 hours, or in a lifetime. This is why speed and numbers are real advantages of asexual reproduction.

Worked example 2: a data question

A gardener plants one strawberry plant. In one year it sends out 4 runners, and each runner forms a new plant that grows to full size by the next year and then does the same.

• Plants at the end of year 1: 1 + 4 = 5 (the original and 4 new plants).
• Each plant produces 4 runners in year 2, so there are 5 × 4 = 20 new plants, and a total of 5 + 20 = 25.

The ratio of plants after year 2 to plants at the start is 25 : 1. All 25 are clones, so one disease that can infect the original can infect them all.

Haploid and diploid (B5.1g)

A chromosome is a long molecule of DNA, with proteins, that carries many genes. The nucleus of a human body cell contains 46 chromosomes. They are not 46 different chromosomes. They are 23 pairs. In each pair, one chromosome came from the mother and one from the father, and the two carry the same genes in the same order.

• Diploid cells contain two sets of chromosomes, so the chromosomes are in pairs. The diploid number is written 2n. Human body cells are diploid, with 2n = 46.
• Haploid cells contain one set of chromosomes, one from each pair. The haploid number is written n. Human gametes (sperm and egg cells) are haploid, with n = 23.

The zygote formed at fertilisation is diploid again: 23 + 23 = 46.

Organism	Diploid number (2n)	Haploid number (n)
Human	46	23
Pea plant	14	7
Fruit fly	8	4
Dog	78	39

The number in a gamete is always half the number in a body cell of the same organism. The diploid number is always even, because chromosomes come in pairs. A common slip is to treat “haploid” as meaning “half the cell”. It means “half the number of chromosomes, one of each pair”.

Worked example 3: counting chromosomes

A pea plant has 14 chromosomes in each leaf cell. State (a) the number of chromosomes in a pollen nucleus and (b) the number in the cell formed when a pollen nucleus fuses with an egg cell.

(a) Pollen nuclei are gametes, so they are haploid: 14 ÷ 2 = 7. (b) Fertilisation adds the two haploid sets: 7 + 7 = 14, so the cell is diploid.

Meiosis (B5.1h)

Meiosis is a type of cell division that produces gametes. It happens only in the reproductive organs: the testes and ovaries in humans, and the anthers and ovules in flowering plants. It starts with a diploid cell and produces haploid cells.

Why halving is needed

Suppose human gametes kept 46 chromosomes. A zygote would then hold 46 + 46 = 92. The next generation would double again. Meiosis halves the number to 23 in each gamete, so fertilisation restores the diploid number of 46. That keeps the number the same from one generation to the next.

What happens

1. The cell copies its DNA, so each chromosome is made of two identical copies joined together.
2. In the first division the pairs of chromosomes are separated, so each new cell has one chromosome from each pair. This is where the chromosome number is halved.
3. In the second division the two copies of each chromosome are separated.
4. The result is four cells, each haploid, and not identical to each other.

Meiosis and variation

When the pairs separate, each pair separates independently of the others. Whether the maternal or the paternal chromosome of a pair goes into a particular gamete is a matter of chance. Each gamete therefore gets a different mixture. Fertilisation adds more variation, because any sperm can fuse with any egg. Meiosis is a source of genetic variation, which is why brothers and sisters, other than identical twins, differ from each other.

Mitosis and meiosis compared

	Mitosis	Meiosis
Where	Body (somatic) cells	Reproductive organs
Divisions	One	Two
Cells produced	Two	Four
Chromosome number	Stays the same (diploid to diploid)	Halved (diploid to haploid)
Offspring cells	Genetically identical	Genetically different
Used for	Growth, repair, asexual reproduction	Making gametes

Worked example 4: how many different gametes?

Each human gamete contains one chromosome from each of the 23 pairs, and each of the two chromosomes in a pair is equally likely to be chosen. How many different combinations of chromosomes are possible in one person’s gametes? Ignore any other source of variation.

• For each of 23 pairs there are 2 choices, so the number of combinations is 2 × 2 × … × 2, with 23 twos.
• 2²³ = 8 388 608.

Fertilisation combines one gamete from each parent. The number of possible zygotes from the same two parents is 8 388 608 × 8 388 608 = 2⁴⁶ ≈ 7.04 × 10¹³. With so many possibilities, it is very unlikely that two children have the same set of chromosomes.

Worked example 5: probability

A fruit fly has 4 pairs of chromosomes. What is the probability that a gamete contains all four chromosomes that the fly received from its father?

• For each pair the chance of getting the paternal chromosome is 1/2.
• The pairs separate independently, so multiply: (1/2)⁴ = 1/16, which is 0.0625.

Common errors

• Writing that asexual offspring are “exactly” the same in every way. They have the same genetic information, but their phenotype can still differ if the environment differs.
• Saying sexual reproduction is “better”. It has advantages and disadvantages, and the answer depends on the organism and environment.
• Saying that meiosis makes two cells. It makes four.
• Confusing haploid with “half a chromosome”. Haploid means one of each pair.
• Saying that gametes have 23 pairs. They have 23 single chromosomes.
• Explaining only that fertilisation “adds the numbers”. State that the diploid number is restored.

Next steps

Test yourself with the practice questions, then check what you know with the diagnostics and the checklist.

Official syllabus

OCR GCSE (9-1) Biology A (Gateway Science) specification J247, Version 4.0 (August 2026), for first assessment in 2018, published by OCR: statements B5.1f, B5.1g and B5.1h.