Reproduction

6. Inheritance, Variation and Evolution

There are two ways of making new organisms. Sexual reproduction mixes genetic information from two parents and produces offspring that are all different. Asexual reproduction copies one parent and produces offspring that are all identical to it.

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186 questions6 subtopicsFoundation tier

What reproduction covers

Every subtopic below has its own questions, mark schemes and notes.

4.6.1.1Sexual and asexual reproduction

4.6.1.2Meiosis

Meiosis is the cell division that makes gametes. It happens only in the reproductive organs, it involves two divisions rather than one, and it produces four genetically different cells, each with half the number of chromosomes the parent cell started with.

4.6.1.3Advantages and disadvantages

Neither kind of reproduction is simply better. Sexual reproduction buys variation, which is insurance against a changing environment. Asexual reproduction buys speed and needs no mate. Some organisms do both, switching between them as conditions change.

4.6.1.6Genetic inheritance

Genes come in different versions called alleles, and you carry two of each. Which version shows up depends on whether the alleles are dominant or recessive. Genetic diagrams let you predict the possible offspring of a cross and work out how likely each outcome is.

4.6.1.7Inherited disorders

Some disorders are caused by faulty alleles that are passed from parent to child. Polydactyly is caused by a dominant allele and cystic fibrosis by a recessive one, which is why they run through families in very different patterns. Embryos can be screened for them, and that raises questions with no easy answers.

4.6.1.8Sex determination

One pair of chromosomes out of the 23 decides sex. Females have two X chromosomes and males have an X and a Y, and because all eggs carry an X, it is the sperm that decides. A genetic diagram shows why the chance of a boy or a girl is the same.

Key terms

The definitions examiners expect, in the wording that earns the mark.

Gamete — A sex cell, such as a sperm cell or an egg cell, that carries a single set of chromosomes.

Fertilisation — The fusion of a male gamete with a female gamete to form a single new cell.

Clone — An offspring that is genetically identical to its parent.

Variation — Differences between the individuals of the same species.

Meiosis — A cell division involving two divisions that produces four genetically different gametes, each with half the number of chromosomes.

Chromosome pair — Two matching chromosomes, one inherited from each parent, that carry genes for the same characteristics.

Reproductive organs — The ovaries and testes, the only places in a human where meiosis happens.

Natural selection — The process by which individuals better suited to their environment are more likely to survive, breed and pass on their genes.

Selective breeding — Breeding together individuals with a desirable characteristic so that the offspring have it too.

Runner — A stem that grows horizontally over the soil surface and forms a new plant identical to the parent.

Allele — One of the different versions that a gene can take.

Genotype — The combination of alleles an organism has for a gene.

Phenotype — The characteristics an organism actually shows.

Homozygous — Having two alleles for a gene that are the same.

Heterozygous — Having two alleles for a gene that are different.

Punnett square — A grid used to show all the possible combinations of gametes from two parents.

Carrier — Someone who has one copy of a recessive faulty allele but does not have the disorder.

Polydactyly — An inherited disorder, caused by a dominant allele, in which a baby is born with extra fingers or toes.

Cystic fibrosis — An inherited disorder of the cell membranes, caused by a recessive allele, in which thick sticky mucus builds up in the air passages and pancreas.

Embryo screening — Testing the genes of an embryo to find out whether it carries alleles linked to a genetic disorder.

Sex chromosomes — The 23rd pair of chromosomes in a human, which determine whether a person is male or female.

X chromosome — The sex chromosome carried by every egg cell and by half of sperm cells; two of them give female characteristics.

Y chromosome — The sex chromosome carried by half of sperm cells, which causes male characteristics to develop.

Where marks get lost

The mistakes that come up again and again in this topic.

Say the gametes fuse, not that they simply 'meet' or 'join up' - fusion is the word that earns the mark.
Asexual offspring are genetically identical, not just 'similar' or 'the same species'.
Pollen is the male gamete of a flowering plant, not the plant's version of an egg cell.
Sexual reproduction produces variation because two sets of genetic information are mixed - not because the offspring 'grow differently'.
Meiosis makes four cells, not two - losing this is the commonest slip in the whole topic.
Say the chromosomes are copied before the first division; jumping straight to 'the pairs line up' misses a marking point.
The cell produced at fertilisation divides by mitosis, not meiosis.
Gametes are genetically different because the chromosomes are shuffled at random, not because meiosis 'causes mutations'.
'Asexual reproduction is quicker' on its own is thin - say why it matters, for example that many offspring can be produced quickly when conditions are favourable.
Variation does not help an individual survive a change; it means that some individuals in the population are likely to survive it.
For the malarial parasite, get the two hosts the right way round: sexual in the mosquito, asexual in the human.
Fungal spores can be either sexual or asexual - saying 'spores are asexual' throws away the mark.
Genotype is the alleles, phenotype is what you see - mixing these two up is the single most expensive error in this subtopic.
A recessive characteristic needs two recessive alleles; one is never enough.
Write the letters carefully: an upper case and a lower case of the same letter must be clearly different, or the examiner cannot award the mark.
A 3:1 ratio means 3 out of 4, so 75% and 25% - not 1 in 3.
A ratio from a genetic diagram is a probability, so real offspring numbers will not usually match it exactly.
There are no carriers for a disorder caused by a dominant allele - anyone with the allele has the condition.
Cystic fibrosis affects cell membranes; describing it only as 'a lung disease' misses the pancreas and the underlying cause.
A 1 in 4 chance applies to every pregnancy separately; two affected children in a row is unlucky, not impossible.
When a question asks you to evaluate screening, give points on both sides and then a conclusion - one-sided answers are capped.
It is the sperm, not the egg, that determines sex - because all eggs carry an X chromosome.
Label a sex-determination genetic diagram with X and Y, not with letters like B and G for boy and girl.
A 50:50 ratio does not mean a family of four will have two of each; each pregnancy is a separate 50% chance.
Say 22 pairs plus one pair of sex chromosomes, not '46 pairs' - 46 is the number of chromosomes, not pairs.

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