Reproduction and genetics

The fusion of a sperm and an egg

HintTwo half-sets of instructions join to make one complete set.

The whyEach gamete carries half the usual number of chromosomes. Fertilisation restores the full set, so the new cell — the zygote — holds instructions from both parents.

Hormones

HintChemical messengers carried in the blood from glands to target organs.

The whyFrom about the age of eleven the ovaries begin releasing oestrogen and the testes testosterone. These travel in the blood and bring about the changes that make reproduction possible.

pupa

HintInside a hard case the caterpillar rebuilds itself completely.

The whyIn complete metamorphosis the insect passes through egg, larva, this resting stage, then adult. Inside the case the body is entirely reorganised before it emerges.

The anther

HintIt sits on top of a slender stalk, ready for a bee to brush past.

The whyThe anther is the male part of the flower, held up on a filament; together the two form the stamen. Wind-pollinated flowers dangle theirs outside the petals.

The transfer of pollen from an anther to a stigma

HintMoving the male gametes, carried inside tiny grains, to the female part of a flower.

The whyInsect-pollinated flowers advertise with bright petals, scent and nectar. Wind-pollinated ones skip all that and make huge amounts of light, dry pollen instead.

A seed

HintThe part you would plant to grow a new one.

The whyWhile the ovule ripens, the ovary wall around it swells into the fruit. An apple is a swollen ovary and its pips are the fertilised ovules.

To avoid competing with the parent for light, water and minerals

HintSeedlings crowded in one spot fight each other for the same resources.

The whyA seedling landing in its parent shade is up against a much larger plant. Wind, animals, water and explosive pods all carry seeds to fresh ground.

The ovaries

HintA pair of organs low in the abdomen, one on each side of the uterus.

The whyA baby girl is born with all her immature egg cells already present. From puberty onwards, roughly one matures and is released each month.

Ovulation

HintIt happens roughly halfway through a monthly cycle.

The whyOn a 28-day cycle this falls around day 14. If the egg is not fertilised, the thickened lining of the uterus breaks down and a period follows.

The placenta

HintA disc of tissue in the wall of the uterus, joined to the baby by a cord.

The whyIt brings the two blood supplies very close together without mixing them, so dissolved food, oxygen and waste can diffuse across in either direction.

A short section of DNA

HintA chromosome is a long thread; this is just one instruction along it.

The whyGenes sit in order along chromosomes inside the nucleus. Each carries the instructions for one characteristic, such as eye colour, and you inherit one copy from each parent.

Continuous

HintMeasure a whole class and you get a smooth spread, not a few separate groups.

The whyThis kind of variation takes any value in a range and is plotted on a histogram. Blood group is the other kind: it falls into distinct categories shown on a bar chart.