Reproduction and genetics

Fertilisation

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.

The order is pollination, then fertilisation, then seed.

HintDelivery of the grains first, joining of nuclei next, and finally the part you would plant.

The whyThe ovary wall around the ovule swells into a fruit that protects the seed. Once carried away from the parent, the seed germinates and the cycle begins again with a new plant.

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.

Pollination

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.

Six

HintEach pea started life as a separate ovule, and one grain can fertilise only one of them.

The whyEvery ovule needs its own pollen grain: the grain grows a tube down the style and delivers male nuclei to one ovule only. A poorly pollinated flower sets a pod with gaps where unfertilised ovules shrivelled, which is why fruit growers care so much about bee numbers.

Each seed carries a large food store, so far more of them survive

HintThink about what a young plant needs before its first leaves can catch the light.

The whyDispersal is a numbers game played two ways. Wind-blown seeds are cheap and travel far, but nearly all land somewhere useless. A coconut invests everything in one seed with a huge food store and a buoyant, waterproof husk, so the few that beach on a shore can germinate at once. Most plants sit somewhere between the two extremes.

The ovaries

HintLow in the abdomen, one sits 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, and you inherit one copy of each from your mother and one from your father. Each gene is a section of DNA holding instructions your cells use to build and run your body, and it helps shape features such as eye colour, though most features are shaped by several genes working together.

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.