Reproduction and genetics

Fertilisation

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

为什么Each 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

提示Chemical messengers carried in the blood from glands to target organs.

为什么From 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.

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

为什么The 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

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

为什么The 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

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

为什么Insect-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

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

为什么Every 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

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

为什么Dispersal 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

提示Low in the abdomen, one sits on each side of the uterus.

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

Ovulation

提示It happens roughly halfway through a monthly cycle.

为什么On 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

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

为什么It 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

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

为什么Genes 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

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

为什么This 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.