Reproduction and genetics

The fusion of a sperm and an egg

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.

pupa

提示Inside a hard case the caterpillar rebuilds itself completely.

为什么In 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

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.

The transfer of pollen from an anther to a stigma

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.

A seed

提示The part you would plant to grow a new one.

为什么While 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

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

为什么A 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

提示A pair of organs low in the abdomen, one 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. Each carries the instructions for one characteristic, such as eye colour, and you inherit one copy from each parent.

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.