Two kinds of reproduction
Asexual reproduction needs only one parent and produces genetically identical offspring (clones) — fast, and useful in a stable environment, as in bacteria dividing or a plant growing runners. Sexual reproduction needs two parents and the fusion of gametes, producing offspring that are genetically varied — slower, but variation helps a species survive change.
| Asexual | Sexual | |
|---|---|---|
| Parents | one | two |
| Gametes | none | fuse at fertilisation |
| Offspring | identical (clones) | genetically varied |
| Variation | none | yes |
The human reproductive systems
In the male system, the testes make sperm and the hormone testosterone; sperm travel through the sperm duct and are released in semen. In the female system, the ovaries release egg cells (ova) and make oestrogen and progesterone; an egg travels down the oviduct, where fertilisation may occur, to the uterus.
The menstrual cycle and fertilisation
About once a month, the menstrual cycle prepares the female body for pregnancy: an egg is released (ovulation) and the uterus lining thickens ready to receive it. If the egg is fertilised by a sperm in the oviduct, the resulting embryo implants in the lining and develops; if not, the lining breaks down and is lost as the period, and the cycle begins again.
During pregnancy the placenta lets the mother's blood supply the growing fetus with oxygen and nutrients and remove its wastes, without the two blood supplies mixing.
A worked example
A grower can reproduce a prize apple tree sexually (from seed) or asexually (from cuttings). Which method guarantees the new trees have the same fruit, and why?
- Asexual reproduction (cuttings) uses one parent and no gametes.It does not mix genes from two parents.
- The offspring are genetically identical to the parent (clones).No genetic variation is introduced.
- So they will produce the same fruit as the prize tree.Identical genes give identical characteristics.
- Seeds (sexual reproduction) would give varied offspring, which might not.Sexual reproduction mixes genes, giving variation.
Taking cuttings (asexual) guarantees identical fruit, because the offspring are genetic clones of the parent tree.
Hormones of the menstrual cycle
The menstrual cycle is controlled by hormones. Oestrogen, from the ovaries, rebuilds the uterus lining and triggers the release of an egg; progesterone maintains the thickened lining ready for pregnancy, and its fall at the end of the cycle causes the lining to break down (the period). Two hormones from the brain — FSH (which stimulates an egg to mature) and LH (which triggers ovulation) — control the ovaries. This hormonal control is also the basis of the contraceptive pill and fertility treatments.
Controlling fertility and preventing infection
Because reproduction is under hormonal control, fertility can be managed. Contraception prevents pregnancy — by barrier methods (which also help block infection), by hormonal methods such as the pill (which stops ovulation), or by other means. Reproduction can also spread sexually transmitted infections (STIs), some caused by bacteria and some by viruses; barrier protection and, for bacterial STIs, antibiotics help control them. These are studied as matters of health and biology.
Hormones drive the cycle
Oestrogen builds the lining and leads to ovulation; progesterone maintains it. When progesterone falls, the lining is shed — the period. FSH and LH from the brain control the ovaries.