Sexual reproduction begins with a diploid zygote forming when nuclei from two haploid gametes fuse, but that only sets the scene for more complex development.
Sexual reproduction provides for a more efficient mixing of genes in the offspring that result, giving them an advantage in terms of natural selection.
Sexual reproduction requires a reduction division or meiosis to reduce the number of chromosomes before the gametes fuse and restore the total.
The male part of a flower is the stamen, which contains the anthers that actually produce pollen grains. The ovary is located at the centre of most flowers.
Asexual reproduction results in almost identical 'daughter' offspring being produced: this form is assumed to lead to slower evolutionary changes.
Asexual reproduction provides quick and easy reproduction, especially in cases where it may be hard for one individual to find a mate or breeding partner.
Many grasses and rosette plants spread by sending out runners into any vacant space and by trying to overgrow other plants, or to beat them to bare space.
Higher plants reproduce in many ways, using seeds, cuttings, runners, and grafts, though some of these methods may be of more use than others.
Self-pollination is a form of sexual reproduction, even though there is only one 'parent'. Most plants have mechanisms to favour 'outside' pollen.
In many animals, the sex of offspring is determined by the presence or absence of a sex chromosome, but there are several variations on the basic plan.
Pollen fertilizes a flowering plant when a pollen tube grows down, carrying a haploid nucleus to a point where that nucleus can fuse with one in the ovum.
Pregnancy in human beings can be prevented or avoided by using different types of barriers to keep sperm cells away from ova, such as diaphragms and condoms.
Pregnancy can be prevented by stopping women from ovulating (that is, producing ova). This is the way the more common forms of 'The Pill' operate.
Pregnancy can be prevented by stopping men producing sperm cells, typically by means of a vasectomy, which prevents sperm cells entering the semen.
Pregnancy in humans can be ended by abortion which may be caused either by natural conditions (spontaneous abortion), or mechanical or chemical interference.
Pregnancy can be prevented by stopping or disrupting implantation of a fertilized ovum (some forms of 'Pill', including the 'morning-after' Pill).
The health of a fetus may be checked by amniocentesis, which uses a needle to sample cells of the fetus that have been sloughed off into the amniotic fluid.
In 1974, the first test-tube babies were conceived in vitro, after which they were implanted artificially, developed normally, and were born normally.