The following statements apply equally to plants and animals. Sexual reproduction involves the production of sex cells. These sex cells are called gametes and they are made in reproductive organs. The process of cell division that produces the gametes is called meiosis. In sexual reproduction, the male and female gametes come together and fuse, that is their cytoplasm and nuclei join together to form a single cell called a zygote. The zygote then grows into a new individual.
In flowering plants the male gametes are found in pollen grains and the female gametes, called egg cells, are present in ovules. In animals, male gametes are sperm and female gametes are eggs.
In both plant and animals, the male gamete is microscopic and mobile (i.e. can move from one place to another). The sperm swim to the ovum; the pollen cell moves down the pollen tube see Figure below.
The male gametes is small and mobile; the female gamete is larger
The female gametes are always larger than the male gametes and are not mobile. Pollination in seed-bearing plants and mating in most animals bring the male and female gamete close together.
In normal body cells (somatic cells) the chromosomes are present in the nucleus in pairs. Humans, for example, have 46 chromosomes: 23 pairs. Maize (sweet corn) has 10 pairs. This is known as the diploid number. When gametes are formed, the number of chromosomes in the nucleus of each sex cell is halved. This is the haploid number. During fertilisation, when the nuclei of the sex cells fuse, a zygote is formed. It gains the chromosomes from both gametes, so it a diploid cell.
The advantages and disadvantages of sexual reproduction
In plants, the gametes may come form the same plant or from different plants of the same species. In either case, the production and subsequent fusion of gametes produce a good deal of variation among the offspring. This may result from new combinations of characteristics, e.g. petal colour of one parent combined with fruit size of the other. It may also be the result of spontaneous changes in the gametes when they are produced.
Variation can have its disadvantages: some combinations will produce less successful individuals. On the other hand, there are likely to be some more successful combinations that have greater survival value or produce individuals which can thrive in new or changing environments.
In a population of plants that have been produced sexually, there is a chance that at least some of the offspring will have resistance to disease. These plants will survive and produce further offspring with disease resistance.
The seeds produced as a result of sexual reproduction will be scattered over a relatively wide range. Some will land in unsuitable environments, perhaps lacking light or water. These seeds will fail to germinate. Nevertheless, most methods of seed dispersal result in some of the seeds establishing populations in new habitats.
The seeds produced by sexual reproduction all contain some stored food but it is quickly used up during germination, which produces only a miniature plant. It takes a long time for a seedling to become established and eventually produce seeds of its own.
Sexual reproduction is exploited in agriculture and horticulture to produce new varieties of animals and plants by cross-breeding.
It is possible for biologists to use their knowledge of genetics to produce new varieties of plants and animals. For example, suppose one variety of wheat produces a lot of grain but is not resistant to a fungus disease. Another variety is resistant to the disease but has only a poor yield of grain. If these two varieties are cross-pollinated, the F1 (which means ‘first filial generation’) offspring should be disease-resistant and give a good yield of grain (assuming that the useful characteristic are controlled by dominant genes).
A long-term disadvantage of selective breeding is the loss of variability. By eliminating all the offspring who do not bear the desired characteristics, many genes are lost from the population. At some future date, when new combinations of genes are sought, some of the potentially useful ones may no longer be available.
Summary advantages and disadvantages of sexual reproduction
|There is variation in the offspring, so adaptation to a changing or new environment is likely, enabling survival of the species||Two parents are usually needed (though not always-some plants can self-pollinate)|
|New varieties can be created, which may have resistance to disease||Growth of a new plant to maturity from a seed is slow|
|In plants, seeds are produced, which allow dispersal away form the parent plant, reducing competition|