Gametes are supplied from two separate parent organisms which are usually genetically unrelated. This increases the probability of heterozygous loci occurring. Animals tend to be outbreeders even in most of the hermaphrodite species.

Some organisms only self-fertilise and therefore are referred to as inbreeders. They do not show any loss of vigour as a result of inbreeding and therefore are assumed not to have lethal and sub-lethal systems acting within them. Some inbreeders occur in the animal kingdom but these are generally in the lower phyla. In the plant kingdom inbreeders are less infrequent, but the majority of plants are still outbreeders.

Many plants show a tendency to facultative inbreeding, eg the dandelion, which will inbreed if outbreeding fails, this is achieved by a curling of the stigma as it ages which brings it into contact with the pollen released from its own stamens. Facultative inbreeding also occurs in some parasitic organisms which find themselves isolated in their host on some occasions eg Tapeworm.

The effects of these two systems upon the genetic make-up of the animal and plant populations is very significant. The outbreeders tend to show a high level of heterozygosity and are subject to inbreeding depression and this makes it extremely important to maintain a high level of genetic variability in commercial breeding programmes. The inbreeders consist of many closely related homozygous lines which remain independent of each other although they are breeding within the same area. They do not suffer from inbreeding depression (but no-one is sure why!). Commercial breeding programmes for these types will have problems in obtaining new genetic combinations from which to select. You should remember that outbreeders can sometimes inbreed and that inbreeders can sometimes outbreed. Since inbreeding causes increased homozygosity then there is a greater chance for lethal alleles appearing in the homozygous condition. Shull (1909) examined the effects of inbreeding upon maize which is a natural out breeder (it has separate male and female flowers). In the first generation a number of lethal types appeared, subsequently the maize plants separated into distinct lines with each line becoming more uniform for various morphological characters and later all lines showed a decreased vigour and some died out. However, he found that when two inbred lines were crossed the hybrids showed renewed viability, often greater than the original parent strain and this became known as hybrid vigour or heterosis.

DOMINANT FACTORS: The greater the number of favourable dominant factors in an individual then the greater the viability. The idea here is that the hybrid has more of the desirable dominant factors than the inbred lines from which it was bred.

P:AA bb cc DD EE ff X aa BB CC dd EE FF (each has 3 dominant loci)

F1 Hybrid Aa Bb Cc Dd EE Ff (has 6 dominant loci)

Note that dominant factor refers here to a dominant locus on the chromosomes.

It should clearly be possible to produce an inbred line which has complete dominant homozygosity which would then have optimum viability, but this has never been possible and results like those of Shull show little indication of dominant homozygosity appearing during the inbreeding programmes used.

HETEROZYGOSITY: The idea that evolution favours the heterozygote. Natural selection tending to favour the intermediate rather than the extreme phenotypes. The idea implies that the more heterozygous the organism is then the more average it will become and therefore the more likely it is to survive. It does not in itself explain heterosis and has very little evidence to support it.

OVERDOMINANCE: The heterozygous condition at a particular locus may as a result of inter-action between the alleles result in an effect which is greater than the effect of the homozygous dominant condition, which leads to a better performance.

None of the above explanations of heterosis are completely adequate. Perhaps, on the biochemical level, the greater range of protein products expressed in the heterozygote may well have an effect upon the overall performance of an organism, and the lower levels of output of certain key enzymes in the heterozygote may result in better use of materials within the organism.

Many organisms use special techniques to enforce outbreeding and to ensure that inbreeding depression does not occur. Many mammals live in close family/social groups and they tend to breed only within the social group, a good example of this is the house mouse. The mouse community has a dominant male which breeds with the females of the group and with their offspring. If another male enters the group area the dominant male will attack it and drive it off. After some time the dominant male will age and lose control of its territory by some stronger, younger male entering the territory. This new male will then breed with the females in the territory and thus introduce new gene types and alleviate the problem of inbreeding depression. Many mammals have similar arrangements whereby inbreeding is allowed for a short period but outbreeding is forced to occur at intervals.

In chimpanzees a similar system operates and the behaviour of the females is designed to encourage them to mate with a 'foreign' male which has left another family group and is hanging around the new group hoping to be accepted!

In wood lice the egg batches always hatch as single sex groups which means that the offspring are forced to breed with individuals derived from a different egg batch.

Outbreeding plants may be unisexual, ie they are dioecious, eg hops, spinach and asparagus. They may have separate male and female flowers on the same plant, eg maize, where the male flowers are at the top of the main stem while the female flowers are lower down at the stem nodes. There may be differential maturation of the gametes, protandry, where the male gametes develop first as in carrots and raspberry, protogyny, where the female parts reach maturity first as in the avocado pear. Finally, there may be a system of self-incompatibility which prevents pollen germinating, or growing at the normal rate upon stigmas with identical phenotypes.

Any selective breeding programme must take into account the type of breeding system used by the plant being selectively bred and the sources of variability available. You cannot selectively breed without genetic variation!

Table to show breeding systems of crop plants

Crop	inbreeders	outbreeders	both
cereals	wheat	rye
	barley	maize
	oats
	rice
legumes	pea	runner beans	field bean
	soya bean
	peanut
fruit	peach	blackberry
		apple
		banana
		cherry
		pear
		plum
product	cotton	sunflower	oil seed rape
	flax	sugar beet
	tobacco
vegetables	lettuce	Brussel sprouts
	tomato	carrot
	parsnip	kale
	cauliflower	turnip