The number of a gene’s alleles of a given gene pool can have any number, but a few alleles and a small number of gene locations are absolutely sufficient in order to create a vast variety of genotypes.
These observations do not distinguish between dominant and recessive, between advantageous and disadvantageous alleles. In each population, alleles of a high adaptive value and such with a low fitness value coexist. Alleles with unfavourable properties are suppressed in diploid organisms, if at least one of the alleles has the necessary performance. This topic has already been mentioned when discussing heterosis. Individuals that are heterozygous in as many as possible gene locations have the best performance. Why, then, are so many unfavourable alleles kept and how large can their part of a population’s gene pool be? This part is also called the genetic load. The alleles belonging to the genetic load do without doubt not offer any advantage to their host, they do nevertheless constitute a genetic reservoir that may be of advantage under changed living conditions or in different genetic constellations. It is difficult to esteem the proportion of the genetic load that a population can still bear, but it looks as if this value is higher in plants than in animals and higher in polyploids than in diploids. In addition, these ideas consider only active genes. We will, nevertheless, learn in the topic Active Genes that eucaryotes harbour far more DNA in their genomes that required for the normal life cycle. Obviously, far more inactive nucleotide sequences with potential active genes (so-called silent genes) exist than necessary. This makes the genetic load only a fraction of the seemingly surplus genetic information.
It is the high percentage of polyploids in plants that constitutes a buffer against most unfavourable alleles.
The fitness of an individual is rarely determined by a gene or an allele alone. It is usually controlled by numerous, partly coupled, partly uncoupled genes instead. It is therefore spoken of an inclusive fitness. The contribution of single alleles has always to be related to the total constitution of the genome. If just one of its properties is impaired, this does not necessarily mean that all activities of the respective individual are affected.
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