Mutations or slight changes in the chemical structure of a gene occur at random and are caused by a variety of influences. Mutations occur at very low rates for individual genes, but, because of the large total number of genes in organisms, mutations are constantly occurring within a species. The majority of new mutations are deleterious because of their randomness; in fact, mutations can create lethal alleles that have phenotypic expressions which kill the organism. Because natural selection acts on phenotypes and only indirectly on genotypes, only dominant deleterious mutations can be selected against. Recessive mutations are not easily selected against because usually an organism will have one normal dominant allele that masks the deleterious mutated recessive allele. Such mutations remain in populations for long periods of time. The advantage of diploidy is that deleterious recessive alleles can be masked, whereas, in haploidy deleterious mutations are always expressed. Because inbreeding increases the chance that two parents will carry similar deleterious recessive genes, inbreeding increases the chance that two individuals will produce homozygous recessive offspring in which mutated deleterious alleles can be expressed. In unrelated parents, the chance that two individuals will have the same deleterious mutated alleles is very slight and, thus, the chances are very high that any deleterious allele will be masked by a dominant normal allele from the other parent. Although mutations do increase variability in genotypes providing the ultimate raw material for evolution and, therefore, constantly exert a slight influence on the
