Biology 1001A Lecture Notes - Allele Frequency, Sympatric Speciation, Genotype Frequency
9 Feb 2013
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Dominance status of an allele describes its effect on the phenotype of heterozygotes, not whether the allele is helpful, harmful or neutral. Most genetic disorders are associated with recessive alleles because selection weeds out harmful dominant alleles while recessive alleles can. Diploidy reduces the effectiveness of natural selection in eliminating harmful recessive alleles they can be protected from selection by the phenotypic expression on the dominant allele. When a new, favourable allele is dominant, a small proportion of recessive alleles will be sheltered from selection and carried in heterozygous alleles. A beneficial dominant allele will start with a higher initial rate of frequency. A beneficial dominant allele is constrained by diploidy. Most genetic disorders are caused by recessive alleles for this reason. When a new, favourable allele is recessive, eventually selection will remove every copy of the harmful dominant alleles.
