Hence at the gross morphological level there is

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hence, at the gross morphological level there is complete dominance, but at the microscopic level there is incomplete dominance. Codominance – both alleles are separately manifest in the phenotype. An example is human blood groups involving the M and N alleles. Individuals who are homozygous MM will have only M type molecules on their red blood cells and will be type M (exhibiting blood type M is the phenotype – being MM as to alleles carried is the genotype). Individuals who are homozygous NN will have only N type molecules on their red blood cells and will be type N (exhibiting blood type N is the phenotype – being NN as to alleles carried is the genotype). Individuals who are heterozygous MN will have both M type and N type molecules on their red blood cells and will be type MN (exhibiting blood type MN is the phenotype – being MN as to alleles carried is the genotype).
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3 The type of dominance (or lack of dominance) often depends on the level at which the phenotype is examined – consider Tay-Sachs disease (let Norm = the normal allele and let TS = the Tay-Sachs allele). Genotype Phenotype Organismal level Norm/Norm Normal – no disease (complete dominance) Norm/TS Normal – no disease TS/TS Tay-Sachs disease Biochemical level Norm/Norm 100% enzyme activity (incomplete dominance) Norm/TS 50% enzyme activity TS/TS No enzyme activity Molecular level Norm/Norm Only normal enzyme present (codominance) Norm/TS Normal and abnormal enzyme present TS/TS Only abnormal enzyme present Traits may skip a generation due to incomplete penetrance and vary in their expressivity. In general we expect heterozygous individuals to express the dominant trait, but in the case of incomplete penetrance this is not necessarily true. An example in humans is polydactyly, a trait that causes an individual to have extra digits on their hands or feet (Figure 4.5). Penetrance is measured by the percent of the population that possess the allele and exhibit the trait. A penetrance of 60 percent indicates that 60 percent of the dominant alleles are being expressed in a population. Expressivity is the degree to which the traits are expressed. Individuals with the same allele for polydactyly may have different numbers of digits expressed, on various locations on the hands and feet. Unlike other examples of Mendelian inheritance, the molecular basis of incomplete penetrance and expressivity is not well understood, but is believed to be associated with environmental factors. The outcome of traits is influenced by the environment. Environmental conditions may influence the expression of a specific phenotype. The arctic fox has coat color changes due to the environment (Figure 4.6a). In humans, the detrimental effects of PKU can be reduced by adjusting the diet (Figure 4.6b).
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