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Biology 110 - Basic Concepts and Biodiversity

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The Complex Expression Patterns of Multiple Alleles

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Distinguishing between these three types of dominance is sometimes difficult.. Think of complete dominance, incomplete dominance, and codominance as a continuum of dominance relationships among alleles at a gene. At one end is complete dominance, in which the phenotype for only one of the two alleles is expressed in individuals that are heterozygous. At the other end is codominance, in which both alleles are equally expressed in individuals that are heterozygous. In between there are various levels of incomplete dominance, in which individuals that are heterozygous for the alleles display an intermediate phenotype. The key to understanding the difference between the three types is to look at the phenotype of the individuals with heterozygous alleles, then classify the relationship accordingly.

 

Transcript for Complex Expression - Part I

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Multiple Alleles

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The characters that Mendel studied are sometimes referred to as discrete characters because they can only be classified on an "either-or" basis (e.g., purple or white flowers, green or yellow seeds). Many characters cannot be classified in this manner because they vary in a population across a continuum (gradient). For example, the figure above illustrates that skin color in humans is a quantitative character. Quantitative characters usually indicate that the character is controlled by more than one gene (polygenic inheritance).

Figure 7 shows a simplification of the genetics of skin color in humans, with three  genes interacting to determine the level of pigment in an individual's skin. The dominant alleles (A, B, and C) each contribute one "unit" of pigment to the individual, and their effects are cumulative such that individuals with more of these alleles will be darker than those with fewer alleles. The recessive alleles (a, b, and c) do not contribute any units of pigment. Therefore, skin color is related to the number of dominant alleles present in each individual's genotype.

A cross of two completely heterozygous parents produces seven phenotypes in their offspring, ranging from very light to very dark skin. The distribution of skin color in the offspring would resemble a bell-shaped curve because there would be more individuals with intermediate skin colors than either extreme. As the number of genes involved increases, the differences between the various genotypes become more subtle and the distribution fits the curve more closely. Other examples of polygenic inheritance in humans include height, hair color, and eye color. This helps to explain the  variations in these characters that we see in different individuals.

 

Transcript for Complex Expression - Part II

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Figure 7. A simplified example of polygenic inheritance . (Click image to enlarge)

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 Questions?  Either send Send your instructor a message through ANGEL or attend an office hour (the times are posted on ANGEL).Canvas!

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