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Mendelian inheritance refers to the expression of inheritance of monogenic traits, that is traits that are controlled by one gene. We will use flower color as an example. Suppose we have two of the same kind of plant. Plant A has red flowers, and plant B has blue flowers. Red is dominant, and blue is recessive. Therefore the color genes for plant A are either AA or Ab. Plant B is bb. If we cross A with B, the color of the flowers will either be red or blue but not purple. The gene composition can only be AA, Ab, or bb. If it is AA, then the flowers will be red. If it is Ab, then the flowers will be red because the A gene will mask the expression of the b gene. "Ab" does not result in a purple flower. If it is bb, then the flowers will be blue.
However, when we consider polygenic inheritance things become more complicated. Polygenic inheritance refers to the expression of traits controlled by two or more genes and environmental interactions. Polygenic traits do not follow Mendelian patterns of dominance and recessiveness. Skin color is a good example. Not only do multiple genes affect skin color, but the environment does as well. Changing a specific gene or factor may result in only minor changes in the expression of the gene. Suppose one person has black skin and their mate has white. Their offspring may have black skin, white skin, or some shade in between.
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