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12.4 Notes The Mendelian inheritance pattern is rare in nature. Most traits have more than 2 alleles or are controlled by more than 1 gene. Polygenic Inheritance (Multiple genes): When several genes (not just one) affect a trait. Example: eye color is controlled by several genes. Multiple alleles: When there are more than 2 alleles for a trait. Example: Blood types A, B or AB or O. IA IB i IA IA IA A IA I B AB IAi A IB IA IB AB IB I B B IB i B i IAi A IB i B ii O Codominance: When both traits show up (both are fully expressed). If an animal has a gene for black fur and gene for white fur, he has both black and white hair (not gray). B=Black W = White BW= Black and White 25% Black; 25% White; 50% Black and White B B BB W BW W BW WW Incomplete Dominance: When both traits blend because neither trait completely dominates over the other. B= Black W = White BW = Gray 25% Black; 25% White; 50% Gray B B BB W BW W BW WW Environment affects phenotype: Temperature and nutrition, for example, can affect how genes of fur color or height are expressed. Linkage – genes (traits) that are close together on one chromosome are less likely to separate during meiosis and are said to be linked.