Many important traits are determined by more than one gene. These traits are called additive because the contributions from many genes are added to produce the phenotype. These traits, also called multi-genic, polygenic, or quantitative, can be recognized as those traits with continuous variation in phenotype. The inheritance of polygenic traits is complicated by the fact that many genes and environmental conditions contribute to the trait. For agricultural breeding, it is important to calculate the heritability, or the amount of genetic contribution, to multigenic traits. In humans, twins studies are helping to determine the genetic and environmental contribution to important human traits.
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