How do genes have an additive effect in polygenic inheritance?

The traits that are determined by polygenic inheritance are not an effect of dominance and recessivity, and do not exhibit complete dominance as in Mendelian Genetics, where one allele dominates another. Instead, polygenic traits exhibit incomplete dominance so the phenotype displayed in offspring is a mixture of the phenotypes displayed in the parents. Each of the genes that contributes to a polygenic trait has an equal influence and each of the alleles has an additive effect on the phenotype outcome. Hope this helps!

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