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Polygenic traits are traits the combined expressions of multiple genes. In other words, these traits are not determined by just one single gene. Most of the traits that are expressed are determined by many genes, with very few traits determined by one single gene.
EXAMPLE
The color of your eyes, hair, or skin are all polygenic traits.Polygenic traits can show continuous variation within a population. Height is a good example of a polygenic trait because, within a given population, we could have a wide range of continuous differences of that trait. Height is also a multifactorial trait, meaning that it is determined by multiple factors such as the combination of a person’s genes and environment.
EXAMPLE
Nutrition is an example of an environmental factor that can influence height.Penetrance is the varying degree to which someone expresses a trait that's associated with an allele. Incomplete penetrance means that some people who inherit a disease allele will not manifest the disease in their phenotype. For example, mutations in the BRCA1 gene cause familial breast cancer. 80% of people who inherit one of these mutant alleles will contract breast cancer at some point during their lifetime. That means that the penetrance of these alleles is 80%; just because you have the allele doesn't guarantee the phenotypic outcome.
Cystic fibrosis is an example of a trait that would be completely penetrant. This means that 100% of people who are homozygous recessive will have cystic fibrosis.
Polydactyly would be an example of a trait that would be incompletely penetrant. Polydactyly relates to the number of digits that a person has. Some people who carry the genes for polydactyly might have the normal ten fingers, while some people who have that trait might have more than ten fingers. There are varying degrees to which someone expresses this trait.
Pleiotropy is the expression of one gene that affects multiple traits. An example of this is the gene that causes sickle-cell anemia. This gene produces various effects throughout the body and can affect the way the blood carries oxygen, other internal organs, et cetera.
Source: SOURCE: THIS WORK IS ADAPTED FROM SOPHIA AUTHOR AMANDA SODERLIND