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3.1.5: Polygenic Traits

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    136388
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    While Mendelian traits tend to be influenced by a single gene, the vast majority of human phenotypes are polygenic traits. The term polygenic means “many genes.” Therefore, a polygenic trait is influenced by many genes that work together to produce the phenotype. Human phenotypes such as hair color, eye color, height, and weight are examples of polygenic traits. Complex diseases (e.g., cardiovascular diseases, Alzheimer’s, and Schizophrenia) also have a polygenic basis.

    Human hair color is an example of a polygenic trait. Hair color is largely determined by the type and quantity of a pigment called melanin, which is produced by a specialized cell type within the skin called melanocytes. The quantity and ratio of melanin pigments determine black, brown, blond, and red hair colors. MC1R is a well-studied gene that encodes a protein expressed on the surface of melanocytes that is involved in the production of eumelanin pigment. Typically, people with two functional copies of MC1R have brown hair. People with reduced functioning MC1R allele copies tend to produce pheomelanin, which results in blond or red hair. However, MC1R alleles have variable penetrance, and studies are continually identifying new genes (e.g., TYR, TYRP1, SLC24A5, and KITLG) that also influence hair color. Individuals with two non-functioning copies of the gene TYR have a condition called oculocuteaneous albinism—their melanocytes are unable to produce melanin so these individuals have white hair, light eyes, and pale skin.

    In comparison to Mendelian disease, complex diseases tend to be more prevalent in humans. Complex diseases can also run in families, but they often do not have a clear pattern of inheritance. Geneticists may not know all of the genes involved with a given complex disease. In addition to different gene combinations, complex diseases are also influenced by environment and lifestyle factors. Moreover, how much each of these determinants contribute to a disease phenotype can be difficult to decipher. Therefore, predicting medical risk is often a significant challenge. For instance, cardiovascular diseases (CVDs) continue to be one of the leading causes of death around the world. Development of CVDs has been linked to malnutrition during fetal development, high fat and sedentary lifestyles, smoking/drug usage, adverse socioeconomic conditions, and various genes. Human environments are diverse, and public health research including the field of Human Biology can help identify risk factors and behaviors associated with chronic diseases. Large-scale genetic studies can also help elucidate some of these complex relationships.


    3.1.5: Polygenic Traits is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.