Erika has taught college Biology, Microbiology, and Environmental Science. She has a PhD in Science Education.
The traits with the most variation are polygenic. This means that more than one gene controls them, and the interaction of all the genes involved causes the observed appearance. People don't just come as short or tall, or heavy or light; they have a range of heights and weights. Furthermore, people have a spectrum of skin tones and eye colors. Your eyes probably don't look exactly the same shade as your mother's or siblings' eyes even if they are the same color. Height, weight, hair color, eye color, and skin color are examples of polygenic traits. The appearance of each of these traits is due to interactions between two or more genes.
Alleles for a gene can be dominant, recessive, codominant, or show incomplete dominance. The interaction between two alleles of a gene is easy to see when there is only one gene that causes the trait, such as for sickle cell anemia, cystic fibrosis, or ectrodactyly (fused fingers or toes). Either you have the trait or you don't. It is more difficult to see dominance when there is more than one gene involved in making a trait because you have to examine the interaction between multiple alleles.
Polygenic dominance describes a relationship between the alleles of a polygenic trait where some alleles are dominant and others are recessive. Eye color is an example of a polygenic trait that demonstrates polygenic dominance. Some alleles for eye color dominate other alleles, where the presence of even one allele dominates all the other alleles. For example, having one allele for brown eyes means that you will most likely have brown eyes. The way this happen is described further in the next section.
The eye colors blue, brown, and green are determined by two genes. At least for this lesson, there are only two genes. If you want to impress people with random knowledge they are 'gey' and 'bey2'. It is important that you understand that the combination of 'gey' and the combination of 'bey2' alleles work together to determine what eye color you have. The 'bey2' gene determines whether your eyes will be brown or blue. This is the gene that people normally discuss in biology classes; brown (B) is dominant over blue (b).
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Eye color is also determined by other genes. The 'gey' gene is the gene for green eyes where Green (G) is dominant and not green (g) is recessive. However, green color may not always show up because Brown (B) dominates green (G). Green is only seen if you have the bb phenotype in the 'bey2' gene. The most important thing to remember is that the allele for brown eyes (B) dominates over the alleles for blue (b) or green eyes (G) even though eye color is polygenic.
We haven't even considered the explanation for how hazel, grey, black, violet, and yellow eyes are inherited, and we won't. It is likely that our exact eye color is the result of multiple genes (or polygenic) where some alleles are dominant, some are codominant, some are additive, and others show other patterns of inheritance, allowing people to have a spectrum of eye colors.
Polygenic dominance is an interaction between alleles from a polygenic trait (a trait that involves multiple genes) where one allele dominates the other alleles. Eye color is an example of polygenic dominance where the brown (BB, Bb) allele dominates the alleles for blue (bb) and green (GG, Gg).
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High School Biology: Help and Review37 chapters | 570 lessons
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