Are all people either short or tall?
Unlike Mendel's peas, people do not all fall into two categories: short or tall. Most people, in fact, are somewhere in between. Obviously, Mendel's rules are too simple to explain the inheritance of human height.
Another exception to Mendel’s rules is polygenic inheritance, which occurs when a trait is controlled by more than one gene. This means that each dominant allele "adds" to the expression of the next dominant allele.
Usually, traits are polygenic when there is wide variation in the trait. For example, humans can be many different sizes. Height is a polygenic trait, controlled by at least three genes with six alleles. If you are dominant for all of the alleles for height, then you will be very tall. There is also a wide range of skin color across people. Skin color is also a polygenic trait, as are hair and eye color.
Polygenic inheritance often results in a bell shaped curve when you analyze the population (Figure below). That means that most people fall in the middle of the phenotypic range, such as average height, while very few people are at the extremes, such as very tall or very short. At one end of the curve will be individuals who are recessive for all the alleles (for example, aabbcc); at the other end will be individuals who are dominant for all the alleles (for example, AABBCC). Through the middle of the curve will be individuals who have a combination of dominant and recessive alleles (for example, AaBbCc or AaBBcc).
Polygenic traits tend to result in a distribution that resembles a bell-shaped curve, with few at the extremes and most in the middle. There may be 4 or 6 or more alleles involved in the phenotype. At the left extreme, individuals are completely dominant for all alleles, and at the right extreme, individuals are completely recessive for all alleles. Individuals in the middle have various combinations of recessive and dominant alleles.
- In polygenic inheritance, a trait is controlled by more than one gene.
- Examples of polygenic inheritance include height or skin color.
Use the resource below to answer the questions that follow.
- Genetics and Eye Color at http://www.youtube.com/watch?v=MjBZaed9yzM (1:49)
- Is eye color a trait controlled by a single gene as it is often taught in schools?
- Do you think skin color is a polygenic trait? Explain your reasoning, and be as specific as possible.
- What is an albino? What kind of eyes would they definitely NOT have?
- What is known about the melanin levels in people with blue eyes?
- How does polygenic inheritance violate Mendel's rules?
- Give examples of traits governed by polygenic inheritance.