Polygenic Traits Hair Color, Skin Color, Eye Color, and Height

Polygenetic traits- traits controlled by two or more of the SAME genes 1. Offers a great deal of variation 2. Examples: a. hair color (4 genes) b. skin color (3 genes, each gene controls the amount of melanin) c. human height (unknown number of genes) d. eye color (up to 16, some control amount of melanin)

Examples of Polygenic traits 1. Genes are inherited by Mendel’s laws but only characteristics that are monogenic display Mendelian inheritance 2. Most interesting traits are polygenic. All continuously varying quantitative traits must be polygenic. **Only one clear example of a monogenic trait in human beings: red hair!

A. Hair color Same genetic code is found on chromosomes 3, 5, 10, and 18 The more dominant alleles, the darker the hair

B. Skin Color 1. Skin color is now believed to be a polygenic trait governed by three sets of genes. 2. Differences in skin color arise from the quantity and distribution of the pigment melanin in the uppermost layers of skin cells. 3. Exposure to sunlight is an environmental factor that can affect skin color.

Variation in human height is caused by genotype and can be influenced by the environment. (f) For example, if a person has inherited a number of dominant alleles for tallness, he has the potential to grow tall. However if the diet is poor, for example, lacking in protein and calcium, the body cells do not have sufficient nutrients for maximuim development of the growth potential. C. Human Height 1. Variation in human height is caused by genotype and can be influenced by the environment. Example: a person has inherited a number of dominant alleles for tallness & has the potential to grow tall. However if the diet is poor the body cells do not have sufficient nutrients for maximum development of the growth potential.

D. EYE COLOR Determined by 2 factors 1) Eye color happens because of the amount of MELANIN in the iris. A lot of melanin gives brown eyes and less melanin gives green. Little or no melanin in the stroma of the iris gives blue eyes. -varies from light brown to black depending on the amount of MELANIN in the FRONT of the iris -there are no genes for any color other than BROWN

2) Scattering of light in STROMA of the iris -depends on the density of the stroma (located in the front of the iris) -how light is reflected back determines blues, greens, hazel, etc.

The two main genes involved in eye color are: 1) OCA2-comes in two versions, brown (B) and blue (b). The brown version works in the stroma, the blue version does not. Since the blue version doesn't work there, no melanin builds up. So these folks have blue eyes. Green eyes may occur when OCA2 is not at full strength. 2.) HERC2- not actually involved in eye color, but is close to the OCA2 gene and may have an affect on how it functions -people with brown eyes have a hard working OCA2 and a strong HERC2 enhancer “coaxing” it to make more melanin http://genetics.thetech.org/ask-a-geneticist/how-blue-eye-snp-works

Elizabeth Taylor -had VIOLET EYES! -possible gene mutation???

Heterochromia Two different colored eyes. A change in the melanocytes (melanin making cells) in one or both eyes. Reasons? 1.) While fetus was developing the melanocytes were damaged or didn’t make it to where the eyes would be. 2.) Waardenburg syndrome- mutation in the melanocytes 3.) Chimerism- person has 2 different sets of genes (2 fertilized eggs fused) 4.) Mosaicism- a gene gets changed early in some of the melanocytes.

Do eye colors change? *Most European babies are born with light colored eyes because the melanin is not being produced yet. (eyes change between 1-3 years old) *Changes of eye colors during puberty, early childhood, pregnancy, and sometimes after serious trauma do sometimes happen based on chemical reactions and hormonal changes. *Most eye-color changes have been observed or reported in the Caucasian population having hazel and amber eyes **EYE COLOR CHANGES DO NOT OCCUR FROM DAY TO DAY! This is simply a matter of perception and the type of light being reflected from the eye (caused by the color of clothing someone is wearing or type of light such as sunlight or fluorescent lighting)

Fun eye color calculator http://genetics.thetech.org/online-exhibits/what-color-eyes-will-your-children-have