Download NOTES Polygenic Traits

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
1. Same genetic
code is found
on
chromosomes
3, 5, 10, and 18
2. 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.
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
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-willyour-children-have