Epistasis, Pleiotropy, and Polygenic Traits

March 25, 2009

Sometimes the expression of one gene can affect the expression of other genes in an organism.

These interactions are not predicted by Mendel’s Laws.

Examples:EpistasisPolygenic TraitsPleiotropy

At the conclusion of this lesson a student should be able to:Identify and distinguish between epistasis,

pleiotropy, and polygenic traits.

Usually a gene at one location (locus) affects the expression of another gene at a second locus.

Example: Coat color in mammals

i.e. in horses (see right) Brown allele “B” is

dominant to tan allele “b.” Dominant allele “C” allows

pigment to be deposited while recessive allele “c” prevents pigment deposition.

Leads to an incidence of white horses.

Epistasis is the interaction of two or more genes to control a single phenotype.

Is that really a chocolate lab? In Labrador retrievers coat

color is controlled by two alleles: A dominant allele “E” will

produce dark pigment while the recessive allele “e” produces little to no pigment.

The dominant allele “B” determines how much pigment will be produced.

What percentage of the puppies would be chocolate labs if you crossed two parents that were heterozygous , EeBb, for these two genes?

EB Eb eB eb

EBEEB

BEEB

bEeB

BEeB

b

EbEEB

bEEb

bEeB

bEeb

b

eBEeB

BEeB

beeBB

eeBb

ebEeB

bEeb

beeB

beebb

Three out of sixteen possible offspring would show a “chocolate” coloration.

Note: This cross yields four possible phenotypes for only one trait.

E e

E EE Ee

e Ee ee

B b

B BB Bb

b Bb bb

Another option for solving involves using two Punnett squares; however, you must remember which characteristic each allele controls.The first allele controls the color (dark or light). Chocolate labs are darker so we are looking for a dog that contains the dominant “E” allele.

The second allele controls the color intensity (very intense or less intense). Chocolate labs have less intense color than black labs so we are looking for a dog that contains the recessive “b” allele.

¾ * ¼ = 3/16

A polygenic trait is a single trait that is affected by more than one gene.

Usually referred to as quantitative inheritance Examples:

Eye colorHeightHair colorSkin color

Harry Cutting -Photography

In these traits it seems that each dominant allele adds a “dose” of the trait.

People with a lot of the dominant alleles show more of the trait (i.e. darker skin)

When the frequency of dominant alleles is graphed, the result is a bell curve.This means more people have

an intermediate phenotype than either of the extremes.

Dickinson, Boonsri. Eye Color Explained: Why everything you know is wrong

Pleiotropy results when a single gene affects more than one trait.Example:

Sickle cell diseaseCauses sickle-shaped blood cells, contributes to

organ damage, & imparts resistance to malaria.Pigmentation alleles in cats

White coloration (WW or Ww) often causes deafness

Pigmentation alleles in miceYy genotype produces yellow miceYY genotype is lethal!

# of

gen

es#

of g

enes

# of traits affected# of traits affected

11

11

manymany

manymany

The environment can affect an organism’s phenotype.

Examples: Lack of water and sunlight affects flower

formation, leaf retention, root growth, and chlorophyll production.

Soil acidity affects the color of hydrangea flowers.

Diet and exercise affect height, bone density, and body mass composition in humans.

Temperature affects the color of the fur in Himalayan rabbits and Siamese cats.

Gender of parrotfish and clownfish is controlled by the gender ratio of adults in the population.

Siamese cat

Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.

"Biology 103: Chapter 10: Mendel." 18 Sep 2003. Queens University of Charlotte. 14 Apr 2008 <http://campus.queens.edu/faculty/jannr/bio103/helpPages/c10gene.htm>.

Dickinson, Boonsri. "Eye Color Explained: Why everything you know is wrong." Discover 13 Mar 2007 14 Apr 2008 <http://discovermagazine.com/2007/mar/eye-color-explained>.

"Golden Retriever." Canine Genetic DNA Test. HealthGene Molecular Diagnostic and Research Center. 14 Apr 2008 <http://www.healthgene.com/canine/retriever_golden.asp>.

McLean, Phillip. "Pleiotropic Effects and Lethal Genes." Mendelian Genetics. 2000. 14 Apr 2008 <http://www.ndsu.nodak.edu/instruct/mcclean/plsc431/mendel/mendel5.htm>.

"Polygenic Inheritance." The Complex Expression of Multiple Alleles. 2002. Department of Biology, Penn State University. 14 Apr 2008 <http://courses.bio.psu.edu/fall2005/biol110/tutorials/tutorial5.htm>.