Download Complex Inheritance Patterns

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:
 Epistasis
 Polygenic Traits
 Pleiotropy
 At the conclusion of this lesson a student should be
able to:
 Identify and distinguish between epistasis, pleiotropy,
and polygenic traits.
Epistasis is the interaction of two or more genes to control a single
phenotype.
 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.
 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
EB
EEBB
EEBb
EeBB
EeBb
Eb
EEBb
EEbb
EeBb
Eebb
eB
EeBB
EeBb
eeBB
eeBb
eb
EeBb
Eebb
eeBb
eebb
Three out of sixteen possible offspring would show a “chocolate” coloration.
Note: This cross yields four possible phenotypes for only one trait.
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.
E
e
E
EE
Ee
e
Ee
ee
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
B
b
B
BB
Bb
b
Bb
bb
 A polygenic trait is a single trait that is affected by
more than one gene.
 Usually referred to as quantitative inheritance
 Examples:
 Eye color
 Height
 Hair color
 Skin 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 disease
 Causes 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 mice
 Yy genotype produces yellow mice
 YY genotype is lethal!
# of genes
many
1
1
many
# of traits affected
 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>.