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Multiple Alleles, Epistasis, Collaborative, Complementary, Pleiotropic, Modifier
genes.
www.ndsu.nodak.edu/instruct/mcclean
go to intermediate genetics, course topics.
Some traits are determined by a single set of alleles. When more than two allelic
forms of a gene exist these are known as multiple alleles.
Ex: ABO blood types.
A
AB
B
O
A A
A i
A B
B B
B i
I I /I I
I I
I I /I I
I iI i
O is the universal Donor
AB is the universal Acceptor
Most genes have multiple phenotypic effects. This is called pleiotropy.
Pleiotropic alleles are responsible for multiple symptoms inEx: Sickle cell disease. Genetic defect causes crystallization of hemoglobin at high
altitudes/stress resulting in sickle shape of RBCs. These block bvs which leads to
pain, organ damage, paralysis.
In heterozygotes the alleles are codominant-both proteins are made.
(sickle cell trait).
Cystic Fibrosis. Defect in membrane protein involved in Cl- ion transport. Defect
leads to Cl ion build up in extracellular fluid and causes mucus coating cells to build
up in lungs, pancreas, digestive tract resulting in poor absorption of nutrients and
recurring infections.
Epistasis. This is when a gene at one locus (site), affects the phenotypic expression
of a gene at a second site.
Ex: coat color. Organism can have alleles for Black -B/ brown-b but whether
pigment is actually laid down is determined by a second gene, C= Color and c = no
color.
If cc present then organism will be albino regardless of whether BB, Bb or bb
present.
Ex: BbCc x BbCc gives 9:3:3:1 ratio.
The ratios can vary depending upon epistatic interactions.
When two or more genes affect a single phenotype and have an additive effect this
is known as polygenic inheritance.
Ex: skin color. Controlled by three or more separately inherited genes.
Dark: ABC
Light: abc
AABBCC – very dark.
aabbcc – very light.
Environmental factors also play a significant role in determining the phenotype of
an organism. This leads to the question of Nature vs Nurture.
The genotype gives you a range of possible phenotypes which are open to change by
the environment rather than a fixed phenotype. This range of phenotype is called
the norm of reaction. The norms of reaction are the broadest for traits controlled
by two or more genes (polygenic) such as skin color. However, for a specific
phenotype such as Blood group the norm of reaction has no range. But, the numbers
of WBCs and RBCs can vary depending on an individual’s regular level of physical
fitness, infective state or altitude.
As these traits are polygenic and influenced by environmental factors they are
called multifactorial.
The words Genotype and phenotype can be used specifically to look at a particular
trait or more broad aspects of an organism’s genetic make up.
Genotype can be used to look at:
1.
Entire genetic map,
2.
Alleles of a single locus,
3.
Impact of a gene on phenotype and how it is influenced by other genes
and environment.
Phenotype can be used to look at:
1.
Entire organism,
2.
Specifically anatomy, physiology or behavior,
3.
Particular traits.
Collaborative genes.
Chicken Varieties
Phenotype
Wyandotte
Rose Comb
Brahmas
Pea Comb
Leghorns
Single Comb
Rose
Single
Pea
Walnut
Result: The F1 differed from both parents and two new phenotypes not seen in the
parents appeared in the F2. How can this result be explained? The first clue is the
F2 ratio. We have seen this ratio before when the F1 from a dihybrid cross is selfed
(or intermated). This observation suggests that two genes may control the
phenotype of the comb. The gene interactions and genotypes were determined by
performing the appropriate testcrosses.
A series of experiments demonstrated that the genotypes controlling the various
comb phenotypes are as follows.
Phenotypes
Walnut
Rose
Pea
Single
Genotypes
R_P_
R_pp
rrP_
rrpp
Frequency
9/16
3/16
3/16
1/16
It was later shown that the genotypes of the initial parents were:
Rose = RRpp
Pea = rrPP
Therefore, genotypically the cross was:
Collaboration - the interaction between two or more genes to control a single
phenotype resulting in a new phenotype.
The interactions of the two genes which control comb type was revealed because
we could identify and recognize the 9:3:3:1. Other genetic interactions were
identified because the results of crossing two dihybrids produced a modified
Mendelian ratio. All of the results are modifications of the 9:3:3:1 ratio.
Pleiotropic Gene.
- coat color and viability.
During the first years after the rediscovery of Mendel's laws, a number of
experiments were performed that gave results that at first glance did not coincide
with the laws. In 1904, a cross was made between a yellow-coated mouse and a
mouse with a gray coat. The gray- coated mouse was extensively inbred and
therefore was considered to be pure bred.
P gen; Y_ x yy
Gave
F1 gen; 1 Y_ : 1 yy
What allelic relationship do we have here? We know that the gray mouse is
homozygous (because it is a pure line). If gray coat was dominant then we would see
all gray mouse. Since we obtain both yellow and gray mice, yellow must be dominant
to gray. So what are the genotypes of the two mice populations? First, let's provide
gene symbols.
Gene Symbols:
gray = y
yellow = Y
From the above discussion, the genotype of the gray mouse must be yy. What is the
genotype of the yellow mouse? If the mouse was homozygous we would not see any
gray mice from the cross, therefore the genotype must be heterozygous or Yy.
Next a cross was made between two yellow mice. What genetic ratio would we
expect to see? Yy x Yy should give a ratio of 3 yellow:1 gray. The result, though,
was a ratio of 2 yellow to 1 gray mice. How can this result be explained? Let's
first set up a Punnett Square.
Expected Punnett Square
Female Gametes
Male
Gametes
Y
y
Y
y
YY (yellow)
Yy (yellow)
Yy (yellow)
yy ( gray)
As we can see, we should get a 3:1 ratio of yellow to gray mice. Could some genotype
be absent from the progeny. How can we test the genotypes of the yellow mice,
since we already know the genotypes of the gray mice are yy. Testcross!! All
testcross data with the yellow mice give a 1:1 ratio. This ratio is typical of what is
seen with heterozygous individuals. Therefore, all of the yellow mice from the cross
of two heterozygous yellow mice are genotypically Yy. Somehow the YY genotype is
lethal. The 2:1 ratio is the typical ratio for a lethal gene.
Coat Color in Mice
Lethal Gene - a gene that leads to the death of an individual; these can be either
dominant or recessive in nature
An important question is how can a gene controlling coat color cause death in an
organism? Possibly in a single dose the allele causes a yellowing of the coat, but
when expressed in two doses, the gene product kills the animal. Thus, this gene
actually has an effect on two phenotypes.
In this example the gene that causes yellowing of the coat also affects viability and
is termed a pleiotropic gene.
Complementary genes
9:7 Ratio Example: Flower color in sweet pea
If two genes are involved in a specific pathway and functional products from both
are required for expression, then one recessive allelic pair at either allelic pair
would result in the mutant phenotype. This is graphically shown in the following
diagram.
If a pure line pea plant with colored flowers (genotype = CCPP) is crossed to pure
line, homozygous recessive plant with white flowers, the F1 plant will have colored
flowers and a CcPp genotype. The normal ratio from selfing dihybrid is 9:3:3:1, but
epistatic interactions of the C and P genes will give a modified 9:7 ratio. The
following table describes the interactions for each genotype and how the ratio
occurs
Genotype
Flower Color
Enzyme Activities/TH>
9 C_P_
Flowers colored;
anthocyanin produced
Functional enzymes from both genes
3 C_pp
Flowers white;
no anthocyanin produced
p enzyme non-functional
3 ccP_
Flowers white;
no anthocyanin produced
c enzyme non-functional
1 ccpp
Flowers white;
no anthocyanin produced
c and p enzymes non-functional
Because both genes are required for the correct phenotype, this epistatic
interaction is called complementary gene action.
Modifier Genes
Instead of masking the effects of another gene, a gene can modify the expression
of a second gene. In mice, coat color is controlled by the B gene. The B allele
conditions black coat color and is dominant to the b allele that produces a brown
coat. The intensity of the color, either black or brown is controlled by another
gene, the D gene. At this gene, the dominant D allele controls full color whereas the
recessive d allele conditions a dilute or faded expression of the color expression at
the B gene. Therefore, if a cross is made among mice that are BdDd, the following
phenotypic distribution will be seen:
9 B_D_ (black)
3 B_dd (dilute black)
3 bbD_ (brown)
1 bbdd (dilute brown)
The D gene does not mask the effect of the B gene, rather it modifies its
expression.
Modifier genes - genes that have small quantitative effects on the level of
expression of another gene
Copyright © 2000. Phillip McClean