Polygenic Inheritance and Epistasis
POLYGENIC INHERITANCE
Many traits are actually controlled by more than one gene. These are called ______________________.
Examples include human height, intelligence and eye color.
Polygenic traits explain the ______________________ variation that we see in human traits.
Humans eye color, is determined by two genes. The two genes are the OCA2 gene located on
chromosome __________ and the EYCL1 gene on chromosome __________.
The OCA2 gene is the classic eye color gene that students learn about, with a dominant brown allele (B)
and a recessive blue allele (b).
This system does not, however, explain the inheritance of ___________ eyes. The trait of green eyes is
controlled by the EYCL1 gene, which has a dominant green allele (G) and a recessive blue allele (g).
EPISTASIS
Epistasis is the phenomenon where the effects of one gene are modified by one or several other genes,
which are sometimes called ___________________.
The gene whose phenotype is expressed is called ______________, while the phenotype altered or
suppressed is called _________________.
The OCA2 gene(coding for brown) is epistatic over the EYCL1 (green) gene. In other words, the alleles of
OCA2 influence the expression of the alleles of EYCL1.
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If OCA2 genotypes are BB or Bb, the offspring will have____________ eyes, regardless of the
alleles at the EYCL1 gene.
If the OCA2 genotype is bb, the individual will have ___________ eyes or __________ eyes,
depending on the alleles at the EYCL1 gene.
If OCA2 is bb and EYCL1 is GG or Gg, the individual will have___________ eyes.
If OCA2 is bb and EYCL1 is gg, the individual will have ____________ eyes.
This type of epistasis is known as _________________ epistasis.
The following combinations of genetic information show the phenotype possibilities for the various
allele combinations.
Bbgg BBgg BBGg-
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bbGG -
bbGg -
bbgg -
The effect can occur directly at the __________________ level, where one gene could code for a
protein that prevents the production of another protein from the other gene.
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Alternatively, the effect can occur at the ____________________ level. For example, the gene
causing albinism would hide the gene controlling color of a person's hair. In another example, a
gene coding for a widow's peak would be hidden by a gene causing baldness.
EXAMPLES:
Example- 9:7 Ratio (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.
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
___________________________________________________.
Example- 15:1 (Kernel Color in Wheat)
For this type of pathway a functional enzyme A or B can produce a product from a common precursor.
The product gives color to the wheat kernel. Therefore, only one dominant allele at either of the two
loci is required to generate the product.
Thus, if a pure line wheat plant with a colored kernel (genotype = AABB) is crossed to plant with white
kernels (genotype = aabb) and the resulting F1 plants are selfed, a modification of the dihybrid 9:3:3:1
ratio will be produced. The following table provides a biochemical explanation for the 15:1 ratio.
Genotype
Kernel Phenotype
Enzymatic Activities
9 A_B_
colored kernels
functional enzymes from both genes
3 A_bb
colored kernels
functional enzyme from the A gene pair
3 aaB_
colored kernels
functional enzyme from the B gene pair
1 aabb
colorless kernels
non-functional enzymes produced at both genes
If we sum the three different genotypes that will produce a colored kernel we can see that we can
achieve a 15:1 ratio. Because either of the genes can provide the wild type phenotype, this interaction is
called _______________________________________.
Example 3- 12:3:1 Ratio (Fruit Color in Squash)
With this interaction, color is recessive to no color at one allelic pair. This recessive allele must be
expressed before the specific color allele at a second locus is expressed. At the first gene white colored
squash is dominant to colored squash, and the gene symbols are W=white and w=colored. At the second
gene yellow is dominant to green, and the symbols used are G=yellow, g=green. If the dihybrid is selfed,
three phenotypes are produced in a 12:3:1 ratio. The following table explains how this ratio is obtained.
Shapes of Squash Fruit
Genotype
Fruit Color
Gene Actions
9 W_G_
White
Dominant white allele negates effect of G allele
3 W_gg
White
Dominant white allele negates effect of G allele
3 wwG_
Yellow
Recessive color allele allows yellow allele expression
1 wwgg
Green
Recessive color allele allows green allele expression
Because the presence of the dominant W allele masks the effects of either the G or g allele, this type of
interaction is called _______________________________.
Other Examples
1. In a particular species of cats there are 4 eye colour phenotypes: blue, green, amber, and brown. Eye
colour is controlled by 3 genes – genes A, B, and C. If a cat has only recessive alleles (aabbcc), the cat
will have blue eyes. The presence of at least 1 dominant allele for 1 gene, will result in green eyes. (ie:
Aabbcc , and aaBBcc, and aabbCc will all have green eyes.) The presence of at least 1 dominant allele for
2 genes will result in amber eyes (ie. AAbbCc and aaBBCC will both have amber eyes). The presence of at
least 1 dominant allele for all 3 genes will result in brown eyes (AABBCC and AaBbCc will all have brown
eyes). What is the expected phenotypic ratio when a AaBBCc cat mates with a aabbCC cat?
2. The height of spike weed is a result of polygenic inheritance involving three genes, each of which can
contribute 5 cm to the plant. The base height of the weed is 10 cm, and the tallest plant can reach 40
cm.
a. If a tall plant (AABBCC) is crossed with a base-height plant (aabbcc), what is the height of
the F1 plants? Show your work.
b. What are the phenotypes of the offspring?