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.

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- 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.


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 F
1
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.
Enzyme Activities/TH> Genotype
9 C_P_
Flower Color
Flowers colored; anthocyanin produced
Functional enzymes from both genes
3 C_pp
Flowers white; no anthocyanin produced p enzyme non-functional
3 ccP_
1 ccpp
Flowers white; no anthocyanin produced
Flowers white; no anthocyanin produced c enzyme non-functional 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 F
1
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_
3 A_bb
3 aaB_
1 aabb colored kernels colored kernels colored kernels colorless kernels functional enzymes from both genes functional enzyme from the A gene pair functional enzyme from the B gene pair 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
3 W_gg White
3 wwG_ Yellow
1 wwgg Green
Dominant white allele negates effect of G allele
Dominant white allele negates effect of G allele
Recessive color allele allows yellow allele expression
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. a) In the case of the eye genes EYCL1 and OCA2, calculate the phenotype ratios of the expected eye colors of the offspring of two heterozygotes, BbGg.

P: BbGg x BbGg
BG
BG
Bg bG bg
Bg bG bg
Phenotypes:
2. The pigment in our skin is determined by three genes. The phenotype for very dark skin is determined by the expression of all dominant alleles (AABBCC). The phenotype for very light skin is expressed when only recessive genes are expressed (aabbcc). Every dominant gene being expressed adds an additional unit of melanin (pigment) to an individual’s skin colour. a)Draw a Punnett square for an individual with very dark skin (6units) and an individual that has a medium shade of skin (Expresses 2 units for the A gene and 1 unit for B gene).
P:____________________________
Gametes: ______________________ b)What are the phenotypes of the offspring?