1. Extending Mendelian Genetics for two or more genes

2. Epistasis (“stopping”) – a gene at one locus alters the phenotypic expression of a gene at a second locus
Example: Mice…
Black is dominant to brown (B and b)
A second gene determines whether or not pigment will be deposited in the hair. Dominant allele (C) results in color deposition.
If a mouse is homozygous recessive (cc), the color is not deposited, so the mouse is white.
The gene for pigment deposition is said to be epistatic to the gene that codes for black or brown pigment
Epistasis

4. Polygenetic Inheritance
Quantitative Characters – characters that vary in a population along a continuum (in gradations)
Polygenic inheritance – an additive effect of two or more genes on a single phenotypic character
Polygenetic Inheritance

5. Polygenetic Inheritance
Example: skin pigmentation (is controlled by at least three genes)
Dark-skin allele for each gene = A,B,C each contributing one unit of darkness to the phenotype, and are dominant to the alleles a,b,c
AABBCC would be very dark, aabbcc would be very light
Alleles have a cumulative effect, so AABbcc is the same intermediate shade as AaBbCc… both contribute 3 units to skin darkness
Polygenetic Inheritance

7. When a phenotype for a character depends on environment as well as genotype
In humans, for example
Height (nutrition)
Build (exercise)
Skin darkness (Sun-tanning)
Intelligence (experience)
Nature and nurture

8. Norm of Reaction = a range of phenotypic possibilities due to environmental influences
Multifactoral = many factors, both genetic and environmental, collectively influence phenotype
Nature and nurture