Download Extending Mendelian Genetics for two or more genes

EXTENDING MENDELIAN
GENETICS FOR TWO OR
MORE GENES

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

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

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

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

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