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Beyond Mendel
Into the Genetic Frontier
Mendel is the tip of the “genetics
iceberg”
Mendelian Genetics
As people have studied
genetics, they have
realized that the
inheritance of traits is
much more complex than
Mendel’s work with peas
indicated.
Mendelian Genetics
The Rest of Genetics
Polygenic traits

Some traits which have many possible
phenotypes are controlled by more than
one gene.
- the interactions of these genes allows
for the large numbers of phenotypes.
- human examples: Hair, eye and skin
color
The Inheritance of Eye Color
Only partially understood.
 at least 3 different genes with 2 alleles
each are responsible for eye color.
– bey 1 bey 2 and gey
 These genes code for the formation of
the pigment melanin
 The function of only 2 of the 3 genes is
currently understood

The Inheritance of Eye Color as it is
Currently Understood
Known as the 2 gene model
 One gene known as bey 2 has 2 alleles B for
brown eyes and b for blue eyes
 The other gene known as gey has 2 alleles G
codes for green eyes and g for blue eyes

The 2 Gene Model
- B is dominant to all other allelesBBGG,BBGg,BBgg,BbGG
BbGg,Bbgg = Brown eyes
- G is dominant to b
bbGG , bbGg = Green eyes
- bbgg only = blue eyes
Pleiotropy-one gene multiple traits

When a single gene influences multiple
traits this is known as Pleiotropy
example: Marfan’s Syndrome
- a dominant disorder caused by a single gene
- the defective gene causes individuals to be
tall and skinny with very long fingers,
hyper joint mobility, eye and heart defects
More Pleiotropy-Marfans
Pleiotropy is thought to occur when a gene codes
for a protein that is involved in a several processes
Epistasis

The action of one gene influencing the
expression of another.
-Example:
In mice the color is controlled by one gene
with 2 alleles
- the allele for gray (G) is dominant to the
allele for black (g)
- Another gene controls an early stage in the
development of hair pigment
- normal color development (A) is dominant
to no color development(a)
- AA, Aa = color production
- aa = no color production = albino mouse
Epistasis in mice

The gene for color production influences the
expression of the gene for fur color
More Epistasis – Bombay Phenotype

influences the expression of A and B blood
types
- the gene involved is responsible for the
formation of antigens on blood cells.
The dominant form of the gene allows the
formation of antigens
HH or Hh allow antigens to form
The recessive form prevents the formation of
antigens
hh means no antigens form
-If an individual is HH or Hh for blood
antigen formation then:
AA, AO = A blood
BB, BO = B blood
AB = AB blood
-If an individual is hh for blood antigen
formation then:
AA, AO = O blood
BB, BO = O blood
AB = O blood
Environment Can Influence Gene
Expression
example: Siamese cats
- Siamese coloring is a partial
albinism. Most of the cat is white
-Black fur is only expressed in areas
where the temperature is lower than
the rest of the body
- if black hair is shaved and the area kept
then the hair that grows back will be white
- if white hair is shaved and the area is cooled,
then the hair will grow back black
warm