Beyond MendelBeyond MendelInto the Genetic Frontier

Mendelian Genetics

Mendel is the tip of the Mendel is the tip of the “genetics iceberg”“genetics iceberg”

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 traitsPolygenic 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 The Inheritance of Eye Color Color Only partially understood. at least 3 different genes with 2

alleles each are responsible for eye color.

– bey 1 bey 2 and geyThese genes code for the

formation of the pigment melaninThe function of only 2 of the 3

genes is currently understood

The Inheritance of Eye Color The Inheritance of Eye Color as it is Currently Understoodas it is Currently Understood

Known as the 2 gene modelOne gene known as bey 2 has 2 alleles B

for brown eyes and b for blue eyesThe other gene known as gey has 2

alleles G codes for green eyes and g for blue eyes

The 2 Gene ModelThe 2 Gene Model

- B is dominant to all other alleles- BBGG,BBGg,BBgg,BbGG BbGg,Bbgg = Brown eyes

- G is dominant to b bbGG , bbGg = Green eyes

- bbgg only = blue eyes

Pleiotropy-one gene multiple Pleiotropy-one gene multiple traitstraitsWhen 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-MarfansMore Pleiotropy-Marfans

Pleiotropy is thought to occur when a gene codes for a protein that is involved in a several processes

EpistasisEpistasisThe 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 miceEpistasis in mice

The gene for color production influences the expression of the gene for fur color

More Epistasis – More Epistasis – Bombay Bombay PhenotypePhenotype

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 Environment Can Influence Gene ExpressionGene 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 warm 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