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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