Into the Genetic Frontier Beyond Mendel Into the Genetic Frontier

Mendel is the tip of the “genetics iceberg” Mendelian Genetics Mendelian Genetics The Rest of 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.

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 Evolution of eye color- not understood. One theory melanin was useful in tropical climates, but as people moved north, less sun required less melanin. One other idea, the color blue allows more light to pass through the iris reducing depression in the winter due to SADS

The 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 How to figure out eye genetics – dihybrid cross

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 Marfans is the result of a mutation that affects an enzyme that converts phenylalanine into tyrosine 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 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 The Siamese color is recessive and only shows up in homo recessive cats. The same gene for fur color also controls crossed eyes associated with siamese. Albinism is associated with crossed eyes in many species Pleiotropy