Genetics Biology 12

Mendel’s laws are operating  alleles are segregating  independent assortment  fertilization is random but…Mendel didn’t look at how complex things can get!

Post-Mendelian studies have revealed that relationships between genotype and phenotype can be complex degrees of dominance multiple alleles epistasis polygenic inheritance environmental effects

Example of incomplete dominance

0Beddin... Snapdragons

Multiple alleles: Individual - 2 alleles for any autosomal gene - one on each homolog Population - gene can exist in more than two forms

A medical problem - some blood transfusions produce lethal clumping of cells. Don’t worry about details yet...

The Human ABO Blood Group System multiple alleles codominance

Table 14.2

Codominance in the Human ABO Blood Group System Dominance Codominance

Fig. 17.4

Possible Dominance Relationships between Two Alleles Dominance RelationshipHeterozygote Phenotype Complete (Mendelian) same as homozygous dominant Incomplete intermediate between the two homozygotes Codominance phenotypes of the two homozygotes are expressed

A mutant gene that caused complete baldness in humans would be epistatic to a gene that determines hair color. Epistasis: one gene alters or blocks the expression of another independently inherited gene. Disclaimer: Made-up example

A a b b dD ee

Epistasis Normal dihybrid ratio is altered from 9:3:3:1 to 9:3:4:0 C/c gene is epistatic to the B/b gene

Epistasis example: Coat color - Labrador retrievers Gene 1 - production of pigment BB - Black Bb - black bb – chocolate Gene 2 - pigment deposition EE or Ee - pigment deposited ee - no pigment deposited

BBEE BBEe BbEE BbEe bbEE bbEe BBee Bbee bbee

BbEe X BbEe BEBebEbe BE Be bE be BBEE BBEe BbEE BbEe BBee BbEE BBEeBbee BbEe Bbee bbEE bbeebbEe

environment often influences phenotype the norm of reaction = phenotypic range due to environmental effects norms of reactions are often broadest for polygenic characters. Blue require low pH

Polygenic inheritance: additive effects (essentially, incomplete dominance) of multiple genes on a single trait. More than one gene controls the outcome of the phenotype. AA = dark Aa = less dark aa - light And similarly for the other two genes - in all cases dominance is incomplete for each gene. Think of each “capital” allele (A, B, C) as adding a dose of brown paint to white paint.