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

www.florifacts.umn.edu/Trials/Annual%20Beddin...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

Dominance

Codominance

Fig. 17.4

Possible Dominance Relationships between Two Alleles

Dominance Relationship Heterozygote 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

d D

e e

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 pigmentBB - BlackBb - blackbb – chocolate

Gene 2 - pigment depositionEE or Ee - pigment depositedee - no pigment deposited

BBEEBBEeBbEEBbEe

bbEEbbEe

BBeeBbeebbee

BbEe X BbEe

BE Be bE

be

BE

Be

bE

be

BBEE BBEe BbEE

BbEe

BbEe

BBee

BbEE

BBEe Bbee

BbEe

BbEe

Bbee

bbEE

bbeebbEe

bbEe

• 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.