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.
