Non-Mendelian Inheritance Patterns
1) Not all traits follow the Dominant/Recessive inheritance pattern as described by
Mendel.
2) Incomplete Dominance
a. Incomplete Dominance is characterized as a blending between the two
characteristics for a given trait.
b. Ex. Red snapdragons (RR) x White snapdragons (WW) result in an F1
generation that is pink (RW)
c. Incomplete dominance is more common in plants than in animals.
d. Incomplete dominance allows for 3 possible phenotypes with only two
alleles.
e. It also makes it very easy to determine the genotype of an organism just by
looking at the phenotype (only works for traits that follow incomplete
dominance). Since neither trait is truly dominant over the other, the trait
that each allele codes for will only show up if the organism has 2 copies of
the same allele (homozygous/purebred).
f. Punnett Square: Red snapdragon x White snapdragon
3) Codominance
a. Codominance is characterized as the expression of both alleles within an
organism.
b. The expression of both alleles does not have to be equal. However, both
are expressed.
c. Ex. Roan cattle (red and white) and Sickle Cell Disease (Humans)
i. Roan cattle (RW) – cross from a red and a white cow, have both
red and white hairs. NO PINK COWS!
ii. Sickle Cell Disease – caused by a 1 base pair change in the gene
coding for hemoglobin. Causes red blood cells to elongate into a
sickle shape. Very common in African-Americans (about 1 in 10
are carriers).
1. Carriers have both normal and sickle red blood cells, have
more normal RBC’s.
2. Homozygous Sickle Cell have only sickle shaped RBC.
3. Thought to be an adaptation against Malaria.
d. Punnett Squares
4) Multiple Alleles
a. Traits that have more than 1 possible allele at a given locus.
b. Ex. ABO Blood type in humans
i. ABO blood type refers to the presence or absence of certain
carbohydrates on the surface of RBC’s. CHO A, CHO B, both A
and B, or neither.
ii. ABO Blood Type – A is dominant to O, B is dominant to O, and A
is codominant to B. (Letter I/i used in genotype for the affected
protein, immunoglobin)
1. Genotype IAIA , IAi = Type A blood phenotype
2. Genotype IBIB , IBi = Type B blood phenotype
3. Genotype i = Type O blood phenotype (no CHO)
4. Genotype IAIB = Type AB blood phenotype (Both)
c. Punnett Sqaure
5) Pleiotropy
a. Pleiotropy is characterized as genes that have multiple phenotypic effectts.
b. An example in humans is with cystic fibrosis and the sickle-cell disease.
6) Some inheritance patterns extended even beyond focusing on the alleles of a
single gene. There are inheritance patterns that involve 2 or more genes when
determining phenotype.
7) Epistasis
a. Epistasis is characterized as a gene at one locus that can alter the
phenotypic expression of a gene at a second locus.
b. 2 genes involved. Both genes are at different loci.
c. Since 2 loci are involved, Punnet Squares are all dihybrid. All epistatic
interactions will have modified 9:3:3:1 ratios
d. This occurs with the coat color for many mammals.
e. Ex. Mice: Black coat color (B) is dominant to brown (b). Color is
dominant(C) to albino (c). So for a mouse with the genotype cc
(phenotype albino), the coat is white regardless of the genotype at the
other locus. This results in a phenotypic ratio of 9:3:4
f. Eye color in humans follows epistatic interaction.
8) Polygenetic Inheritance
a. Polygenetic inheritance is characterized as an additive effect of two or
more genes on a single phenotypic character. These are usually called
quantitative characters.
b. Ex. Skin pigmentation in humans. Controlled by at least 3 separately
inherited genes