Mendelian Exceptions
Exceptions to Mendel’s Rules
Incomplete Dominance
When the heterozygous genotype of a trait shows neither a dominant or recessive phenotype, but rather a mixture.
Ex. Carnation flowers – dominant red allele and recessive white allele results in pink phenotype
Incomplete Dominance
Incomplete Dominance
Codominance
When the heterozygous genotype expresses both the dominant and recessive phenotypes at the same time
Ex. Blood types – this is how you can have an AB blood type
Codominance
Codominant
Blood Type A Blood Type B
A allele
B allele O allele
Blood Type A Blood Type AB (Codominant)
U.S. National Library of Medicine
Blood Type B
Blood Type O
Multiple Alleles
More than two different alleles can exist for a particular trait, but only two can be present at any given time.
Ex. Blood Types –
This is how you can have A, B, AB, or O type blood.
Phenotype O A
B AB
Genotypes ii
IAIA, IAi
IBIB, IBi IAIB
Blood Type Cross
Parents: AO x BO
A
O
Offspring Phenotypes: ¼ Type A ¼ Type B ¼ Type AB ¼ Type O
B AB BO O AO OO
Blood Type Cross
I
B
A
I
B
B
I I I I I
A
A B
B B
i I i I i
Polygenic Inheritance
When a phenotypic result is controlled by a cumulative result of the effects of many genes
Examples:
Height Weight Skin Color Hair Color Eye Color
Polygenic Inheritance
P generation X aabbcc
(very light)
AABBCC
(very dark)
X AaBbCc
F1 generation AaBbCc
Links with additional material
Mendelian Genetics