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Medelian Exceptions

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					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


				
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Lynne Conner Lynne Conner
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