12.2 Exceptions to Mendelian Inheritance | Inheritance | Biology 11

Mendel's laws of segregation and independent assortment explain many inheritance patterns, but several phenomena deviate from simple Mendelian ratios. These exceptions arise from more complex allelic interactions and gene interactions.

1. Incomplete Dominance

In incomplete dominance, neither allele is completely dominant over the other. The heterozygote shows an intermediate phenotype — a blend of the two homozygous phenotypes.

Classic Example: 4 O'Clock Plant (Mirabilis jalapa)

Cross	Genotype	Phenotype
Red parent	$R_{1} R_{1}$	Red
White parent	$R_{2} R_{2}$	White
$F_{1}$ offspring	$R_{1} R_{2}$	Pink

When $F_{1}$ pink plants are self-crossed: $R_{1} R_{2} \times R_{1} R_{2} \to 1 R_{1} R_{1} : 2 R_{1} R_{2} : 1 R_{2} R_{2}$ Phenotypic ratio: 1 Red : 2 Pink : 1 White

Note: The genotypic and phenotypic ratios are identical (1:2:1) in incomplete dominance, unlike in complete dominance.

2. Codominance

In codominance, both alleles are fully and equally expressed in the heterozygote. There is no blending — both phenotypes appear simultaneously.

Example: ABO Blood Group AB

A person with genotype $I^{A} I^{B}$ expresses both A antigens and B antigens on their red blood cells. Neither allele is dominant over the other.

Genotype	Phenotype
$I^{A} I^{A}$ or $I^{A} i$	Blood Group A
$I^{B} I^{B}$ or $I^{B} i$	Blood Group B
$I^{A} I^{B}$	Blood Group AB (Codominance)
$ii$	Blood Group O

Incomplete Dominance vs. Codominance

Feature	Incomplete Dominance	Codominance
Heterozygote phenotype	Intermediate blend	Both parental phenotypes expressed
Example	Pink flowers in Mirabilis	AB blood group
Allele expression	Partial	Full and equal

3. Multiple Alleles

Mendel worked with genes having only two alleles. However, many genes have more than two allelic forms in a population — this is called multiple allelism.

A single individual still carries only two alleles (one per homologous chromosome), but the gene pool contains more than two variants.

Example: ABO Blood Group System

The ABO gene has three alleles: $I^{A}$ , $I^{B}$ , and $i$ .

$I^{A}$ and $I^{B}$ are codominant to each other
Both $I^{A}$ and $I^{B}$ are dominant over $i$

Number of possible genotypes with 3 alleles: $\frac{n ( n + 1 )}{2} = \frac{3 \times 4}{2} = 6 genotypes$

The 6 genotypes are: $I^{A} I^{A}$ , $I^{A} i$ , $I^{B} I^{B}$ , $I^{B} i$ , $I^{A} I^{B}$ , $ii$

4. Pleiotropy

Pleiotropy occurs when a single gene affects multiple, seemingly unrelated phenotypic traits.

Example: Sickle Cell Anemia

A single point mutation in the $β$ -globin gene (substituting valine for glutamic acid at position 6) causes:

Abnormal sickle-shaped red blood cells
Reduced oxygen-carrying capacity
Increased blood viscosity and clotting
Damage to spleen, kidneys, and other organs
Anemia and fatigue

All of these diverse effects stem from one gene mutation — a hallmark of pleiotropy.

5. Epistasis

Epistasis is an inter-genic interaction where one gene (the epistatic gene) masks or suppresses the expression of another gene (the hypostatic gene) at a different locus.

Epistasis differs from dominance: dominance involves alleles at the same locus, while epistasis involves genes at different loci.

Types of Epistasis

Type	Description	Phenotypic Ratio (dihybrid)
Dominant Epistasis	Dominant allele of one gene masks expression of another gene	12:3:1
Recessive Epistasis	Homozygous recessive at one locus masks expression of another gene	9:3:4
Duplicate Dominant	Dominant allele at either locus produces same phenotype	15:1

Example of Recessive Epistasis: Coat Color in Labrador Retrievers

Gene B controls pigment (B = black, b = brown)
Gene E controls pigment deposition (E = pigment deposited, ee = no pigment → yellow)
When a dog is ee, no pigment is deposited regardless of the B gene → yellow coat

Summary Table

Exception	Key Feature	Example
Incomplete Dominance	Intermediate blended phenotype in heterozygote	Pink flowers in Mirabilis jalapa
Codominance	Both alleles fully expressed in heterozygote	AB blood group
Multiple Alleles	>2 alleles for one gene in population	ABO blood group ( $I^{A}$ , $I^{B}$ , $i$ )
Pleiotropy	One gene → multiple unrelated traits	Sickle Cell Anemia
Epistasis	One gene masks another non-allelic gene	Coat color in Labradors

1. Incomplete Dominance

In incomplete dominance, neither allele is completely dominant over the other. The heterozygote shows an intermediate phenotype — a blend of the two homozygous phenotypes.

Classic Example: 4 O'Clock Plant (Mirabilis jalapa)

Cross	Genotype	Phenotype
Red parent	$R_{1} R_{1}$	Red
White parent	$R_{2} R_{2}$	White
$F_{1}$ offspring	$R_{1} R_{2}$	Pink

When $F_{1}$ pink plants are self-crossed: $R_{1} R_{2} \times R_{1} R_{2} \to 1 R_{1} R_{1} : 2 R_{1} R_{2} : 1 R_{2} R_{2}$ Phenotypic ratio: 1 Red : 2 Pink : 1 White

Note: The genotypic and phenotypic ratios are identical (1:2:1) in incomplete dominance, unlike in complete dominance.

2. Codominance

In codominance, both alleles are fully and equally expressed in the heterozygote. There is no blending — both phenotypes appear simultaneously.

Example: ABO Blood Group AB

A person with genotype $I^{A} I^{B}$ expresses both A antigens and B antigens on their red blood cells. Neither allele is dominant over the other.

Genotype	Phenotype
$I^{A} I^{A}$ or $I^{A} i$	Blood Group A
$I^{B} I^{B}$ or $I^{B} i$	Blood Group B
$I^{A} I^{B}$	Blood Group AB (Codominance)
$ii$	Blood Group O

Incomplete Dominance vs. Codominance

Feature	Incomplete Dominance	Codominance
Heterozygote phenotype	Intermediate blend	Both parental phenotypes expressed
Example	Pink flowers in Mirabilis	AB blood group
Allele expression	Partial	Full and equal

3. Multiple Alleles

Mendel worked with genes having only two alleles. However, many genes have more than two allelic forms in a population — this is called multiple allelism.

A single individual still carries only two alleles (one per homologous chromosome), but the gene pool contains more than two variants.

Example: ABO Blood Group System

The ABO gene has three alleles: $I^{A}$ , $I^{B}$ , and $i$ .

$I^{A}$ and $I^{B}$ are codominant to each other
Both $I^{A}$ and $I^{B}$ are dominant over $i$

Number of possible genotypes with 3 alleles: $\frac{n ( n + 1 )}{2} = \frac{3 \times 4}{2} = 6 genotypes$

The 6 genotypes are: $I^{A} I^{A}$ , $I^{A} i$ , $I^{B} I^{B}$ , $I^{B} i$ , $I^{A} I^{B}$ , $ii$

4. Pleiotropy

Pleiotropy occurs when a single gene affects multiple, seemingly unrelated phenotypic traits.

Example: Sickle Cell Anemia

A single point mutation in the $β$ -globin gene (substituting valine for glutamic acid at position 6) causes:

Abnormal sickle-shaped red blood cells
Reduced oxygen-carrying capacity
Increased blood viscosity and clotting
Damage to spleen, kidneys, and other organs
Anemia and fatigue

All of these diverse effects stem from one gene mutation — a hallmark of pleiotropy.

5. Epistasis

Epistasis is an inter-genic interaction where one gene (the epistatic gene) masks or suppresses the expression of another gene (the hypostatic gene) at a different locus.

Epistasis differs from dominance: dominance involves alleles at the same locus, while epistasis involves genes at different loci.

Types of Epistasis

Type	Description	Phenotypic Ratio (dihybrid)
Dominant Epistasis	Dominant allele of one gene masks expression of another gene	12:3:1
Recessive Epistasis	Homozygous recessive at one locus masks expression of another gene	9:3:4
Duplicate Dominant	Dominant allele at either locus produces same phenotype	15:1

Example of Recessive Epistasis: Coat Color in Labrador Retrievers

Gene B controls pigment (B = black, b = brown)
Gene E controls pigment deposition (E = pigment deposited, ee = no pigment → yellow)
When a dog is ee, no pigment is deposited regardless of the B gene → yellow coat

Summary Table

Exception	Key Feature	Example
Incomplete Dominance	Intermediate blended phenotype in heterozygote	Pink flowers in Mirabilis jalapa
Codominance	Both alleles fully expressed in heterozygote	AB blood group
Multiple Alleles	>2 alleles for one gene in population	ABO blood group ( $I^{A}$ , $I^{B}$ , $i$ )
Pleiotropy	One gene → multiple unrelated traits	Sickle Cell Anemia
Epistasis	One gene masks another non-allelic gene	Coat color in Labradors