Variations on Mendel’s Laws: Patterns of Inheritance Beyond Mendel

Variations on Mendel’s Laws

Incomplete Dominance Results in Intermediate Phenotypes

Mendel’s laws describe inheritance patterns in sexually reproducing species, but not all traits follow the simple dominant-recessive relationships he observed. In some cases, the phenotype of heterozygotes is intermediate between those of the two homozygotes, a phenomenon known as incomplete dominance.

• Complete dominance: The phenotype of the heterozygote is identical to one of the homozygotes. Example: Mendel’s pea plants.

• Incomplete dominance: The F1 hybrid shows a phenotype that is intermediate between the two parental varieties.

• Example: Crossing red-flowered (RR) and white-flowered (rr) snapdragons produces pink-flowered (Rr) offspring in the F1 generation.

Genetic Cross Example

• P generation: RR (red) × rr (white)

• F1 generation: All Rr (pink)

• F2 generation: 1 RR (red) : 2 Rr (pink) : 1 rr (white)

Checkpoint: The cross does not support the blending hypothesis because the parental phenotypes reappear in the F2 generation, showing that alleles remain distinct.

Many Genes Have More Than Two Alleles That May Be Codominant

Some genes have more than two alleles in the population, and certain alleles may be codominant, meaning both are fully expressed in heterozygotes. The ABO blood group in humans is a classic example.

• Three alleles: IA, IB, i

• Four phenotypes: A, B, AB, O

• IA and IB are codominant; i is recessive.

• Example: Individuals with genotype IAIB have AB blood, expressing both carbohydrate A and B on red blood cells.

ABO Blood Group Table

Blood Type	Genotype(s)	Carbohydrates on RBCs	Antibodies in Blood	Reaction with Anti-A	Reaction with Anti-B
A	IAIA or IAi	Carbohydrate A	Anti-B	Clumping	No reaction
B	IBIB or IBi	Carbohydrate B	Anti-A	No reaction	Clumping
AB	IAIB	A and B	None	Clumping	Clumping
O	ii	Neither	Anti-A, Anti-B	No reaction	No reaction

A Single Gene May Affect Many Phenotypic Characters (Pleiotropy)

Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits. A well-known example in humans is sickle-cell disease.

• Sickle-cell disease affects hemoglobin structure, red blood cell shape, causes anemia, and organ damage.

• Sickle-cell and normal alleles are codominant at the molecular level.

• Carriers (heterozygotes) have increased resistance to malaria.

Additional info: Pleiotropy is common in genetic disorders, where a single mutation can have widespread effects throughout the body.