Exceptions to Rule 2 - Biology

1. Fundamental Concepts

Definition: Mendel's rules of inheritance describe the patterns of genetic traits passed from parents to offspring, but there are exceptions where these rules do not apply.
Rule 2 Exception: Incomplete dominance and codominance are examples where the phenotype of the heterozygote is neither fully dominant nor recessive.
Codominance Example: Blood types in humans (AB blood type results from codominance of A and B alleles).

2. Key Concepts

Incomplete Dominance: $$(Aa) \rightarrow \text{{Intermediate}}$$

Codominance: $$(Aa) \rightarrow \text{{Both traits expressed}}$$

Application: Used to explain variations in traits that do not follow simple dominant/recessive patterns

3. Examples

Example 1 (Basic)

Problem: If a red-flowered plant (RR) is crossed with a white-flowered plant (WW), what would be the expected phenotype of the F1 generation?

Step-by-Step Solution:

The genotype of the F1 generation will be Rr.
Since this is an example of incomplete dominance, the phenotype will be pink flowers.

Validation: The F1 generation shows an intermediate phenotype between red and white, which is pink.

Example 2 (Intermediate)

Problem: In chickens, the allele for black feathers (B) is incompletely dominant over the allele for white feathers (b). What would be the expected phenotype of the offspring if a black-feathered chicken is crossed with a white-feathered chicken?

Step-by-Step Solution:

The genotype of the offspring will be Bb.
Since this is an example of incomplete dominance, the phenotype will be gray feathers.

Validation: The F1 generation shows an intermediate phenotype between black and white, which is gray.

4. Problem-Solving Techniques

Punnett Square Method: Use Punnett squares to visualize possible genotypes and phenotypes.
Phenotype Observation: Observe the physical traits of organisms to determine the presence of incomplete dominance or codominance.
Genetic Ratios: Understand the ratios of different genotypes and phenotypes in the offspring to predict outcomes.