Exceptions to Rule 1 - Biology

1. Fundamental Concepts

Definition: Mendel's rules of inheritance are based on the idea that traits are passed from parents to offspring in a predictable manner. However, there are exceptions where these rules do not apply.
Rule 1 (Law of Segregation): Each organism inherits one allele for each trait from each parent, and these alleles segregate during gamete formation.
Exceptions: Certain genetic phenomena violate this rule, such as incomplete dominance, codominance, multiple alleles, and pleiotropy.

2. Key Concepts

Incomplete Dominance: $F_1$ hybrids show an intermediate phenotype between the two parental phenotypes.

Codominance: Both alleles are fully expressed in the heterozygote.

Pleiotropy: A single gene affects multiple phenotypic traits.

3. Examples

Example 1 (Basic)

Problem: In snapdragons, flower color is determined by incomplete dominance. Red flowers ($RR$) and white flowers ($rr$) produce pink flowers ($Rr$). If you cross two pink flowers, what is the expected genotype ratio?

Step-by-Step Solution:

Set up the Punnett square with $Rr \times Rr$.
The genotypes will be $RR$, $Rr$, $Rr$, $rr$.
The expected ratio is $1RR : 2Rr : 1rr$.

Validation: The observed ratio matches the expected ratio of $1:2:1$.

Example 2 (Intermediate)

Problem: In humans, the $ABO$ blood group system involves multiple alleles. If a person with type $AB$ blood has one parent with type $A$ blood and another with type $B$ blood, what are the possible genotypes of the child?

Step-by-Step Solution:

Parent genotypes: Type $A$ could be $AA$ or $AO$, and Type $B$ could be $BB$ or $BO$.
For simplicity, assume the parents are $AO$ and $BO$.
The possible genotypes of the child are $AB$, $AO$, $BO$, and $OO$.

Validation: The child can have blood types $A$, $B$, $AB$, or $O$.

4. Problem-Solving Techniques

Punnett Square Method: Use a Punnett square to visualize all possible combinations of alleles.
Phenotype Analysis: Observe the physical traits and use them to infer the genotype.
Genetic Ratios: Apply the ratios derived from Mendelian genetics to predict outcomes.