Non-Mendelian Genetics
Interactive Punnett Squares detailing Incomplete Dominance (Pink Flowers), Codominance (Roan Cattle), and Multiple Alleles (ABO Blood Types).
BEYOND MENDEL: NON-MENDELIAN INHERITANCE
While Mendel's laws describe simple dominant and recessive traits, many patterns of inheritance are far more complex. These traits do not follow the expected 3:1 phenotypic ratios and often involve interactions between multiple alleles or genes.
KEY PATTERNS OF INHERITANCE
1. **Incomplete Dominance**: The heterozygote has an intermediate phenotype (e.g., Red x White = Pink). 2. **Codominance**: Both alleles are expressed simultaneously (e.g., AB Blood Type). 3. **Multiple Alleles**: A gene has more than two possible versions (e.g., ABO blood group system). 4. **Polygenic Inheritance**: A single trait is controlled by multiple genes (e.g., human skin color or height), resulting in a continuous range of phenotypes.
HOW TO USE THIS VISUALIZATION
1. **Cross the Flowers**: Breed snapdragons with Red and White alleles to see the intermediate Pink offspring. 2. **Analyze Blood Types**: Cross parents with and alleles to see how both antigens appear on the cell surface. 3. **Graph the Population**: Observe the bell curve distribution generated by polygenic traits. **Try This**: Cross two Pink flowers. Notice how the Red and White phenotypes reappear in a 1:2:1 ratio. Why does this prove that the alleles didn't 'blend' but remained distinct?
AP EXAM CONNECTION
Unit: Unit 5: Heredity (Topic 5.4)
Learning Objective: SYI-3.B
COMMON MISCONCEPTIONS
- Thinking codominance and incomplete dominance are the same.
- Believing 'dominant' means 'most common' (it only refers to the expression of the allele).
- Assuming all traits have only two possible alleles.
KEY TAKEAWAYS
- Incomplete dominance results in a blend.
- Codominance expresses both alleles.
- Polygenic traits show continuous variation.
- Ratios vary from standard Mendelian predictions.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): A cross between a black chicken and a white chicken produces offspring with both black and white feathers. Is this incomplete dominance or codominance?
Show Answer & Explanation
Answer: Codominance.
Explanation: Because both original phenotypes are visible simultaneously in the offspring, the alleles are codominant.
Q2 (CONCEPTUAL): Human height is controlled by dozens of different genes. What is the name for this pattern of inheritance?
Show Answer & Explanation
Answer: Polygenic inheritance.
Explanation: Traits that show continuous variation (a spectrum) are almost always controlled by the additive effects of multiple genes.
