Law of Dominance - Biology

1. Fundamental Concepts

Definition: The Law of Dominance states that in a heterozygous organism, one allele (the dominant allele) will be expressed while the other (the recessive allele) will not.
Dominant Allele: An allele whose trait always shows up in the organism's phenotype if present.
Recessive Allele: An allele whose trait is hidden when a dominant allele is present.

2. Key Concepts

Basic Rule: $${\text{{P}}} \cdot {\text{{p}}} = {\text{{P}}}$$ (In a heterozygous pair, the dominant trait P masks the recessive trait p.)

Dominance Example: $${\text{{Tall}}} ({\text{{T}}}) \cdot {\text{{Short}}} ({\text{{t}}}) = {\text{{Tall}}} ({\text{{T}}})$$ (If an organism has one tall gene T and one short gene t, it will be tall.)

Application: Used to predict traits in offspring based on parental genotypes

3. Examples

Example 1 (Basic)

Problem: If a pea plant with genotype $$\text{{Tt}}$$ is crossed with another plant with genotype $$\text{{Tt}}$$, what are the possible genotypes of the offspring?

Step-by-Step Solution:

Create a Punnett square:

              |   | T | t |            --|---|---|---|              T | TT | Tt |            --|---|---|---|              t | Tt | tt |

The possible genotypes are $$\text{{TT}}$$, $$\text{{Tt}}$$, and $$\text{{tt}}$$.

Validation: Since T is dominant over t, plants with genotypes $$\text{{TT}}$$ and $$\text{{Tt}}$$ will be tall, and only those with $$\text{{tt}}$$ will be short.

Example 2 (Intermediate)

Problem: Given two pea plants with genotypes $$\text{{Tt}}$$ and $$\text{{tt}}$$, determine the probability that their offspring will be short.

Step-by-Step Solution:

Create a Punnett square:

              |   | T | t |            --|---|---|---|              t | Tt | tt |            --|---|---|---|              t | Tt | tt |

The possible genotypes are $$\text{{Tt}}$$ and $$\text{{tt}}$$. Since $$\text{{tt}}$$ is the only genotype for a short plant, the probability is $$\frac{1}{2}$$.

Validation: In this cross, half of the offspring will have the genotype $$\text{{tt}}$$ and thus be short.

4. Problem-Solving Techniques

Punnett Square Method: Use a Punnett square to visualize all possible combinations of alleles from parents.
Probability Calculation: Calculate the probability of specific genotypes by counting favorable outcomes over total possible outcomes.
Phenotype Prediction: Determine the phenotype based on the dominance of alleles.