Law of Unit Character Inheritance - Biology

1. Fundamental Concepts

Definition: The Law of Unit Character Inheritance, also known as the Law of Segregation, states that each individual possesses two factors (alleles) for each characteristic, which segregate during gamete formation.
Dominant and Recessive Alleles: One allele may be dominant over another, which is recessive. Dominant alleles are expressed in the phenotype when present, while recessive alleles are only expressed if both alleles are recessive.
Punnett Square: A visual representation used to predict the probability of inheriting certain traits from parents.

2. Key Concepts

Basic Rule: $${\text{{Each parent contributes one factor to the offspring.}}}$$

Dominance and Recessiveness: $${\text{{If an organism has one dominant and one recessive allele, it will exhibit the trait associated with the dominant allele.}}}$$

Punnett Square Application: $${\text{{Used to determine the genotypes and phenotypes of offspring based on parental genotypes.}}}$$

3. Examples

Example 1 (Basic)

Problem: If a pea plant with genotype $$\text{{Tt}}$$ (where $$\text{{T}}$$ is the dominant allele for tall plants and $$\text{{t}}$$ is the recessive allele for short plants) is crossed with a homozygous recessive plant ($$\text{{tt}}$$), what are the possible genotypes and phenotypes of the offspring?

Step-by-Step Solution:

Create a Punnett square:

              T | t              -|-              t| Tt | tt

The possible genotypes are $$\text{{Tt}}$$ and $$\text{{tt}}$$. Since $$\text{{T}}$$ is dominant, all offspring will have the phenotype of tall plants.

Validation: All offspring have the genotype $$\text{{Tt}}$$ or $$\text{{tt}}$$; since $$\text{{T}}$$ is dominant, all offspring will appear tall.

Example 2 (Intermediate)

Problem: Two heterozygous pea plants ($$\text{{Tt}}$$) are crossed. What are the possible genotypes and phenotypes 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 $$\text{{tt}}$$. The phenotypes will be tall ($$\text{{TT}}$$ and $$\text{{Tt}}$$) and short ($$\text{{tt}}$$).

Validation: The ratio of genotypes is 1:2:1 (TT:Tt:tt). The ratio of phenotypes is 3:1 (tall:short).

4. Problem-Solving Techniques

Punnett Square Construction: Use a grid to systematically list all possible combinations of alleles from the parents.
Phenotype Identification: Determine the phenotype by identifying which alleles are dominant and recessive.
Probability Calculation: Calculate the probability of specific genotypes and phenotypes using the Punnett square.