Question

Describe Mendel's 'law of dominance'.

Hint:

Law of Dominance: In heterozygotes, the dominant allele is expressed, and the recessive allele is masked. Example: Tall (T) is dominant over dwarf (t) in pea plants.

Answer 1

Step 1: Background.
Gregor Mendel, the father of genetics, conducted experiments on garden pea plants (Pisum sativum) to study the inheritance of traits. From his experiments, he proposed three laws of inheritance: \begin{enumerate} \item Law of Dominance \item Law of Segregation \item Law of Independent Assortment \end{enumerate} Here, we focus on the Law of Dominance.

Step 2: Statement of the law.
The Law of Dominance states that when two different alleles of a character are present in an organism (heterozygous condition), only one allele expresses itself (dominant), while the other allele remains masked (recessive).

Step 3: Example of Mendel's experiment.
Mendel crossed pure tall plants (\(TT\)) with pure dwarf plants (\(tt\)): \[ TT \times tt \;\;\;\; $\Rightarrow$ \;\;\;\; F_1 \text{ generation: all } Tt \] All plants in the F₁ generation were tall, showing that tallness (T) is dominant over dwarfness (t).

Step 4: Punnett Square.
\[ \begin{array}{|c|c|c|} \hline & T & T
\hline t & Tt & Tt
\hline t & Tt & Tt
\hline \end{array} \] All F₁ hybrids are tall (Tt), proving that the dominant trait (Tallness) masks the recessive trait (Dwarfness).

Step 5: Importance of the law.
\[\begin{array}{rl} \bullet & \text{Explains why certain traits appear in the F1 generation and why recessive traits reappear in the F2 generation.} \\ \bullet & \text{Foundation of classical genetics, used to predict inheritance patterns.} \\ \bullet & \text{Helps in plant and animal breeding to select desirable traits.} \\ \end{array}\]

Step 6: Limitation.
The law of dominance does not explain incomplete dominance or codominance, where both alleles may express partially or equally (e.g., flower colour in snapdragon, ABO blood groups in humans).