Question

Describe the Mendel's law of dominance.

Hint:

The dominant allele expresses its trait in both homozygous (TT) and heterozygous (Tt) conditions, while the recessive allele only expresses its trait in the homozygous recessive condition (tt).

Answer 1

Mendel's Law of Dominance is one of the three fundamental principles of inheritance that were established by Gregor Mendel in the 19th century. According to Mendel's law of dominance, when an organism inherits two alleles for a particular trait, one allele may be dominant and the other recessive. The dominant allele will mask the expression of the recessive allele in the organism's phenotype, meaning that the trait controlled by the dominant allele will be expressed, while the trait controlled by the recessive allele will not. ### Key Terms: \begin{enumerate} \item Dominant Allele: The allele that expresses its trait even in the presence of a different allele. It is usually represented by a capital letter (e.g., T for tall in pea plants). \item Recessive Allele: The allele that is masked in the presence of a dominant allele. It only expresses its trait when an organism has two copies of the recessive allele. It is represented by a lowercase letter (e.g., t for short in pea plants). \item Homozygous: When an organism inherits two identical alleles for a trait (e.g., TT or tt). \item Heterozygous: When an organism inherits two different alleles for a trait (e.g., Tt). \end{enumerate} ### Explanation with Example: Consider the example of pea plants, which were used by Mendel in his experiments. In this case, the height of the plant is governed by a single gene with two alleles: - The dominant allele (T) causes tall plants. - The recessive allele (t) causes short plants. Mendel crossbred plants with different genotypes and observed the following: - **Homozygous Dominant (TT):** These plants were tall because the dominant allele T was expressed. - **Homozygous Recessive (tt):** These plants were short because both alleles were recessive, and no dominant T allele was present to mask the effect. - **Heterozygous (Tt):** These plants were also tall because the dominant allele T masked the effect of the recessive allele t. ### Mendel's First Experiment (Monohybrid Cross): Mendel crossed a homozygous tall plant (TT) with a homozygous short plant (tt). All the F1 offspring in this cross had the genotype Tt and exhibited the dominant tall phenotype. This demonstrated that the dominant allele (T) masks the recessive allele (t) in the heterozygous state. When Mendel crossed two F1 plants (Tt), the resulting F2 generation had a 3:1 phenotypic ratio, with 75% of the plants being tall (dominant phenotype) and 25% being short (recessive phenotype). This result confirmed the principle of dominance and suggested that the alleles segregate independently during gamete formation.