CBSE Class 12 Biology NCERT Solutions: Chapter 5 - Principles of Inheritance and Variation

Gregor Mendel chose pea plants (Pisum sativum) for his groundbreaking experiments on inheritance due to several key advantages:

• Distinct Contrasting Characters: Pea plants exhibit numerous easily distinguishable contrasting traits, such as plant height (tall vs. dwarf), seed shape (round vs. wrinkled), seed color (green vs. yellow), flower color (purple vs. white), pod shape (inflated vs. constricted), and pod color (green vs. yellow). This allowed Mendel to track the inheritance of specific characters clearly.
• Bisexual Flowers and Self-Pollination: Pea flowers are bisexual, meaning they contain both male (stamens) and female (pistil) reproductive organs. This naturally allows for self-pollination, ensuring that the offspring produced are true-breeding for their traits.
• Ease of Cross-Pollination: Mendel could easily perform controlled cross-pollination by emasculation. This process involves removing the stamens from a flower before they mature, preventing self-pollination, and then manually pollinating the stigma with pollen from another desired plant.
• Short Life Span and High Yield: Pea plants have a relatively short life cycle, allowing Mendel to observe the results of multiple generations within a reasonable timeframe. Furthermore, each plant produces a large number of seeds per generation, providing ample data for statistical analysis.

Here's a differentiation between the given genetic terms:

1. Dominance and Recessive
   • Dominance: A dominant allele or factor expresses its phenotypic effect even when only one copy is present in the genotype (i.e., in a heterozygous condition). It masks the effect of the recessive allele. Examples in pea plants include tallness (T) and violet flower color (P).
   • Recessive: A recessive allele or factor expresses its phenotypic effect only when two copies are present in the genotype (i.e., in a homozygous condition). Its effect is masked by the dominant allele in a heterozygous state. Examples in pea plants include dwarfness (t) and white flower color (p).
2. Homozygous and Heterozygous
   • Homozygous: An organism is homozygous for a particular trait if it possesses two identical alleles for that trait. For example, a plant can be homozygous dominant (like TT for tallness) or homozygous recessive (like tt for dwarfness). Homozygous individuals produce only one type of gamete regarding that trait.
   • Heterozygous: An organism is heterozygous for a particular trait if it possesses two different alleles for that trait. For example, a plant with genotype Tt is heterozygous for height. Heterozygous individuals produce two different types of gametes (e.g., T and t).
3. Monohybrid and Dihybrid
   • Monohybrid Cross: This type of cross involves parents that differ in only one pair of contrasting characters. It is used to study the inheritance of a single gene. For example, crossing a tall pea plant with a dwarf pea plant.
   • Dihybrid Cross: This type of cross involves parents that differ in two pairs of contrasting characters. It is used to study the inheritance of two genes simultaneously. For example, crossing a pea plant with round and yellow seeds with a plant having wrinkled and green seeds.

A locus refers to the specific physical location of a gene or other DNA sequence on a chromosome. A diploid organism heterozygous for a locus means it has two different alleles for that gene (e.g., Aa).

When an organism is heterozygous for multiple loci, the alleles at these different loci segregate independently during meiosis, leading to the formation of various combinations of alleles in the gametes.

The number of different types of gametes that can be produced by a diploid organism heterozygous for 'n' number of loci is given by the formula $2^n$.

In this case, the organism is heterozygous for 4 loci (n = 4).

Therefore, the number of types of gametes that can be produced is:

$$2^4\; =\; 2\; \backslash times\; 2\; \backslash times\; 2\; \backslash times\; 2\; =\; 16$$

Thus, a diploid organism heterozygous for 4 loci can produce 16 different types of gametes.