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

Gregor Mendel chose the pea plant (Pisum sativum) for his groundbreaking experiments on inheritance due to several advantageous characteristics:

• Distinct Contrasting Characters: Pea plants exhibit numerous easily observable and distinct 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 traits clearly.
• Bisexual Flowers and Self-Pollination: Pea flowers are bisexual, meaning they contain both male (stamen) and female (pistil) reproductive organs. This naturally facilitates self-pollination, ensuring that the plants produce offspring with the same traits generation after generation, maintaining purity of the lines.
• Ease of Cross-Pollination: Mendel could also easily perform cross-pollination. By carefully removing the anthers (emasculation) from a flower before they matured and dusting the stigma with pollen from another desired plant, he could control the cross-breeding process.
• Short Generation Time and High Yield: Pea plants have a relatively short life cycle, allowing Mendel to observe the results of crosses over multiple generations within a reasonable timeframe. Furthermore, each plant produces a large number of seeds, providing ample data for statistical analysis.

Here's a differentiation between the given genetic terms:

1. Dominance and Recessive
   • Dominant: An allele or factor that expresses its phenotypic effect even when present with a contrasting allele. It masks the expression of the recessive allele. For example, in pea plants, the allele for tallness (T) is dominant over the allele for dwarfness (t).
   • Recessive: An allele or factor that expresses its phenotypic effect only when present in the homozygous state (i.e., in the absence of a dominant allele). For example, the allele for dwarfness (t) is recessive to the allele for tallness (T).
2. Homozygous and Heterozygous
   • Homozygous: An individual having two identical alleles for a particular gene or locus (e.g., TT for tallness or tt for dwarfness). Such individuals produce only one type of gamete regarding that gene.
   • Heterozygous: An individual having two different alleles for a particular gene or locus (e.g., Tt for tallness). Such individuals produce two different types of gametes, each carrying one of the alleles.
3. Monohybrid and Dihybrid
   • Monohybrid Cross: A genetic cross between two parents that differ in only one specific pair of contrasting characters. For example, a cross between a pure tall pea plant (TT) and a pure dwarf pea plant (tt) is a monohybrid cross.
   • Dihybrid Cross: A genetic cross between two parents that differ in two specific pairs of contrasting characters. For example, a cross between pea plants with round yellow seeds (RRYY) and wrinkled green seeds (rryy) is a dihybrid cross.

A locus refers to the specific physical location of a gene on a chromosome. A diploid organism heterozygous for a locus means it possesses two different alleles for that gene (e.g., Aa). During meiosis, these alleles segregate, allowing for the formation of gametes carrying either 'A' or 'a'.

If an organism is heterozygous for multiple loci, the number of different types of gametes it can produce depends on the number of heterozygous loci. For each heterozygous locus, there are two possible allele combinations that can be passed into a gamete.

The formula to calculate the number of different types of gametes produced by a diploid organism heterozygous for 'n' loci is given by 2ⁿ.

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