Basic Principles of Inheritance: Class 11 NCERT Biotechnology Guide

Mendel’s Experiments: The Foundation of Inheritance

Gregor Johann Mendel, known as the father of genetics, conducted pioneering work using pea plants (Pisum sativum). He chose seven pairs of contrasting traits such as tall vs dwarf plants and round vs wrinkled seeds. Mendel produced pure lines by self-pollinating plants over generations. Then, he performed artificial cross-pollination by transferring pollen between flowers using a brush.

By growing large numbers of progeny and carefully recording traits over generations, Mendel formulated two key laws:

• Law of Segregation: Allele pairs separate during gamete formation.
• Law of Independent Assortment: Genes for different traits assort independently.

Though published in 1866, Mendel’s work was rediscovered in 1900 and remains the basis of modern genetics.

Activity: Try artificial cross-pollination on pea plants to observe trait inheritance firsthand.

Understanding Genotype and Phenotype

Inheritance involves two important concepts:

• Genotype: The genetic constitution of an organism, i.e., the alleles it carries.
• Phenotype: The observable physical or biochemical characteristics influenced by genotype and environment.

For example, in pea plants, the genotype could be TT or Tt (alleles for tallness), while the phenotype is the actual height (tall or dwarf).

Understanding this distinction is crucial for solving inheritance problems in Class 11 NCERT Biotechnology.

Dominant and Recessive Characters Explained

In inheritance, traits are controlled by alleles that can be dominant or recessive:

• Dominant Allele: Expressed in the phenotype even if only one copy is present (e.g., T for tall).
• Recessive Allele: Expressed only when two copies are present (e.g., t for dwarf).

If an organism has one dominant and one recessive allele (heterozygous), the dominant trait appears.

Example:

• Genotype TT or Tt → Tall (dominant)
• Genotype tt → Dwarf (recessive)

This concept helps explain why some traits skip generations or appear unexpectedly.

Monohybrid Cross and Law of Segregation

A monohybrid cross studies the inheritance of a single trait. Mendel’s experiments with tall (T) and dwarf (t) pea plants illustrate this.

• Parental (P) generation: Pure tall (TT) × pure dwarf (tt)
• F1 generation: All tall plants (Tt)
• F2 generation: Cross F1 × F1 → Genotypic ratio: 1 TT : 2 Tt : 1 tt; Phenotypic ratio: 3 tall : 1 dwarf

This supports the Law of Segregation which states that allele pairs segregate during gamete formation so each gamete carries only one allele.

Worked example: Cross Tt × Tt:

Phenotype: 3 tall : 1 dwarf.

Dihybrid Cross and Law of Independent Assortment

A dihybrid cross examines inheritance of two traits simultaneously. Mendel studied seed shape (round R vs wrinkled r) and seed colour (yellow Y vs green y).

• Parental: Pure round yellow (RRYY) × pure wrinkled green (rryy)
• F1 generation: All round yellow (RrYy)
• F2 generation: Cross F1 × F1 results in a phenotypic ratio of 9:3:3:1

This demonstrates the Law of Independent Assortment: alleles of different genes assort independently during gamete formation.

Note: This law applies only to genes located on different chromosomes or far apart on the same chromosome.

Test Cross: Determining Unknown Genotype

A test cross helps determine whether an individual with a dominant phenotype is homozygous dominant or heterozygous.

Procedure: Cross the individual with a homozygous recessive (aa) individual.

• If all offspring show dominant phenotype, the unknown parent is homozygous dominant (AA).
• If offspring show a 1:1 ratio of dominant to recessive phenotypes, the unknown parent is heterozygous (Aa).

Diagrammatic representation:

This method is widely used in genetics to identify genotypes in breeding experiments.

Comparison of Key Genetic Terms

Understanding the following pairs is essential for Class 11 NCERT students:

This table helps clarify common confusions in genetics.