Basic Principles of Inheritance: Class 11 NCERT Biotechnology Guide

Understanding Genotype and Phenotype in Inheritance

Inheritance starts with the concepts of genotype and phenotype:

• Genotype is the genetic constitution of an organism, representing the alleles inherited from parents.
• Phenotype is the observable physical or biochemical traits influenced by genotype and environment.

For example, if a pea plant has the genotype $Tt$ (where $T$ = tall allele, $t$ = dwarf allele), the phenotype is tall because the tall allele is dominant.

This distinction is fundamental in genetics and helps explain how traits appear in offspring even if the underlying genes differ.

Mendel’s Laws: Foundation of Basic Principles of Inheritance

Gregor Mendel’s experiments with pea plants laid the foundation for inheritance principles:

• Law of Segregation: Each organism carries two alleles for a trait, which segregate during gamete formation, so each gamete carries only one allele.
• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation.

Example: Monohybrid Cross

Crossing two heterozygous tall pea plants ($Tt imes Tt$) results in:

Phenotypic ratio: 3 Tall : 1 Dwarf

These laws explain how traits are inherited in predictable patterns.

Dominant and Recessive Characters Explained

In inheritance, alleles can be dominant or recessive:

• Dominant alleles express their trait even if only one copy is present (heterozygous condition).
• Recessive alleles express their trait only when two copies are present (homozygous recessive).

Understanding dominance helps predict offspring traits and is essential for solving genetic problems.

Polyploidy: Multiple Sets of Chromosomes in Plants

Polyploidy is a condition where organisms have more than two complete sets of chromosomes, common in plants:

• Diploid (2n): Two sets of chromosomes (e.g., rice, sorghum).
• Triploid (3n), Tetraploid (4n), Hexaploid (6n), Octoploid (8n): Higher multiples, often leading to larger cells and organs.

Polyploid plants are often more resilient and important in agriculture and breeding.

Test Cross: Identifying Unknown Genotypes

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

Diagrammatic Example:

Test crosses are vital tools in genetics to reveal hidden genotypes.

Monohybrid and Dihybrid Crosses: Predicting Genetic Outcomes

Genetic crosses help predict offspring traits:

• Monohybrid cross: Involves one trait with two alleles.
• Dihybrid cross: Involves two traits, each with two alleles.

Monohybrid Cross Example

Crossing heterozygous parents ($Aa imes Aa$):

Dihybrid Cross Example

Crossing parents heterozygous for two traits ($AaBb imes AaBb$):

Phenotypic ratio: 9:3:3:1

• 9 both dominant traits
• 3 dominant first, recessive second
• 3 recessive first, dominant second
• 1 both recessive

These crosses demonstrate Mendel’s laws and help solve inheritance problems.