Principles of Inheritance and Variation

Notes

Introduction

Genetics is the branch of Biology that deals with inheritance and variation.

Inheritance: Transmission of characters from parents to offspring.

Variation: Differences among individuals of the same species.

Father of Genetics

Gregor Johann Mendel

Experiment:

Garden Pea (Pisum sativum)

1856–1863

Studied 7 pairs of contrasting characters

Why did Mendel choose Pea Plant?

Easy cultivation

Short life cycle

Naturally self-pollinated

Artificial cross-pollination possible

True breeding varieties available

Large number of offspring

Seven Contrasting Characters

Important Terminology

Gene → Unit of heredity

Allele → Alternative forms of gene

Locus → Position of gene on chromosome

Genotype → Genetic constitution

Phenotype → Physical appearance

Homozygous → TT / tt

Heterozygous → Tt

Dominant allele → Expresses in heterozygous condition

Recessive allele → Expresses only in homozygous condition

Mendel's Monohybrid Cross

Example:

Tall (TT)

×

Dwarf (tt)

↓

F1

All Tall (Tt)

↓

Self Pollination

F2

Genotypic Ratio

1 TT : 2 Tt : 1 tt

Phenotypic Ratio

3 Tall : 1 Dwarf

Mendel's Laws

1. Law of Dominance

One allele is dominant over another.

Example:

T dominates over t

2. Law of Segregation

Alleles separate during gamete formation.

Also called

Law of Purity of Gametes

3. Law of Independent Assortment

Genes located on different chromosomes assort independently.

Applicable in dihybrid cross.

Dihybrid Cross

Example

Round Yellow

RRYY

×

Wrinkled Green

rryy

↓

F1

All Round Yellow

RrYy

↓

F2 Phenotypic Ratio

9 : 3 : 3 : 1

Most Important Ratio

9 Round Yellow

3 Round Green

3 Wrinkled Yellow

1 Wrinkled Green

Test Cross

Unknown dominant

×

Homozygous recessive

Purpose

Determine genotype.

Back Cross

F1

×

Any parent

Deviations from Mendel

Incomplete Dominance

Example

Snapdragon

Red × White

↓

Pink

Ratio

1 Red

2 Pink

1 White

Codominance

Example

Human ABO Blood Group

IA and IB express together.

AB blood group

Both antigens present.

Multiple Alleles

One gene

More than two alleles.

Example

ABO blood group

IA

IB

i

Pleiotropy

…