Mendelian Inheritance

Introduction

Mendelian inheritance refers to the set of genetic principles discovered by Gregor Mendel, often called the Father of Modern Genetics. In the 19th century, Mendel conducted experiments on pea plants (Pisum sativum) and discovered predictable patterns in how traits are passed from one generation to the next. His findings laid the foundation for the study of genetics.

Seven Contrasting Characters in Pea Plants

Mendel studied seven easily observable traits in pea plants, each with two contrasting forms:

Trait	Dominant Character	Recessive Character
Plant height	Tall (T)	Dwarf (t)
Flower position	Axial (A)	Terminal (a)
Pod shape	Inflated (I)	Constricted (i)
Pod color	Green (G)	Yellow (g)
Seed shape	Round (R)	Wrinkled (r)
Seed color	Yellow (Y)	Green (y)
Flower color	Violet (P)	White (p)

Key Terms in Mendelian Genetics

• Gene: A unit of heredity that determines a specific trait

• Alleles: Different forms of a gene (e.g., tall and dwarf)

• Dominant Trait: Expressed even if only one copy is present

• Recessive Trait: Expressed only when both copies are present

• Homozygous: Both alleles are the same (TT or tt)

• Heterozygous: Alleles are different (Tt)

• Genotype: Genetic makeup of an organism

• Phenotype: Physical expression of a trait

Mendel’s Laws of Inheritance

1. Law of Dominance

When two different alleles are present, one (dominant) masks the effect of the other (recessive).

Example: In pea plants, tall (T) is dominant over dwarf (t), so Tt is tall.

2. Law of Segregation

During gamete formation, the two alleles for each trait separate, and each gamete receives only one allele.

Example: A Tt plant produces gametes with either T or t, not both.

3. Law of Independent Assortment

Genes for different traits are inherited independently of each other.

Example: The inheritance of seed shape does not affect the inheritance of flower color.

Monohybrid Cross – Tall × Dwarf Pea Plants

A monohybrid cross studies the inheritance of one trait at a time. Here’s how Mendel explained the inheritance of plant height.

Step 1: Parent Generation (P)

• Tall plant (TT) × Dwarf plant (tt)

Step 2: Gamete Formation

• Tall plant produces gametes: T

• Dwarf plant produces gametes: t

Step 3: First Filial Generation (F₁)

• All offspring (Tt) are Tall because tall (T) is dominant.

	T	T
t	Tt	Tt
t	Tt	Tt

Genotype: 100% Tt

Phenotype: 100% Tall

Step 4: Self-Pollination of F₁ Plants

• Tt × Tt

Step 5: Punnett Square for F₂ Generation

	T	t
T	TT	Tt
t	Tt	tt

Genotypic Ratio: 1 TT : 2 Tt : 1 tt

Phenotypic Ratio: 3 Tall : 1 Dwarf

Mendelian Ratios

• Monohybrid F₂ Generation:

  • Genotype → 1:2:1

  • Phenotype → 3:1

Applications of Mendelian Inheritance

• Predicting traits in offspring

• Breeding programs in agriculture

• Understanding genetic disorders

• Foundation for modern biotechnology

Summary Table of Mendel’s Work

Term	Definition	Example
Dominant Trait	Masks the effect of recessive trait	Tall over dwarf
Recessive Trait	Expressed only in homozygous condition	Dwarf (tt)
Genotype	Genetic makeup	Tt, TT, or tt
Phenotype	Physical appearance	Tall or dwarf

Conclusion

Mendelian inheritance is a cornerstone of biology. By understanding Mendel’s laws and how traits are passed from one generation to the next generation. This knowledge remains essential for fields ranging from agriculture to medicine.

FAQs

1. Why did Mendel choose pea plants?
   They have clear traits, short generation time, and can be easily controlled for pollination.

2. What is the difference between genotype and phenotype?
   Genotype is the genetic code, while phenotype is the visible expression of that code.

3. Are Mendel’s laws always applicable?
   No. Some traits show incomplete dominance, codominance, or are influenced by multiple genes.

References

• Mendel, G. (1865). Experiments on Plant Hybridization.

• Campbell, N. A., & Reece, J. B. (2017). Biology (11th Edition). Pearson.

• Griffiths, A. J. F. et al. (2015). Introduction to Genetic Analysis.