Mendelian Inheritance

Introduction

Mendelian inheritance, also known as classical genetics, is the foundation of understanding how traits are transmitted from one generation to the next. It is named after Gregor Johann Mendel, an Austrian monk and scientist who conducted pioneering experiments with pea plants in the mid-19th century. Mendel’s work revealed that inheritance follows specific patterns determined by discrete units, later termed genes, which exist in alternative forms called alleles.

Through careful experimentation, Mendel established the principles of dominance, segregation, and independent assortment, providing a framework for modern genetics. Today, Mendelian inheritance remains a cornerstone of genetics, helping scientists understand hereditary diseases, breeding, and molecular biology.

Gregor Mendel and His Experiments

Early Life and Scientific Approach

Gregor Mendel was born in 1822 in what is now the Czech Republic. He was educated in natural sciences, mathematics, and physics, which contributed to his methodical approach to research. His interest in inheritance led him to study pea plants (Pisum sativum), chosen for their short life cycle, easily distinguishable traits, and controlled pollination capabilities.

Selection of Traits

Mendel focused on seven distinct traits in pea plants:

Flower color (purple or white)
Flower position (axial or terminal)
Seed color (yellow or green)
Seed shape (round or wrinkled)
Pod color (green or yellow)
Pod shape (inflated or constricted)
Plant height (tall or dwarf)

Each trait had two contrasting forms, making them ideal for studying patterns of inheritance.

Experimental Method

Mendel’s approach was systematic:

He cross-pollinated plants with contrasting traits.
Collected and analyzed the offspring (F1 generation).
Self-pollinated the F1 generation to produce an F2 generation.
Counted and categorized phenotypes in the progeny to detect patterns.

Key Observations

When two contrasting traits were crossed, only one trait appeared in the F1 generation.
The “lost” trait reappeared in the F2 generation in a 3:1 ratio.
Traits segregated independently and recombined in predictable patterns.

These observations led Mendel to formulate three foundational principles of inheritance.

Mendel’s Laws of Inheritance

1. Law of Dominance

The law of dominance states that in a heterozygous organism, one allele may mask the effect of another allele.

Dominant allele: Expressed in the phenotype even if only one copy is present.
Recessive allele: Expressed only when two copies are present (homozygous).

Example: Crossing a tall pea plant (TT) with a dwarf plant (tt) produces F1 offspring that are all tall (Tt), illustrating that the tall allele is dominant.

2. Law of Segregation

The law of segregation states that during gamete formation, the two alleles for a trait separate, so each gamete carries only one allele.

Each parent contributes one allele to the offspring.
This explains the reappearance of recessive traits in the F2 generation.

Example: Crossing two heterozygous tall plants (Tt × Tt) results in offspring with genotypes:

TT (homozygous dominant)
Tt (heterozygous)
tt (homozygous recessive)

The expected phenotypic ratio is 3 tall : 1 dwarf.

3. Law of Independent Assortment

The law of independent assortment states that alleles for different traits segregate independently of one another during gamete formation.

This applies when traits are controlled by genes on different chromosomes or sufficiently far apart on the same chromosome.
Independent assortment explains the variety of combinations observed in offspring.

Example: Crossing pea plants with two traits, such as seed shape (round or wrinkled) and seed color (yellow or green), results in a 9:3:3:1 phenotypic ratio in the F2 generation.

Monohybrid and Dihybrid Crosses

Monohybrid Cross

A monohybrid cross involves a single trait with two alleles.

Parental Generation (P): TT × tt (tall × dwarf)
F1 Generation: All Tt (heterozygous tall)
F2 Generation: TT : Tt : tt in a 1:2:1 genotypic ratio
Phenotypic ratio: 3 tall : 1 dwarf

Monohybrid crosses illustrate dominance and the law of segregation.

Dihybrid Cross

A dihybrid cross involves two traits simultaneously, each with two alleles.

Example: Seed shape (R = round, r = wrinkled) and seed color (Y = yellow, y = green)
Parental Generation: RRYy × rrYY
F2 offspring exhibit a 9:3:3:1 phenotypic ratio:
- 9 round yellow
- 3 round green
- 3 wrinkled yellow
- 1 wrinkled green

Dihybrid crosses demonstrate the law of independent assortment.

Extensions of Mendelian Genetics

Mendelian inheritance forms the foundation for more complex genetic patterns observed in nature.

1. Incomplete Dominance

Occurs when neither allele is completely dominant, resulting in an intermediate phenotype.
Example: Crossing red (RR) and white (WW) snapdragons produces pink (RW) flowers.

2. Codominance

Both alleles are expressed equally in the heterozygote.
Example: Human blood group AB shows codominance, with both A and B antigens present.

3. Multiple Alleles

A gene may have more than two alleles within a population.
Example: ABO blood group system has three alleles: IA, IB, and i.

4. Polygenic Inheritance

Traits influenced by multiple genes, producing continuous variation.
Example: Human height, skin color, and eye color are polygenic traits.

5. Sex-Linked Inheritance

Traits carried on sex chromosomes exhibit sex-linked patterns.
Example: Hemophilia and color blindness are X-linked recessive disorders.

6. Epistasis

One gene may mask or modify the expression of another gene at a different locus.
Example: Coat color in Labrador retrievers depends on two genes, one for pigment production and another for pigment deposition.

Applications of Mendelian Genetics

Mendelian principles have broad applications in medicine, agriculture, and research.

1. Medicine

Genetic Counseling: Predicting inheritance patterns of hereditary diseases.
Diagnosis of Genetic Disorders: Identifying carriers of conditions such as cystic fibrosis, sickle cell anemia, and Tay-Sachs disease.
Personalized Medicine: Using knowledge of genetic traits to tailor treatments.

2. Agriculture

Plant Breeding: Selecting traits such as disease resistance, high yield, and drought tolerance.
Animal Breeding: Enhancing desired traits in livestock and pets.

3. Research and Biotechnology

Gene Mapping: Locating genes on chromosomes using inheritance patterns.
Molecular Genetics: Understanding gene function and regulation.
CRISPR and Genetic Engineering: Editing genes based on knowledge of dominant and recessive alleles.

Punnett Squares and Probability

Mendelian inheritance can be predicted using Punnett squares, which illustrate possible genotypes of offspring.

Monohybrid Punnett Square: Visualizes inheritance of a single trait.
Dihybrid Punnett Square: Visualizes inheritance of two traits simultaneously.
Helps calculate the probability of genotypes and phenotypes in progeny.

Exceptions and Modifications to Mendelian Ratios

While Mendelian principles provide a framework, many traits deviate due to:

Linkage: Genes located close together on the same chromosome tend to be inherited together.
Environmental Influence: Phenotype may be affected by environmental factors, e.g., temperature, nutrition.
Mutations: Changes in DNA sequence can alter expected inheritance patterns.

Mendelian Genetics in Humans

Human traits often follow Mendelian patterns:

Dominant Disorders: Polydactyly, Huntington’s disease
Recessive Disorders: Cystic fibrosis, albinism
Sex-Linked Traits: Red-green color blindness, Duchenne muscular dystrophy
Genetic Testing: Enables identification of carriers and affected individuals for family planning.

Modern Understanding of Mendel’s Work

Mendel’s work was initially unrecognized but later became the cornerstone of genetics.

Discovery of chromosomes and meiosis provided a cellular basis for Mendel’s observations.
The identification of DNA as the hereditary material explained how alleles encode traits.
Modern genetics integrates Mendelian principles with molecular biology, genomics, and epigenetics.