Courses for Kids
Free study material
Offline Centres
Store Icon

Mendelian Ratio

share icon
share icon

Mendelism: An Introduction

MVSAT 2024

Have you ever thought about why children look similar to their parents? Or sometimes they differ. Gregor Mendel has conducted various experiments such as monohybrid and dihybrid cross and gave three laws of inheritance that explain the inheritance pattern but there are also various exceptions to Mendel's law that do not follow Mendel law.


Mendelism is used to illustrate the principles of inheritance given by Gregor Mendel. These are also known as Mendel's law.

Mendel's Experiment

Mendel conducted several experiments on pea plants for years. He considered seven contrasting traits of pea plants.

A brief explanation of Mendel's experiment is discussed here:

Monohybrid Experiment - In this experiment, Mendel takes two pea plants that have all the same traits except one Such as yellow and green seeds.

First Mendel crossed the two pea plants known as the parent plants and obtained the first filial generation. In the first filial generation, all seeds were yellow in phenotypic appearance.

Then he crossed the first filial generation and obtained the second filial generation in which 75% of seeds were yellow and the remaining 25% of seeds were green.

Monohybrid Cross

Monohybrid Cross

Dihybrid Experiment - A Dihybrid cross is a cross between two plants that have all the same traits except two. Two traits are contrasting in nature.

Such as green, and yellow seeds, and round and wrinkled seeds.

First Mendel took two yellow and round, green and wrinkled pea plants, crossed them, and obtained the first filial generation.

All plants in the first filial generation were round and yellow.

Then he again crossed two plants of the first filial generation and obtained the second filial generation. 56% of plants were round and yellow. 18% of plants were yellow and wrinkled, and 18% of plants were green and round whereas 6% of plants were round and wrinkled.

Dihybrid Cross

Dihybrid Cross

Mendel's Ratio

Monohybrid cross

Dihybrid cross

Phenotypic ratio



Genotypic ratio



Mendel's Law

Mendel's laws are also known as the law of inheritance.

There are three laws of inheritance given by Mendel.

  1. Law of dominance

  2. Law of segregation

  3. Law of independent assortment

Law of Dominance

This is the first law of Mendel. It states that

In heterozygous conditions, one allele dominates the other allele. This means in heterozygous conditions only one allele will express itself over the other.

Law of Segregation

This is the second law of Mendel and it states that when there are two traits in one hybrid pair, the two characters do not mix and each allele segregates independently.

It is a universally accepted law without any exceptions. This law is also known as the purity of gametes.

Law of Independent Assortment

The law of independent assortment states that alleles of two or more different genes get separated into gametes independently of each other.

This means each gamete receives an allele of one gene independent of the allele of another gene.

Deviation From Mendel's Law

There are many deviations from Mendel's law.

  1. Incomplete Dominance - In this, no one allele completely dominates the other allele of a gene in a heterozygous condition. Both alleles express themselves and new phenotypes are formed which are intermediate in both of the alleles. Examples are the pink color of the snapdragon flower.

  2. Codominance - In this both alleles express themselves simultaneously in heterozygous conditions. Examples of codominance are the ABO blood group in humans in which both alleles A and B express themselves.

  3. Multiple Alleles - this condition is present in those genes that have more than two alleles. In this, more than two alternative forms of a gene are present on the homologous chromosomes.

  4. Pleiotropy - is a condition in which one gene affects more than one character or phenotypic trait. It is also known as a qualitative trait

  5. Polygenic Inhertence - this is a condition in which one phenotypic trait is controlled by more than one allele. This is also known as quantitative inheritance. Examples of this type of inheritance are human height or skin color.

Interesting Facts

  • Mendel was a mathematician who first described genetics.

  • Mendel took more than seven years to discover and prove the law of inheritance

  • Mendel's work was rejected initially and didn't bring any recognition or success for him,

  • Later his work was rediscovered by three scientists and gave him recognition.

Important Questions

1. What is Mendel's famous ratio?

Ans: Mendel's famous ratio is 3:1, for a phenotype for a monohybrid cross.

2. What is the factor in Mendelism?

Ans: Mendel factor is called a gene. Heredity is the result of discrete units of inheritance.

3. What is the meaning of allele?

Ans: Allele is the alternating form of a gene. An individual inherits two alleles, one from each parent.

Practice Questions

  1. Who developed the three main laws of inheritance?

  2. What is the law of inheritance?

  3. What are the three principles of Mendelian genetics?

  4. What cross makes a 1:1:1:1 ratio?

  5. What is universal inheritance?

Key Features

  • Mendel conducted several experiments on pea plants and concluded his experiments in the form of three laws and tried to explain some concepts of genetics.

  • Only the Second law of inheritance is followed in every condition and there is an exception to both the first and the third law.

  • The phenotypic ratio of the monohybrid cross is 3:1 and the genotypic ratio is 1:2:1.

  • The phenotypic ratio of the dihybrid cross is 9:3:3:1 and the genotypic ratio is 1:2:1:2:4:2:1:2:1.

Want to read offline? download full PDF here
Download full PDF
Is this page helpful?

FAQs on Mendelian Ratio

1. Why did Mendel choose pea plants for his experiment?

Mendel chose pea plants for the following reasons- 

  1. The flowers of these plants were bisexual means have both androecium and gynoecium in a single flower

  2. They have a shorter life span and plants are easy to maintain 

  3. Pure varieties of pea plants were easily available

  4. Hybrids are fertile

  5. Contrasting traits of pea plants are easily available

  6. The different physical characteristics were easy to recognise and easily available.

  7. Pea plants produce a large number of offspring at one cross and this makes the calculation easy

2. Explain the concept of polygenic inheritance.

Polygenic inheritance is one of the exceptions to Mendel's law. One trait is influenced by more than one independent allele. Each allele has a cumulative or additive effect. Multiple genes produce an additive effect. Polygenic variation is characterised by the continuous variation of the phenotype of the trait. There are various examples of polymeric inheritance such as human height, and skin color of humans. There are almost 400 genes responsible for height. And the environment greatly influences the expression of these genes.

3. Explain the phenomenon of incomplete dominance in snapdragon flowers.

Snapdragon is also known as the dog flower. Here the dominant allele is R and is responsible for red color flowers, and r is the recessive allele responsible for white color flowers. Now R expresses itself in homozygous as well as in heterozygous conditions whereas r expresses itself only in homozygous conditions. One  Parent flower is red and the other parent flower is white, in the F1 generation, all flowers were pink due to the incomplete dominance of the red flower.

Competitive Exams after 12th Science