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AGRI Grovestudies

Mendel’s Laws of Genetics by Grovestudies

 Johan Gregor Mendel, also known as the “father of genetics”, started a decade-long research project in 1856 to study the patterns of inheritance. He carried out his experiments on inheritance with the common pea plant taking 7 features of the plant, which included:

Stem height

Pod shape

Pod colour

Flower position 

Flower colour 

Seed colour

Seed shape

He carried out these experiments until 1865.

He started the experiment by breeding plants with two different features, such as tall vs short height, till they were pure breeding.

What are Phenotypes and Genotypes?

Phenotype:

 A phenotype could be defined as an observable characteristic of an organism, which is a result of the interaction of its genotype and environment.Examples: Skin colour, eye colour

Genotype:

 The genetic makeup of a cell, which determines its characteristics, is called the genotype of a cell.Example: the pea plant’s tall dominant genes

He selected a pea plant for his experiments:

The pea plant can be easily grown and maintained.

They are naturally self-pollinating but can also be cross-pollinated.

It is an annual plant, therefore, many generations can be studied within a short period of time.

It has several contrasting characters.

Mendel conducted 2 main experiments to determine the laws of inheritance. These experiments were:

Monohybrid Cross Experiment

Dihybrid Cross Experiment

Monohybrid Cross:- 

In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them. He found the first generation offsprings were tall and called it F1 progeny. Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1.

Dihybrid Cross:-

In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow. This meant that dominant traits were the round shape and yellow colour.

He then self-pollinated the F1 progeny and obtained 4 different traits wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the ratio 9:3:3:1.

Conclusions from Mendel’s Experiments:-

The genetic makeup of the plant is known as the genotype. On the contrary, the physical appearance of the plant is known as phenotype

The genes are transferred from parents to the offsprings in pairs known as allele.

During gametogenesis when the chromosomes are halved, there is a 50% chance of one of the two alleles to fuse with the other parent.

When the alleles are the same, they are known as homozygous alleles and when the alleles are different they are known as heterozygous alleles.

Mendel’s laws

The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are:

1. Law of Dominance

2. Law of Segregation

3. Law of Independent Assortment

Law of Dominance

This is also called Mendel’s first law of inheritance. According to the law of dominance, hybrid offsprings will only inherit the dominant trait in the phenotype. The alleles that are suppressed are called as the recessive traits while the alleles that determine the trait are known as the dormant traits.

The Law of Segregation:

 According to the law of segregation, only a single copy of two gene copies present in a cell is distributed in each gamete (sperm or egg cell) that it produces and the allocation of these gene copies is random. 

The basic concepts of the law of segregation are  :-

A gene exists in multiple forms of an allele.

In meiosis, the allelic pair of a cell separates and each gamete has a single allele. 

Every living organism inherits two alleles for each trait; one is dominant and the other is recessive.

The Law of Independent Assortment:

According to the law of independent assortment, the alleles of two or more different genes are sorted into gametes (sperm and egg cells), independently of one another. 

It can be simplified stating that the allele a gamete receives from one gene does not influence another allele received for another gene.

Key Points on Mendel’s Laws

The law of inheritance was proposed by Gregor Mendel after conducting experiments on pea plants for seven years.

The Mendel’s laws of inheritance include law of dominance, law of segregation and law of independent assortment.

The law of segregation states that every individual possesses two alleles and only one allele is passed on to the offspring.

The law of independent assortment states that the inheritance of one pair of genes is independent of inheritance of another pair.