Mendel's Law of Inheritance

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. He carried out breeding experiments with all the features and noted down similar observations in inheritance, which concluded to the following features:

• One feature, e.g. yellow pods, will always conceal the other form, e.g. green pods, in the F1 or the first generation of the cross. The visible feature was termed as the dominant trait whilst the concealed feature was termed as the recessive trait. 

• The plants were allowed to self-fertilise in the second generation, which made the recessive trait reappear in a small amount of plant. Every 3 plants showed the dominant trait, whilst 1 plant showed the recessive trait.

• The conclusion that Mendel reached was that the features in plants were inherited independently and no individual feature had an influence on other features or traits. For example, The trait stem height did not influence other traits, such as pod shape and flower colour, in any way.

The observations gave rise to two of Mendel’s laws of genetics:

The Law of Dominance: Mendel’s law of dominance states that when parents with pure contrasting traits are crossed together, only the dominant trait will appear as the phenotype of the next generation. The recessive trait will be concealed.

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. As per this law, the offspring will acquire one gene copy from each parent. The law of segregation is also known as Mendel's second law of inheritance.

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.

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

Di-hybrid Cross Experiment

After crossbreeding single traits, Mendel continued with his research and carried out a dihybrid cross with two traits, each having two alleles. 

The total of four traits that Mendel chose from the pea plant for crossbreeding were homozygous and dominant for round (RR) yellow (YY) seeds and homozygous and recessive for wrinkled (rr) green (yy) seeds. These were taken as the parent generation (P).

When they crossed, the first generation (F1) of plants were all heterozygous. It can be represented in the following manner:

RRYY x rryy

The genotype of the F1 generation plants was RrYy. These were called di-hybrids.

After the F1 generation plants were crossed between them, there were multiple variants of the plant observed in the F2 generations. The di-hybrid cross yielded nine plants with round and yellow seeds, three plants with round and green seeds, three plants with wrinkled and yellow seeds, and one plant with wrinkled and green seeds at a ratio of 9:3:3:1. With these observations, Mendel stated the law of independent assortment, the third law of inheritance.

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.

The Principle of the Law of Independent Assortment

The principle of the law of independent assortment explains how different genes independently separate from one another when gametes or reproductive cells develop. It occurs during meiosis cell division of a eukaryotic cell.

Each eukaryotic cell has a total number of chromosomes; 23 of them are from the father and the rest of them are from the mother. During meiosis, a pair of homologous or identical chromosomes are divided in half to form haploid cells and this separation or assortment is completely random. This means all paternal chromosomes are separated into another as well as all maternal chromosomes will not be separated in one cell.

Another feature of the law of independent assortment is genetic recombination, which scrambles maternal and paternal genes, ensuring that they are independently assorted from one another.