Mendels Law of Inheritance - Segregation & Dominance

What are Mendel’s laws of genetics?

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, eg: yellow pods will always conceal the other form eg: 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 and which made the recessive trait reappear in a small amount of plant. Every 3 plants which 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 on 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 have 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 of chromosomes and 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.

FAQ (Frequently Asked Questions)

1. What are the rules for Mendel’s Inheritance?

A: The rules of Mendel's inheritance:

  • In a cross between pure contrasting traits, the dominant trait will be observed in the phenotype of the organism whilst the recessive trait will be concealed

  • Only a single gene copy is allocated in a gamete cell and this is carried out in a random manner

  • A set of genes or more is sorted into gametes completely independent of one another.

2. What are homozygous and heterozygous states of a cell?

A: Homozygous: A cell could be noted as homozygous for a particular gene when identical alleles of the gene are present on both homologous chromosomes. The cell is called a homozygote.

Heterozygous: Any diploid cell is heterozygous at a gene locus when it contains two different alleles, one wild and one mutant type. The cell is called a heterozygote.

3. Why did Mendle choose a pea plant?

A: There are many reasons why Mendle chose the pea plant for his experiment

  • They show observable traits with contrasting forms

  • They have a short life cycle

  • They produce many offsprings in one cross

  • Their life cycle is completed in one season

  • Pollination can be easily manipulated

  • They do not take up much space

4. What are incomplete dominance and codominance?

A: Incomplete dominance: Incomplete dominance takes place when two alleles produce an intermediate phenotype as both are present. 

Codominance: Codominance occurs when two alleles are present and both are simultaneously expressed.

5. What are Mendel’s three laws?

A: Gregor Johan Mendel proposed three laws of inheritance:

  1. The law of dominance

  2. The law of segregation

  3. The law of independent assortment