Genetics

Danish botanist Wilhelm Johannsen coined the word "gene" in 1909 to describe the fundamental physical and functional units of heredity. In 1905 William Bateson coined the term genetics from the word gene.

Mendel selected garden pea for his experiment because it has short life cycle, so can easily can study many generation in short period of time. Artificial self-pollination can easily be done in garden pea plant.

Genetics is the branch of science which involves the study of genes, heredity, and variation in living organisms. In genetics we study the process of inheritance of characters from parents to offspring.

The characters which are not expressed in F₁ generation are recessive character. Example when yellow pod pea plant is crossed with green pod plant. The offspring in F₁ are all plant with yellow pod. Therefore, green pod is a recessive character.

Alternative form of the same gene is called allele. Example gene for height of a plant is denoted by T for tall plant and t for dwarf (short) plant. So, gene T and t are alleles.

Mendel selected seven (7) pair of contrasting characters. These are plant height, flower colour, position of flower, shape of pod, colour of pod, shape of seed and colour of seed.

When F₁ hybrids are crossed with any of the parent in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene.

50% plants will be heterozygous tall and 50 % are dwarf if gene T determines the height of the plant.

The ratio of tall (Tt) and dwarf (tt) is 1:1

Mendel selected seven (7) pair of contrasting characters. These are plant height, flower colour, the position of flower, the shape of the pod, the colour of pod, the shape of seed and colour of seed. But he has not chosen the colour of root for his experiment.

Genotypes are formed in F₂ generation of monohybrid cross are in the ratio 1:2:1. 1 plant is pure tall (TT), 1 plant is a pure dwarf (tt) and 2 plants are hybrid tall (Tt). T is a dominant gene for tallness and t is a recessive gene for dwarfness.

Gregor Mendel is known as the "Father of Genetics". He had performed an experiment with plants in 1857 that led to increased interest in the study of genetics. Mendel studied at the University of Vienna from where he performed many scientific experiments with plants and postulated ‘Laws of heredity’.

Mendel selected garden pea for his experiment because it has a short life cycle, so can easily can study many generations in a short period of time. Artificial self-pollination can easily be done in garden pea plant.

A dominant character is an inherited characteristic which appears in an F1 offspring. It is transferred from a parent through a dominant allele. For example, tall (T) is a dominant gene over its allele (alternative form of the same gene) that is (t).

Transmission of genetic character from one generation to another generation is known as heredity. Transmission of hereditary characters from parental generation to offspring generation is called heredity. The term ‘Heredity’ was given by Spencer (1863).

The laws of inheritance presented by Mendel were ignored for 35 Years. Hugo De Vries of Holland Carl Correns of Germany, and Erick Von Tschermak, they worked separately and rediscovered the laws of inheritance of Mendel in 1900.

Full name of Mendel: Gregor Johann Mendel.

He was an Austrian monk who discovered the basic principles of heredity through experiments on the garden pea.

After performing his breeding experiments on garden pea, Mendel postulated three laws:

I. Law of dominance

II. Law of segregation or law of purity of gametes

III. Law of independent assortment

A test cross was first introduced by Gregor Mendel, to identify whether an organism showing a dominant character is homozygous or heterozygous for a particular allele. The organism in question is crossed with an organism that is homozygous for the recessive character, and the offspring of the test cross is studied.

Out cross is a cross in which F1 plant (Tt) is crossed with dominant parent plant (TT). All the offspring plants obtained by this cross are 100% tall. Among all tall 50%, offspring are homozygous (TT) tall and 50% are heterozygous (Tt) tall plants.

Law of independent assortment can be explained by monohybrid cross because to explain the law of independent assortment, two pairs of contrasting characters such as the height of the plant and seed colour are required.

● In a dihybrid cross, two plants having two or more contrasting characters such as characters for example height of a plant and shape of a seed, are crossed then inheritance of one character is unaffected by inheritance of another character.

● Example: let’s observe the cross between a tall plant with round seeds (TTYY) and a dwarf plant with wrinkled seeds (ttyy). Tall plant with round seed is dominant characters.

The reason behind Mendel success are as follows:

(a) He studied only one character of a garden pea at a time in his breeding experiment.

(b) Mendel analyzed all the data statistically to obtain the result of experiments of hybridization.

(c) Mendel wisely selected garden pea (Pisum sativum) plant for his experiment.

Mendel selected garden pea plant for his experiment because the following characters present in garden pea:

● Garden pea is an annual plant, it became easy to study many generations in a shorter period of time.

● Garden pea have bisexual flowers, it was very easy to obtain pure or homozygous plant through the process of self-pollination.

● Artificial cross-pollination was easily done by emasculation (removal of anthers).

● Various pairs of contrasting characters are found in pea plant.

● Gregor Johann Mendel was born at Silicon village of Heizendarf state in Austria on 22nd July 1822.

● He became a monk of a church of Brunn city of Austria in 1843 after finishing his studies.

● He performed breeding experiments on garden pea (Pisum sativum) for seven years in the garden of the church (1856-1863).

● He presented the conclusions of experiments in 1865 as his research papers in front of Brunn society of Natural history.

● In 1866 these results were published as “Experiments on plant hybridization” in the annual report of the society.

● Based on the results of experiments on garden pea, he formulated ‘Laws of inheritance’ which were known as ‘Mendelism’.

● He died on 6 January 1884.

● Law of Dominance was based on the results of monohybrid cross (crossing plants with one contrasting characters).

● When two homozygous plants, with one contrasting character, is crossed, the character which is expressed in an F₁ generation is known as a dominant character while the character which is not expressed in an F₁ generation is known as recessive character.

● Example: If pure or homozygous tall plants (TT) are crossed with pure homozygous dwarf plant (tt), then we obtains all the plants in F₁ generation (100%) as tall plants.

The appearance of all tall pants proves the law of dominance.

● The expression of dominant character in living is very importantly because many harmful genes cannot express their character due to their recessive nature.

● Due to Mendel’s law of segregation, the concept of gene came into being.

● New characters produced in the hybrid is due to our knowledge by the law of Mendel.

● Useless characters can be removed and useful characters can be brought together in the same species.

● Disease resistant and more yielding crops can be developed by the use of Mendel’s laws of inheritance.

● Law of Segregation is also known as Law of Purity of gametes. According to this law, during gamete formation in the F₁ hybrid plant both the alleles are got separated from each other and goes into separate gametes.

● Hence, it is known as the law of segregation or law of purity of gametes.

● Each gamete contains only one allele for one character. Thus no mixing of characters, gametes are always pure.

● Example: If homozygous tall plant (TT) is crossed with the homozygous dwarf plant (tt), then all the offspring of F₁ generation are heterozygous tall plants.

● In the heterozygous plant, both the alleles remain with each other but do not get mixed with each other.

● Both alleles are separated and go into separate gametes. Due to this reason dwarf plant again reappear in the F₂ generation.

Appearance of dwarf plant in F₂ generation confirms segregation of gametes.

The three ‘laws of inheritance’ such as the law of dominance, the law of segregation and law of independent assortment are known as Mendelism.

● Law of independent assortment can explain by a dihybrid cross. In a dihybrid cross, two plants having two or more contrasting characters such as characters for example height of a plant and shape of a seed, are crossed then inheritance of one character is unaffected by inheritance of another character.

● In F₂ generation formation of plants like dwarf plant with yellow seed and tall plants with green seeds, show an independent assortment of characters.

● Example: let’s observe the cross between a tall plant with round seeds (TTYY) and a dwarf plant with wrinkled seeds (ttyy). Tall plant with round seed is dominant characters.

Four types of plants are appeared in the F₂ generation

After performing his breeding experiments on garden pea, Mendel postulated three laws called ‘Laws of Inheritance’. These are:

(a) Law of dominance

(b) Law of segregation or law of purity of gametes

(c) Law of independent assortment

●Law of dominance states that When two homozygous plants, with one contrasting character, is crossed, the character which is expressed in an F₁ generation is known as a dominant character while the character which is not expressed in an F₁ generation is known as recessive character.

● Example: If pure or homozygous tall plants (TT) are crossed with the pure homozygous dwarf plant (tt), then we obtain all the plants in an F₁ generation (100%) as tall plants.

● Law of Segregation or Law of Purity of gametes. According to this law, during gamete formation in the F₁ hybrid plant both the alleles are got separated from each other and goes into separate gametes.

● Hence, it is known as the law of segregation or law of purity of gametes.

● Each gamete contains only one allele for one character. Thus no mixing of characters, gametes are always pure.

● Example: If homozygous tall plant (TT) is crossed with the homozygous dwarf plant (tt), then all the offspring of F₁ generation are heterozygous tall plants.

● Law of independent assortment states that when two plants having two or more contrasting characters such as characters for example height of a plant and shape of a seed, are crossed then inheritance of one character is unaffected by inheritance of another character.

● In F₂ generation formation of plants like dwarf plant with yellow seed and tall plants with green seeds, show an independent assortment of characters.