The branch of biology under which heredity and variation are studied is called genetics. Gregor Johann Mendel laid the foundation of modern genetics with his scientific discoveries. That is why Gregor Johann Mendel is called the Father of Genetics.
Every living being found in the world has many such qualities, which are transmitted from parents to their children from generation to generation. Such qualities are called hereditary characteristics or ancestral qualities. The transmission of basic characteristics of organisms from one generation to the next is called heredity. Due to the transmission of basic qualities, the qualities of every living being are similar to the qualities of its ancestors. These traits are transmitted from one generation to the next through the gametes of the parents. Therefore, the transmission of ancestral traits from parents to their offspring through gametes from generation to generation is called heredity.
Laws of heredity: –
Gregor John Mendel was the priest of a Christian monastery in Brunn (Austria) and laid the foundation of modern genetics with his scientific discoveries. He published the findings of his experiments on pea plants in the Annual proceedings of the natural history society of Brunn in the year 1866, but it remained unnoticed in the scientific world for years. After his death, the results of his experiments were recognized by scientists in the year 1900.
Mendel chose seven pairs of garden pea (Pisum Sativum) traits for his experiments, one trait of each pair having the ability to suppress the other trait during the experiment. He called the first trait Dominant and the second trait Recessive. Mendel expressed these factors responsible for inheritance of traits in the form of a symbol. Among the pairs of traits, he expressed the factor of dominant trait in capital letters and the factor of recessive trait in small letters. Example:- “T” for tallness, whereas, ‘t’ for dwarfism.
According to Mendel, every germ cell has two factors to express the same trait. When these two factors are equal, the condition is called homozygous and when they are opposite, the condition is called heterozygous.
Mendel first studied the inheritance of one pair of opposite traits and then two pairs of opposite traits, which are called monohybrid cross and dihybrid cross respectively.
Monohybrid cross: –
When hybridization is done between two plants on the basis of one unit trait, it is called monohybrid cross. In a hybrid cross, Mendel selected two subspecies of pea plants, one of which was tall and the other dwarf in pairs of opposite traits and when they were crossed with each other, it was observed that in the first generation (F1 generation) The plants that were produced were all tall. All these first generation plants are called F1 plants. He then grew the plants obtained from F1 generation by self pollination and found that the phenotypic ratio of tall and short plants found in the second generation F2 was 3: 1. This type of ratio is also called monohybrid ratio. Of the three tall plants, one was pure tall (TT) and two were mixed or hybrid tall (Tt). A dwarf plant that was produced from the F2 generation was a pure dwarf.
If we get the third generation i.e. F3 from the F2 plant, we will see that pure tall plants (TT) always produce tall plants. Similarly, pure dwarf plants (tt) always produce dwarf plants, but if mixed tall plants (Tt × Tt) are crossed, then like the F2 generation, the phenotypic ratio of tall and dwarf plants will be 3: 1.
Of the three tall and one dwarf plants of F2 generation, one pure tall (TT), two mixed tall (Tt) and one pure dwarf (tt) were produced whose ratio is 1 : 2 : 1. By crossing F3 with pure tall, pure tall plants are obtained. Only plants are obtained. Example :- TT × Tr → TT
When crossed with F3 pure stunted plants, only pure stunted plants are obtained. Example :- tt × tt → tt
But by crossing mixed tall (Tt) with mixed tall (Tt) again tall and dwarf plants are obtained in the ratio of 3:1. In this-
TT – Homozygous tall
Tt – Heterozygous tall
tt – Homozygous dwarf
Its ratio is Phenotypic ratio – 3:1 (3 tall and 1 dwarf)
Genotypic ratio – 1 : 2 : 1 (1 pure tall, 2 mixed tall and 1 pure dwarf).
Dihybrid Cross: –
In this, two pairs of opposite traits are crossed. Mendel crossed plants produced with round and yellow seeds and green and wrinkled seeds for dihybrid crosses. In this, round and yellow seeds are dominant. Both the plants are represented by RRYY and rryy respectively. It is clear that the gametes of the first plant will have RY factors and the gametes of the second plant will have ry factors. When artificial cross pollination was done between these two plants, the plants obtained from the seeds produced were all round and yellow hybrid seeds. Here wrinkled and green color was a recessive trait. So they remained hidden in the F1 generation, but round and yellow color was the dominant trait, so they appeared. Now self-pollination was allowed to occur in this F1 generation plants and F2 generation plants were obtained. According to the separation rule, four types of seeds were formed, the ratio of which was as follows – Round + Yellow seed = 9.
Round + Green Seed = 3
wrinkled + yellow seeds = 3
wrinkled + green seeds = 1
The ratio between round-yellow and wrinkled-yellow seeds was 3:1. The ratio between round-green and wrinkled-green seeds was also 3:1.
Mendel’s laws: – On the basis of monohybrid cross and dihybrid cross, Mendel propounded some rules related to heredity, which are known as Mendel’s law of Inheritance. Among these rules, the first and second are based on monohybrid cross and the third rule is based on dihybrid cross.
- Law of Dominance: – Under this, Mendel crossed a pair keeping in mind the opposite traits, then the trait present in the first generation remained dominant. Example: When a tall pea plant was crossed with a dwarf plant, only tall plants grew in the first generation. Due to this, according to the rules, tall was dominant and dwarf was recessive.
- Mendel’s Law of Segregation: – According to this rule, at the time of formation of gametes, the factors of a pair of genes (genes) get separated and only one of these factors reaches the gamete. Both the factors never enter the gamete together. This rule is also called the law of purity of gametes. Example:- When a tall pea plant is crossed with a dwarf plant, only tall plants grow in the F1 generation, but again when the flowers of the same generation are self-pollinated, the plants in the F2 generation are of both types. . Here the ratio of tall and dwarf plants is found to be 3:1.
- Law of independent assortment: – According to this law, different pairs of factors found in an organism are independent of each other and can freely mix to form new organisms. Can.
Sex Determination in Human: –
The total number of chromosomes in humans is 46. Each child receives one chromosome of each pair of homologous chromosomes from the mother through the egg and the other from the father through the sperm. In spermatogenesis, two types of sperm are formed by meiosis, half of which contain the X-chromosome of the 23rd pair (ie 22 + X ).
Whereas, in women, eggs having the same type of chromosome i.e. (22 + X) and (22 + X) are found. At the time of fertilization, if the egg meets the sperm having On the contrary, if an egg is fertilized by a sperm having a Y-chromosome, then the 23rd pair in the zygote will be XY and the child formed from it will be a boy. Therefore, the male chromosome is responsible for determining the sex of the child.
Variation: –
Variation is the quality of an organism that shows its difference from the basic form of the same quality of its ancestors or other members of its own species.
Reasons for variation: –
Genes determine the hereditary characteristics of all organisms. Diversity is transmitted from one generation to the next through genes. Therefore, duplication of genes is the main reason for diversity. Duplication of genes is essential for the division of cells and cell division is essential for reproduction. Therefore, it is due to reproduction that variation is transmitted from one generation to the next. Variations are generally seen only in the offspring produced by sexual reproduction.
Visible variations are generally less common in the generations resulting from asexual reproduction, such as vegetative propagation.
Types of variations: – There are two types of variations…
Germinal Variation, and somatic variation.
Germinal Variation: –
Such variations occur due to changes in germ cells. Such variations are inherited from one generation to the next. For this reason, genetic variation is also known as genetic variation. Some of such differences are visible from birth, such as eye and hair colour, while some differences appear after birth, for example, physical structure, body length, etc.
Somatic Variation: –
Such variations can appear due to many reasons, such as influence of climate and environment, types of food available, interactions with other living organisms, etc. Such variations are not due to changes in the properties of chromosomes or genes. Therefore, such variations are not inherited from one generation to the next. Such differences are acquired. For this reason they have no importance in biological evolution.
Sources of Genetic Variation: –
Genetic variation in organisms occurs due to mutation and they play an important role in the development of new species. Change in the structure and position of genes present on a chromosome is the cause of mutation. Another reason for genetic diversity is genetic recombination. Due to genetic recombination, the properties of genes in the chromosomes of the offspring may differ from those of their parents. Such new qualities can be helpful in adapting organisms to their environment. Sometimes such new qualities may not help the organisms to adapt to the environment. In such a situation, due to mutual competition, disease etc., such organisms become extinct in the race of evolution and the remaining organisms transmit such beneficial qualities to their offspring. In this way nature selects some organisms with new qualities and expels others.
Importance of Mendel’s laws of inheritance: –
- Recessive traits do not appear in the heterozygous state.
- Dominant traits appear in both homozygous & heterozygous states.
- Useful traits can be developed by hybridization method.
- Eugenics is based on Mendelian rules.
- Gene concept is confirmed by the law of segregation.
- By using Mendel’s laws, disease-resistant and high-producing crop plant varieties are developed.
- Through the hybridization method, useless traits can be removed and useful traits can be brought together in a single species.
Dr. (Prof.) Amarendra Kumar.
