Mendelian Genetics

Mendel’s Theory of Genetics

Biological genetics, in simple words, is the passing of traits from parents to their offspring. This passing can occur through sexual reproduction or asexual reproduction. The traits are passed onto offspring as genetic information. There are different types of biological genetics. One such type is the Mendelian Genetics, which, discovered in 1900, changed the whole domain of genetics and inheritance forever.


Pre-Mendelian Concept of Heredity

A number of standpoints had already emerged before the Mendelian concept of genetics was discovered. In general, it was believed that the “essences” of parents used to blend during coitus, which was the main reason for inheritance. This theory is termed as the “theory of blending inheritance,” and most of the pertinent views in the pre-Mendelian era were based on this theory: -

  1. Moist Vapour Theory: This theory was advocated by Pythagoras in which he believed that the male body produced some sort of a moist vapour during coitus, which helped in the development of the body parts of the embryo. 

  2. Reproductive Blood Theory: This theory was propounded by Aristotle. He was of the belief that both the males and the females produced reproductive blood. But the male reproductive blood was purer than the female reproductive blood. When the two reproductive drops of blood coagulated to form the embryo, it was due to the male’s pure blood that the characteristics of the male contributed more than the impure blood of the female.

  3. Preformation Theory: This theory was given by Swammerdam. He believed that the organism already existed or was pre-formed in the eggs or sperm in a very minute form. This miniature was called the homunculus, which required fertilization to speed up its growth.

  4. Theory of Epigenesis: The preformation theory was discarded by Wolff, a German scientist. He came up with another theory-the theory of epigenesis- in which he believed that the organism did not develop as a homunculus in sperms or eggs. But, the formation of the body parts of the embryo took place step by step. It was only after the fertilization that this formation began.

  5. Theory of Acquired Characters: As per a famous French biologist Lamarck, a new character is passed on to the progeny of an individual once it has been acquired by the same individual. This theory was later rejected by a biologist who experimented on at least 20 generations of a rat.

  6. Theory of Pangenesis: Charles Darwin, the father of evolution, theorized that miniature and invisible body parts exist in the blood called gemmules and are transmitted to sex organs and assembled in the gametes. After the fertilization process, these gemmules develop into natural body parts and organs.

  7. The Germ Plasm Theory: This theory is propounded by a German biologist called August Weismann. He theorized that there were generally types of body tissues- germplasm and somatoplasm. Germplasm tissues were the reproductive tissues that helped in the production of gametes. On the other hand, somatoplasm was tissues other than the reproductive ones. 


What was Mendel’s Experiment?

Gregor Mendel experimented on crossbred pea plants with single traits over various generations. In this breeding experiment, he crossed a pair of pea plants, with each having a different trait. Example, if one plant was short, the other was tall; if one had the shorter stem, the other pea plant had a longer stem; if one had round peas; the other plant had wrinkled peas, and if one plant bore white flowers the other pea plant bore purple-colored flowers.

On crossing, Mendel found out that the next generation called F1 consisted of whole individuals showing one trait only. In the next stage, the F1 generation was interbred, and Mendel found that the new F2 generation showed a different result. The traits were in the ratio of 3:1, wherein every three individuals showed similar traits of one parent. 

This led Mendel to formulate that the genes in the human body could be combined in three possible forms, and these combinations were made up of different genetic factors or hereditary units- AA, aa, and Aa. The plants in the first stage were AA or aa, i.e., homozygous. The F1 generation Aa and the F2 generation was aa, AA, or Aa. 

This led to the formulation of Mendel’s Laws of Inheritance which summarized and concluded his study –

  1. Law of Segregation: This law states that for any trait, every pair of genes called alleles from both the parent splits and one gene from each parent transmits to the offspring. The passing of genes of any trait is a matter of chance.

  2.  Law of Independent Assortment:  This law states that different pairings of genes or alleles of different traits pass on to the offspring without actually depending on each other. Therefore, the inheritance of one region does not affect the inheritance of another region.

  3. Law of Dominance: While mating, each offspring acquires the trait of one parent only. If a dominant trait or factor is present in a parent, the offspring will exhibit the dominant trait. Recessive factors can only be acquired if both the factors in a gene are recessive in nature.

FAQ (Frequently Asked Questions)

1. Explain The History of Mendel’s Theory of Genetics.

Gregor Mendel was an Austrian monk and a biologist. He discovered a new theory of inheritance in 1865 through a pea plant experiment. With his experiment, he deduced two generalizations, which later came to be known as Mendel’s Principle of Inheritance. His research paper was published in 1865 but was largely unknown and ignored by scientists and biologists of his time. It was only in 1900 that his works were later rediscovered and studied. This pretty many sumps up the Mendelian theory of genetics.

2. Explain The Emergence of Post-mendelian Concepts of Heredity.

Although Mendel’s theory of genetics was rediscovered as late as 1900, it caused much controversy and debate amongst scientists and biologists. It also led to the emergence of the post-Mendelian concepts of heredity, which was referred to as the Mendelian Deviations. These deviations included concepts that challenged Mendelian genetics like multi alleles, codominance, linkage, incomplete dominance, and many more.