Question

What is Mendelism?

Answer 1

Gregor Johan Mendel ($1822 - 1884$) was the first who demonstrated the mechanism of transfer of characters from one generation to another. He is also known as the father of genetics. He also gave some general rules which were later raised to the status of principles or laws of inheritance. These laws constitute the foundations of genetics. Mendel was Austrian born Botanist, a teacher and the rules were formulated by him in $1865$. He carried out hybridization experiments on garden peas for $7$ years from $1856 - 1863$ and confirmed the purity of his experiments through inbreeding.

Complete explanation:
Mendelism or Mendel’s Principles of Inheritance are the rules which were first discovered by Mendel. There are four laws of inheritance. These laws are based on monohybrid and polyhybrid crosses.
One Gene Inheritance: A gene controls every character that has at least two alleles (monogenic inheritance). One gene inheritance is the study of inheritance of a single pair of alleles (factors) of a character at a time. It is also known as monohybrid cross. On the basis of his observations of monohybrid cross, Mendel proposed a set of postulates which resulted into the formulation of three laws of inheritance which are as follows:
1. Principle of Paired Factors: In an organism, a character is represented by at least two factors. The two factors lie at the same locus on two homologous chromosomes. These characters may represent the same (homozygous, e.g., TT in case of pure tall Pea plants, tt in case of dwarf Pea plants) or alternate expressions (heterozygous, e.g., Tt in hybrid tall Pea plants) of the same character. Factors representing the same or alternate form of character are called alleles or allelomorphs.
2. Law or Principle of Dominance: It states that in a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive). This law explains the expression of only one of the parental characters in a monohybrid cross in the $F1$ generation and the expression of both in $F2$ generation. It also explains the phenotypic ratio of $3:1$ obtained at the $F2$ generation.
3. Law of Segregation: This law states that the two factors of a character present in an individual keep their identity distinct, separate at the time of gametogenesis and then get randomly distributed to different gametes, then they get paired again in different offspring as per the principle of probability. It is based on the fact that the factors do not show any blending and both the characters are recovered as such in the $F2$ generation though one is not seen in the $F1$ generation. It produces the phenotypic monohybrid ratio of $3:1$ ($3$ tall and $1$ dwarf) and genotypic ratio of $1:2:1$ ($1$ pure tall, $2$ hybrids tall and $1$ dwarf).
4. Principle or law of Independent Assortment: According to this law the two factors of each character assort or separate independently at the time of gamete formation and get randomly rearranged in the offspring. They produce both parental and new combinations of traits. This law is studied by the means of dihybrid cross. It produces a phenotypic ratio of $9:3:3:1$ in $F2$ generation and a genotypic ratio of $1:2:1:2:4:2:1:2:1$.

Note:
Mendel took $34$ pairs of varieties of Pea plants, then $22$ but at last worked with only $7$ pairs of varieties. They differed in characters such as flower color, flower position, height, pod shape, pod color, seed shape, seed color etc. All the varieties were pure lines. Mendel performed various types of cross breeding and then he allowed the offspring to self-breed. His experiments have large sampling sizes which give greater credibility to his data. Further he was first to use mathematical logic in solving biological problems.