The Mendelism: Basic principles; The Chromosomal basis of Mendelism ​ (Chromosomal theory of heredity)​

The Mendelism: Basic principles; The Chromosomal basis of Mendelism ​ (Chromosomal theory of heredity)​

February 17, 2022

The Mendelism: Basic principles; The Chromosomal basis of Mendelism ​ (Chromosomal theory of heredity)​


  • Study of mechanism of heredity and variation in the living being is the genetics. 
  • “Genetics” word was derived from the Greek word ‘gen’ which means to become grow into and it was coined by Bateson in 1906 to describe the study of physiology of heredity and variations
  • “Process of transmission of characters from one generation to next, either by gametes-(sperm & ova) in sexual reproduction or by asexual reproductive bodies in asexual reproduction is called inheritance or heredity”.
  • Heredity is the cause of similarities between individuals
  • Transmission of those characters from parents to the offspring which are not seen in the parents is called variation
  • Variations cause difference among the individuals


Mendel’s Law (Gregor John Mendel 1822-1884)

  • Gregor Mendel was the first investigator who laid the foundation of Modern Genetics. By his famous experiments in pea plant he concluded that the inheritance is governed by certain factors which occur in the cells of each parent. 
  • He thought that each parent has two such factors, while their sex cells (sperm or pollens, ovum or eggs) have only such factor. 
  • He published his data in 1866 in volume 4 of the proceedings of the Natural Science SocietyIn this paper he proposed the law of inheritance
  • His work remained unappreciated till 1900 when independently three different groups of scientist namely Hugo de Vries in Oenothera; Carl Correns in Pea, maize and Tschermak in various flowering plants drawn the same conclusion like Mendel’s. 
  • Mendel’s original paper was republished in Flora, in 1901. Bateson confirmed Mendel’s work by a series of hybridization experiments.

Mendel’s selection of the experimental plants

Because of the following reasons Mendel selected pea as experimental plants

  • Variation: The pea plant possessed several no of detectable differences and at a time only single or two or three characters can be considered.
  • Reproduction: Pea showed sexual mode of reproduction due to that the inheritance of characters could be properly studied.
  • Controlled mating: The pea was able to mat in controlled or well planned conditions and that is why it was possible to maintain careful records of the offspring of many generations.
  • Short half life: Pea plant showed very short half life.
  • Large no of Progeny: Pea produced large no of progeny after each successive mating because it helped in deducing correct conclusions.
  • Convenience in handling: Pea plants can be raised and maintained conveniently and inexpensively in the laboratory.

Terminology used in the literature 

  • Allele (Alleloporph): One of two or more forms that can exist at a single gene locus, distinguished by their differing effects on the phenotype. Alternate form of genes (phenotypes)
  • Homozygous: The organism having two identical allele genes for a particular character. It is considered as genetically ‘pure for that particular character. 
  • Heterozygous: An individual containing both dominant and recessive genes, or allele pair for a particular character is known as heterozygous or hybrid. 
  • Cross: The deliberate mating of two parental types of organism in genetic analysis.
  • F1 First filial generation: Latin word filin, meaning the son. The first generation of a given cross is known as F1 generation.
  • F2 or Second filial generation: The second generation which is resulted by interbreeding or selfing of Foffspring is known as F2 generation.
  • Progeny: Offspring individuals
  • Monohybrid: When the cross takes place between the parents differing in a single pair of contrasting characters resulted into a monohybrid individual.
  • Dihybrids cross: The cross between the two parents differing in two pair of contrasting characters. 
  • Phenotypes: Observable characters of particular genotype
  • Genotype: The genetic makeup or constitution of an individual. 
  • Haploid (Monoploid): An individual or cell containing a single complete set of chromosome. 
  • Genome: a complete set of chromosome, or of chromosome genes, inherited as a unit from one parent, or the entire genotype of a cell or individual. 
  • Chromosome: The nucleoprotein structures which are generally more or less rod-like during nuclear division. The genes are arranged on the chromosome in a linear fashion. Each species has a characteristic number of chromosomes. Chromosome play most important role in inheritance.
  • Chromatin: A DNA, RNA, histone and non-histone protein containing thread-like coiled structure of interphase nucleus is called chromatin. 
  • Back cross: The cross of a progeny individual with its parents is known as back cross. 
  • Test cross: The cross of an individual (generally of dominant phenotype) with the parents having the recessive phenotype. Generally used to determine whether an individual of dominant phenotype is homozygous or heterozygous for given allele, or to determine the degree of linkage.  
  • Pure Line (True breeding line: A strain of individuals homozygous for all genes being considered or individual that shows no genetic variation from one generation another generation.
  • Genes: The fundamental physical and functional unit of heredity
  • Punnett square: A checkerboard grid designed to determine all possible genotype produced by given cross. Genotype of the gametes of one sex is entered across the top, those of the other down one side. Zygote genotype produced by each possible mating are then entered in the appropriate squares of the grid.     

Mendel’s Law of Inheritance

  • Law of Dominance
  • Law of Segregation
  • Law of Independent assortment

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