Principles of Inheritance and Variation


Genetics term was given by William Bateson.

Genetics is the collective study of heredity and variations.

Population: It is not a mere collection of a large number of people but it always consists of a single species that has a definite period of existence and well-defined rates of birth, death, sexes, and age structure.

Heredity: Transmission of genetic characters from parent to offspring is called heredity.

Inheritance: It is the basis or pattern of Heredity.

Variation: Individuals of the same species have some differences or dissimilarities, these differences are called variations.

History of Researches Linked to Genetics

Father of Genetics – Gregor Johann Mendel.

Father of Modern Genetics – William Bateson.

Father of Experimental Genetics – Morgan

Morgan performed an experiment on Drosophila and proposed various concepts like Linkage, Sex Linkage, Crossing over, Crisscross inheritance, Linkage map, or Genetic map on Drosophila.

Father of human genetics and biochemical genetics – Archibald Garrod

Garrod discovered the first human metabolic disorder which is called Alkaptonuria (black urine disease). In this disease, the patient has enzyme homogentisic acid oxidase deficiency. He gave the concept ‘One mutant gene – one metabolic block’.

Genetics Related Terms

Factors: Unit of heredity which is responsible for inheritance and appearance of characters. These factors were referred to as genes by Johannsen (1909). Mendel used the term “element” or “factor“.

Morgan first used symbols to represent the factor. Dominant factors are represented by capital letters while recessive factors are by small letters.

Allele: Alternative forms of a gene that are located in the same position (loci) or the pair of genes on the homologous chromosome is called Allele.

Representation of allele on homologous chromosome
Representation of allele on the homologous chromosome

The term allele was coined by Bateson.

Characters: Morphological and physiological expression of an organism is called characters.

Examples – Plant height, Flower color.

Trait: Alternative form of character is called a trait.

Example – Plant height (Dwarf, Tall)

Gene/Factor/Element: It is the basic unit of inheritance that is responsible for the transfer of characters from one generation to another generation.

Dominant: It is a character that does not allow the expression of contrasting characters in a hybrid.

Example – In a heterozygous tall plant (Tt) here, the “T” gene is dominant.

Recessive: It is that gene whose expression is suppressed by the dominant allele.

Example – In heterozygous tall plant (Tt) here, the “t” gene is recessive

Homozygous: A zygote is formed by the fusion of two gametes having identical factors called homozygotes and the organism developed from this zygote is called homozygous.

Example – TT, RR, tt

Heterozygous: A zygote is formed by the fission of two different types of gamete carrying different factors is called heterozygote (Tt, Rr), and individually developed from such zygote is called heterozygous.

The term homozygous and heterozygous is coined by Bateson.

Hemizygous: If an individual contains only one gene of a pair then the individual is said to be Hemizygous. The male individual is always Hemizygous for the sex-linked genes.

Example – Heterogametic organism (Like in man having X/Y chromosome)

Pure Breed: It is a homozygous individual having the same genes (both recessive or both dominant) for a character on its pairs of homologous chromosomes.

Phenotype: It is the external and morphological appearance of an organism for a particular character.

Genotype: The genetic constitution or genetic make-up of an organism for a particular character.

Genotype and phenotype terms were coined by Johannsen.

Phenocopy: If different genotypes are placed in different environmental conditions then they produce the same phenotype. Then these genotypes are said to bePhenocopy of each other.

Hybrid vigor or Heterosis: Superiority of offspring over its parent is called Hybrid vigor and it develops due to Heterozygosity.

  • Hybrid vigour can be maintained for long time in vegetatively propagated crops.
  • Hybrid vigour can be lost by inbreeding (selfing) because inbreeding induces the Homozygosity in offsprings.
    • Loss of Hybrid vigour due to inbreeding, is called as inbreeding depression.

Eugenics: Improvement of the human beings by the use of the principle of inheritance is called Eugenics, Like restrictions in mating two disease carrier individuals.

Euthenics: Improvement of human beings by improving their environmental conditions is called Euthenics.

Euphenics: Improvement of human beings by the use of genetic engineering and medical science is called Euphenics.

Cross-Pollination: In this type of pollination heterozygosity increases and hybrid vigor increases.

Self-Pollination or Inbreeding: In this type of pollination heterozygosity decreases and hybrid vigor decreases, it’s also called inbreeding depression.

Do you know how the hybrid vigor is maintained?

Hybrid vigor is maintained by vegetative propagation.

Forward Genetics: Forward genetics is an approach based on the molecular genetics of determining the genetic basis which is responsible for a phenotype. In this genetic bases are responsible for a particular phenotype. It is studied from phenotype to genotype.

Example – Mendelian genetics

Reverse Genetics: It is experimental molecular genetics that enables researchers to elucidate gene function by analyzing the changes made to phenotypes (of cells or in an organism) caused by genetically engineering in a specific nucleic acid sequence (within a DNA or RNA).

Example – Genomics (the study of the genome)

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