Heredity and Variations

Heredity

The continuous transfer of genes from one generation to another and the study of expression is called heredity and the traits which are transferred from one generation to another are called inherited traits. New traits develop through both heredity and variations.

The word genetics was coined by William Bateson in 1905 from the Greek word gen which means ‘to be’ or ‘to increase’. But the father of genetics is called the Austrian priest Gregor John Mendel.

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Variations

Despite the passing of inherited traits from parents to the offspring, there are some traits in the offspring which are completely different from the parents, these are called Variations, that is, the differences found in homogeneous members are called differences. There are 6 types of variations –

Germinal or hereditary variations
Somatic or environmental or acquired variations
Discontinuous variations
Continuous variations
Determinate variations
Indeterminate variations

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Types of Variations

There are 6 types of variations, which are mainly kept in three pairs –

1. Germinal or hereditary variations

These variations take place in the germ cells i.e. germplasm and are transferred from one generation to another. They are hereditary. For example, hair and eye color.

2. Somatic or environmental or acquired variations

These variations are mainly due to environmental conditions, such as food, temperature or any other external factor. They are not inherited. For example, the difference in skin color.

3. Discontinuous variations

The sudden change in the body of any organism is called discontinuous variation. They are numerical or qualitative. Numerical variation can be positive or negative while qualitative variation shows new features.

4. Continuous variations

Small systematic variations found in homogeneous organisms from generation to generation, by which new traits are displayed, are called continuous variations.

5. Determinate variations

Variations found continuously in biological evolution, which keep on developing in a certain direction. They are called deterministic variations. They are beneficial or harmful.

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6. Indeterminate variations

Unlimited variations in any direction are called undetermined variations. They do not have any significance.

Reasons of Variations

Variations result from environmental influences, use or non-use of organs, or changes in genes.

Environmental Effects – Changes in the organism by light, heat, nutrition, air pressure etc. They are not inherited.
Changes in gene structure – The variations resulting due to this are inherited and after that the gene structure changes.
1. Sexual reproduction – it is due to the following reasons
  1. Dual parentage – as a result of mixing of parental chromosomes from generation to generation.
  2. Meiosis – formation of a new gene structure due to the irregular division and exchange of homologous chromosomes during cell division and independent separation of gametes at fertilization.
2. Mutation – It is due to the following reasons
  1. Gene Mutation – The chemical structure of a gene of a particular trait or its position on a chromosome is changed.
  2. Chromosomal Mutations – Changes in the structure of chromosomes due to errors such as transfer, reversal, deletion, frequency etc. at the time of meiosis.
  3. Chromosome group mutation – Variation arises due to change in the number of chromosomes.

Importance of Variations

Variations are the basis of biological evolution. Beneficial variations are inherited from generation to generation, resulting in the development of a new species. Both Heredity and Variations are important for the development of new organisms and new traits.

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