Genetics: Principles of Heredity

What is Heredity?

In Biology, heredity is the term used for transmission of the traits from one generation to the next generation. It is due to heredity that the offsprings look similar to their parents. It also explains why dogs always give birth to puppies and never to kittens. The process of heredity is universal among all living organisms. Genetic variation refers to the variation in a population or species. Genetics is the study of heredity and variation in living organisms.

Transmission genetics and cytogenetics have helped scientists investigate the biological basis of heredity. In transmission genetics, organisms are crossed to study the inheritance pattern in offsprings. Cytological techniques help in understanding cellular reproduction. With the advancement of molecular biology and its tools and techniques, geneticists are able to understand the genetic basis of the inheritance of traits and variations present in various organisms.

Genetics

Genetics is the science which deals with the mechanisms responsible for similarities and differences among closely related species. The term ‘genetic’ was coined by William Bateson in 1905. It is derived from the Greek word ‘genesis’ meaning grow into or to become. So, genetics is the study of heredity and hereditary variations, it is the study of the transmission of body features: i.e., similarities and difference, from parents to offspring and the laws related to this transmission.

Also Read: Behavioural genetics

Variation

Any differences present between individuals of any species, caused either by genetic difference or by the effect of environmental factors, is called variation. Variation can be shown in physical appearance, metabolism, behaviour, learning and mental ability, and other obvious characters.

Types of Variation

Variations can be categorised into two types:

Genotypic variations: – Genotypic variations refer to the differences in the genome, it may be due to structure or number of chromosomes present in the cells or difference in the genetic constituents of the chromosomes. Skin, hair, eye colour, height are some of the genotypic variations in animals. A variation can only be confirmed as genotypic by doing cross-breeding experiments.

Somatic variations: – Somatic variations are not hereditary. These are not due to changes in the alleles or chromosomes. These are due to various factors such as nutrition, climate and due to other social interactions.

Heredity

Heredity refers to the transmission of traits from parents to offsprings. Heredity is responsible for the resemblance among individuals of the same species.

Mendel’s Laws of Heredity

Gregor Johann Mendel is known as the father of genetics. He was the first to show the inheritance pattern of traits from one generation to the next generation. He did his research on the garden pea, Pisum sativum. He selected 7 pairs of contrasting traits like the red and yellow colour of the pod, round and wrinkled seeds, tall and short plants, etc. and crossbred the plants to understand their inheritance pattern. Mendel gave three fundamental laws of inheritance.

1. Law of dominance: States that in the heterozygous condition of the genotype for a pair of alleles, the alleles which express itself phenotypically is dominant and the one which can’t express is recessive.

2. Law of segregation: States that although the alleles of a character remain together for a long time, they do not mix with each other and separate at the time of gametogenesis so that each gamete receives only one allele of a trait, which is either dominant or recessive. When tall pea plants of the F1 generation (obtained by crossing homozygous tall and dwarf pea plants), are self-fertilised, we get tall and dwarf plants in the ratio of 3:1.

3. Law of Independent assortment: States that when more than a pair of characters are taken into consideration, alleles of a character can undergo any sort of combination to give rise to a phenotype differing from both the parents.

Notations used in Breeding Experiments

The dominant trait – Upper case letter, e.g. Tallness is represented by ‘T’

The recessive trait – Lower case letter, e.g. Dwarfness by ‘t’

Homozygous – A pair of same alleles, e.g. TT (homozygous dominant) or tt (homozygous recessive)

Heterozygous – Having different alleles of a trait, e.g. Tt

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