Principles of Inheritance and Variation

The branch of biology which deals with principles of inheritance and its practices is called genetics In the mid-nineteenth century, Gregor Mendel was the first to study this phenomenon systematically. He conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms. He proposed that the ‘factors’ or genes regulate the characters and are found in pairs known as alleles. He observed the expression of the characters in the offspring. Some characters are dominant and are expressed when factors are in heterozygous condition. This law is called Law of Dominance. Some characters are recessive and are expressed in homozygous conditions only. Hence, characters are segregated during the formation of gametes. This law is called Law of segregation. When Mendel studied the inheritance of two characters together, it was found that the factors independently assort and combine in all permutations and combinations. This law is called Law of Independent Assortment. The genetic makeup of a character is called genotype while the physical expression of the characters is called phenotype. The Mendel’s laws were extended in the form of ‘Chromosomal Theory of Inheritance’. The Linkage maps show the arrangement of genes on a chromosome. Many genes are linked to sexes also, and called as sex-linked genes. The male and female sexes have a set of chromosomes which are common, and another set which are different. The chromosomes which are different in two sexes are called sex chromosomes. The remaining set of chromosome is called autosome. Sex determination is a phenomenon by which the sex of an individual can be defined. In humans, a normal female has 22 pairs of autosomes and a pair of sex chromosomes (XX). A male has 22 pairs of autosomes and a pair of sex chromosome as XY. The study of human genetics by the use of pedigree charts is called pedigree analysis. Mendelian disorders are caused due to alteration or mutation in a single gene. Mutation is defined as change in the genetic material. A point mutation is a change of a single base pair in DNA Some of the common Mendelian disorders are Haemophilia, Sickle cell anaemia, Colour blindness, Phenylketonuria, Thalassaemia etc. Some of the common chromosomal disorders are Down’s syndrome, Klinefelter’s syndrome, Turner’s syndrome etc.

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