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 The mechanism of transmission of characters from the parental generation to the offspring is called heredity. |
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 The differences seen among members of a species and their offspring are described as variations. |
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 Genetics is the study of heredity, variations and the environmental factors responsible for these variations. |
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 The earlier belief on heredity was that characteristics of the two parents mix and express in the offspring. These ideas came to be known as the blending theory of inheritance. |
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 Pythagoras suggested that new individuals are formed by a combination of vapours from different organs of an animal. |
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 Aristotle believed that inheritance occurs due to mixing of the female semen (menstrual fluid) with the male semen that provides a vital ingredient. |
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 Leeuwenhock and others held the opinion that a miniature human being is already present inside the sperm and it only enlarges into an offspring on meeting with the ovum. |
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 Darwin believed that tiny particles called pangenes are formed in the different parts of the body, which migrate to the gametes. |
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 The blending theory on inheritance became replaced by the particulate theory of inheritance due to the contributions made by John Gregor Mendel (1822-1884). |
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 Mendel conducted a series of experiments on the pea plant Pisum sativum and collected enormous amount of data. |
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 He analysed the data and came to three major conclusions, which are now described as the principle of dominance, law of segregation and law of independent assortment. |
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 Mendel also designed a test cross to verify his own results. The test cross is also useful for ascertaining unknown genotypes. |
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 The rediscovery of Mendelism in 1900 and the subsequent development in the field of genetics, have established the importance of chromosomes in the inheritance of characters. |
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 The chromosome theory of inheritance was put forth by Boveri and Sulton 1902, according to which the Mendelian factors (genes) are located on the chromosomes and it is the chromosomes which segregate or assort independently during meiosis. |
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 The patterns of inheritance, which cannot be explained on the basis of Mendel's laws alone, are described as non-Mendelian inheritance. |
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 It is now known that certain genes can bring about a modification of the normal phenotypic expression of a given gene. Such a phenomenon is known as gene interactions. |
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 Intragenic (inter allelic) interactions occur between two alleles of the same gene. Common examples are incomplete dominance or co-dominance. |
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 Intergenic (non-allelic) interactions occur between alleles of different genes. Epistasis, pleiotropy and quantitative inheritance are the common examples. |
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 Incomplete dominance, both the alleles of a given trait fail to dominate over each other as a result they express as a blend. |
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 The inheritance of flower colour in the 4'Oclock plant and feather colour in and alusian fowl, are common examples. |
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 Multiple alleles is a phenomenon where more than two alleles occur for a given trait. |
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 The inheritance of ABO blood groups is an example for multiple alleles. There are 3 alleles responsible for 4 blood groups A, B, AB and O. |
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 The inheritance of ABO blood groups is also an example for co-dominance where each of the two alleles responsible for blood group AB, produce respective antigen and neither of them checks the expression of the other. |
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 Epistasis is a pattern of inheritance where a pair of alleles situated on a locus prevent the expression of a pair of alleles situated at another locus. |
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 The inheritance of feather colour in white fowls is a classical example of epistasis. |
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 The phenotypic ratio in the F2 generation in epistatic condition is 13:3 as against 9:3:3:1. |
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 Pleoitropy is a phenomenon where a given gene has multiple phenotypic effect, influencing not only the trait expressed by it but also the traits expressed by other genes. |
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 In the sweet pea plant the genes that control flower colour, also control the seed coat colour. |
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 Quantitative inheritance is a pattern of inheritance where a single phenotypic trait is influenced by more than one pair of alleles. |
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 Many kinds of possible genotypes occur and as a result a side range of phenotypes express. Such a trait is hence called polygenic or multifactor trait. |
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 The inheritance of skin colour in man is known to be under the control of at least three pairs of alleles Aa, Bb and Cc located on different chromosomes. |
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