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Mendel's laws: definitions

Genetics - a branch of biology that deals with the study of heredity and variations

Variations - differences amongst the individuals of the same species and the offspring of the offspring of the same parents. Variations can be due to mutations.

Gene is functional unit of heredity.

Heredity includes those traits or characteristics which are transmitted from generation to generation and it is therefore fixed for a particular individual.

Allelomorphs or alleles indicate alternative forms of the same genes.

Backcross - the f1 individuals obtained in a cross are usually selfed to get f2 progeny. They can also be crossed with the one of the other two parents from which they were derived; such a cross of f1 individual with either of the two parents is known as a backcross.

Test cross - when f1 is backcrossed with recessive parent is called testcross.

Laws of Mendel

Principle of segregation (Law of purity of gametes)
A gamete may carry either the dominant or the recessive factor but not both as we find in f1 individuals. This suggests that there is no blending of mendelian factors in f1, but that they stay together and only one is expressed. The gametes which are formed are always pure for a particular character. This is why it is called either as principle of segregation or law of purity of gametes.

Principle of independent assortment (Mendel's second law)
The two character under consideration are assorting (give rise to different combination) in an independent manner. This principle is known as independent assortment.

Law of dominance
Out of the 2 alleles, the one which is able to express itself is the dominant allele while the other is the recessive allele. In the presence of recessive allele and dominant allele, only dominant allele will express itself.

Law of alleles
Mendel proposed that each genetic character is controlled by a pair of unit factor, now commonly called alleles or allelomorphic factor.


An article submitted by [email protected] for Biology-Online.org on September21, 2008.

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