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7.10 The Concept of "Factor"
The period in which Mendel conducted his experiments and drew
his conclusions about the principles of heredity and variations, the world
was totally ignorant about ’genes’ (as the units of heredity) and
the ’chromosomes’ (as the carriers of hereditary units). Mendel
believed that some definite hereditary units or particles represent
the characters. He called these hereditary units ’factors’.
Thus, tallness is due to a factor for tallness and dwarfness
is due to the factor for dwarfness. Since tallness or dwarfness
are two expressions or forms of the same character,(i.e. height),
it is also understood that their factors also are two forms of the
same gene. Such two factors representing different forms of the
same character (or its gene) are described as alleles.
We now know these ’factors’ as genes. The term
’gene’ was first coined by Johansen in 1909. Chromosomes were
discovered by Strasburger in 1875. Now it is established that genes are
definite units of the DNA molecule of each chromosome. Genes are
present in a linear fashion on the chromosomes. Each chromosome carries
several genes ’linked’ together in ’linkage groups.’ Hence, genes are
not inherited individually as was thought by Mendel but are passed on
to the next generation as ’linkage groups’ (i.e., as whole chromosomes).
This fact alters Mendel’s principle of independent assortment. However,
the principle of segregation remains unchanged even today. The studies
on genetics have shown that genes influence certain biochemical reactions
which in turn are responsible for the production of characters in organisms.
However, a single character is not (always) produced by a single gene
as believed by Mendel. On the contrary, several genes influence the same
basic biochemical reaction and are responsible for the expression of a
SUMMARY : MENDEL’S LAWS OF INHERITANCE
(1) Genetics is the study of the principles of heredity and variations. (2) Hybridization experiments using garden pea plants by Mendel, and his conclusions and explanations regarding the nature of inheritance of each character are commonly known as Mendelism. (3) Mendel established the phenomena of dominance and recessiveness through monohybrid experiments. (4) Mendel formulated the law of segregation of characters on the basis of the results of monohybrid crosses. (5) The law of independent assortment of characters was formulated by Mendel on the basis of the results of dihybrid crosses. (6) The monohybrid cross in Mendelian experiments gives a phenotypic ratio of 3 : 1 and genotypic ratio of 1:2:1 in the F2 generation. (7) The dihybrid cross gives the phenotypic ratio of 9 : 3 : 3 :1 and genotypic and genotypic dihybrid ratios are the products of their respective monohybrid ratios. (9) Mendel devised the test cross (back cross) method to verify the genotype of F1 hybrid as well as for testing correctness of his laws of heredity. (10) Mendel established the concept of ’factor’ which is now known as gene.
Table of Contents7.0 Introduction
7.1 Gregor Mendel
Mendel's Experiment on Sweet Pea
Law of Dominance
Law of Segregation
Law of Independent Assortment
Test Cross or Back Cross
The Concept of "Factor"