As previously stated, genetics is the principal science that underlies plant breeding. Gregor Mendel made significant contributions to the development of the discipline of genetics, albeit in absentia. He derived several postulates or principles of inheritance, which are
often couched as Mendel’s laws of inheritance.
Mendelian postulates Because plant breeders transfer genes from one source to another, an understanding of transmission genetics is crucial to a successful breeding effort. The method of
breeding used depends upon the heredity of the trait being manipulated, among other factors.
According to Mendel’s results from his hybridization studies in pea, traits are controlled by heritable factors that are passed from parents to offspring, through the reproductive cells. Each of these unit factors occurs in pairs in each cell (except reproductive cells or gametes).
In his experiments, Mendel discovered that in a cross between parents displaying two contrasting traits, the hybrid (F1) expressed one of the traits to the exclusion of the other. He called the expressed trait dominant and the suppressed trait recessive. This is the phenomenon of dominance and recessivity. When the hybrid seed was planted and self-pollinated, he observed that both traits appeared in the second generation (F2) (i.e., the recessive trait reappeared), in a ratio of 3 : 1 dominant : recessive individuals.
Mendel concluded that the two factors that control each trait do not blend but remain distant throughout the life of the individual and segregate in the formation of gametes.
This is called the law of segregation. In further studies in which he considered two characters simultaneously, he observed that the genes for different characters are inherited independently of each other. This is called the law of independent assortment. In summary, the two
key laws are as follows:
Law I: Law of segregation: paired factors segregate during the formation of gametes in a random fashion such that each gamete receives one form or the other.
Law II: Law of independent assortment: when two or more pairs of traits are considered simultaneously, the factors for each pair of traits assort independently to the gametes.
Mendel’s pairs of factors are now known as genes, while each factor of a pair (e.g., HH or hh) is called an allele (i.e., the alternative form of a gene: H or h). The specific location on the chromosome where a gene resides is called a gene locus or simply a locus (loci for plural).
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