The study of
science of genetics begins with the work of Gregor Mendel. He was a clergyman
and a teacher. He performed his own experiment on garden pea. He restricted his
attention to the single character and kept pedigree records for each plant.
Mendel studied the nature of inheritance in plants and presented his work in
1885 in the paper “Experiments
on Plant Hybridisation “. The importance of his work did not get noticed
until 1890s, after his death. The advantages of garden pea (Pisum sativum) as
experimental system were:
The characters were readily observable.
Plants were easy to grow and self fertilise before the
flowers opened
Mendel worked with 14 pure breeding varieties and selected
seven pairs of characters and propounded the following laws:
The law of dominance: Mendel stated that the hereditary
characters are determined by pair of factors (genes) in an individual. In a
monohy--brid cross all the plants in F₁ generation showed only one character
but in F₂ the other character appears. The character in F₁ generation is dominant and the one
which expresses in F₂ generation is recessive e.g., the cross between two
homozygous parents (P), one with two dominant alleles for tall plant (TT) and
the other with two recessive alleles for dwarf plant (tt). The phenotype of the
offspring in F₂
generation is dominant and the one which expresses in F₂ generation is recessive e.g., the
cross between two homozygous parents (P), one with two dominant alleles for
tall plant (TT) and the other with two recessive alleles for dwarf plant (tt).
The phenotype of the offspring in F₁ generation is tall plant but the
genotype is Tt. The F₁ heterozygote produces four kinds of gametes. When this plant
self-pollinates, the T and t egg and sperm cells combine randomly to form ¼TT
(tall), 2/4 Tt (tall) and ¼ tt (dwarf) offspring. The F2 generation has 1:2:1
genotypic ratio and 3:1 phenotypic radio.
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