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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