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PRINCIPLES OF INHERITANCE AND VARIATIONGenetics
Genetics is a branch of biology dealing with inheritance and variation
of characters from parents of offspring.
Inheritance
Process by which characters are passed on from parent to progeny
Variation
Degree by which the progeny differs from its parents
Mendel’s Experiments
● Gregor Johann Mendel known as the father of genetics proposed the
laws of inheritance.
● He used garden pea as his sample.
● Large sampling size gave credibility to his collected data.
● Garden pea plant possessed certain completely opposite traits.
Example − tall and dwarf plants
● He worked on the following seven traits of garden pea:
S. No. Character Dominant Recessive
1 Stem height Tall Dwarf
2 Flower colour Violet White
3 Flower position Axial Terminal
4 Pod shape Inflated Constricted
5 Pod colour Green Yellow
, 6 Seed shape Round Wrinkled
7 Seed colour Yellow Green
● True breeding pea lines were obtained by continuous self pollination
for several generations.
● Fourteen true breeding pea lines were selected as pairs, which were
similar except for one character with contrasting traits.
● Artificial cross pollination (hybridisation) was performed on such
varieties to obtain first hybrid generation known as the first filial
progeny or F1.
Inheritance of One Gene
● After hybridisation, the F1 generation so obtained resembled only one
of its parents (say, all tall; no dwarf).
● When 2 plants from F1 generation were self pollinated, the second filial
progeny or F2 generation was obtained.
● Revival of unexpressed trait (dwarf) was observed in some F2 progeny.
Both traits, tall and dwarf, were expressed in F2 in ratio 3:1.
● Mendel proposed that something is being passed unchanged from
generation to generation. He called these things as ‘factors’ (presently
called genes).
● Factors contain and carry hereditary information.
● Alleles − Slightly different form of same factor
Two alleles code for a pair of two contrasting traits. (e.g., tall and dwarf)
Monohybrid Cross
● Cross that considers only a single character (e.g., height of the part)
, ●
● Studying the cross:
○ TT, tt, and Tt are genotypes while the traits, tall and dwarf, are
phenotypes.
○ T stands for tall trait while t stands for dwarf trait.
○ Even if a single ‘T’ is present in the genotype, phenotype is ‘tall’.
When ‘T’ and ‘t’ are present together, ‘T’ dominates and
suppresses the expression of ‘t’. Therefore, T (for tallness) is
dominant trait while t (for dwarfness) is recessive trait.
○ TT and tt are homozygous while Tt is heterozygous.
○ From the cross, it can be found that alleles of parental pair
separate or segregate from each other and only one allele is
transmitted to the gamete.
○ Gametes of TT will have only T alleles; gametes of tt will have
only t alleles, but gametes of Tt will have both T and t alleles.
● Punnett square
○ Graphical representation to calculate the probability of all
possible genotypes of offsprings in a genetic cross
○ Possible gametes are written on two sides, usually at top row
and left columns, and combinations are represented in boxes.
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