5. PRINCIPLES OF INHERITANCE AND VARIATION
INHERITANCE OF ONE GENE
Monohybrid cross: A cross involving 2 plants differing in one
character pair. E.g. Mendel crossed tall and dwarf pea plants
to study the inheritance of one gene.
Monohybrid
genotypic ratio:
1 Homozygous tall (TT)
2 Heterozygous tall (Tt)
1 Homozygous dwarf (tt)
=
1:2:1
Mendel
made similar observations for other pairs of traits. He proposed that some factors were inherited from parent to
offspring. Now it is called as genes.
Do not use T for tall and d for dwarf
because it is difficult to remember whether T & d are
alleles of same gene or not.
The
F1 (Tt) when self-pollinated, produces gametes T and t in equal proportion.
During fertilization, pollen grains of T
have 50% chance to pollinate eggs of
T & t. Also, pollen grains of t
have 50% chance to pollinate eggs of
T and t.
1/4th of the
random fertilization leads to TT (¼ TT).
1/2
(2/4) of the random fertilization leads to Tt (½ Tt).
1/4th
of the random fertilization leads to tt (¼ tt).
Tt x
Tt
Binomial
expression = (ax + by) 2
Hence (½ T + ½ t) 2 = (½ T + ½ t) (½ T + ½ t)
=
¼ TT + ¼ Tt + ¼ Tt + ¼ tt
=
¼ TT + ½ Tt + ¼ tt
Mendel
self-pollinated the F2 plants. He found that dwarf F2
plants continued to generate dwarf plants in F3 & F4.
He concluded that genotype of the dwarfs was homozygous- tt.
Back cross and Test cross
§ Backcross: Cross
between a hybrid and its any parent.
§ Testcross:
Crossing of an organism with dominant phenotype to a recessive individual. E.g.
Test cross is used to
find out the unknown genotype of a character. E.g.
Mendel
conducted test cross to determine the F2 genotype.
Mendel’s Principles or Laws of Inheritance
§ Characters
are controlled by discrete units called factors.
§ Factors
occur in pairs.
§ In
a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive).
2. Second Law (Law of Segregation)
“During gamete formation,
the factors (alleles) of a character pair present in parents segregate from
each other such that a gamete receives only one of the 2 factors”.
Homozygous parent
produces similar gametes.
Heterozygous
parent produces two kinds of gametes.
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