Sunday, July 26, 2020

Principles of Inheritance and Variation - Notes | Class 12 | Part 2: 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 phenotypic ratio: 3 Tall: 1 Dwarf = 3:1

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.

Hence monohybrid test cross ratio= 1:1
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

1. First Law (Law of Dominance)
§ 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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