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”.

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