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)
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.
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 Inheritance1. 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.