Principles of Inheritance and Variation - Notes | Class 12 | Part 2: Inheritance of One Gene

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 and Genotypic Ratios

• 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 genes.
• Do not use T for tall and d for dwarf because it is difficult to remember whether T & d are alleles of the 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 a 50% chance to pollinate eggs of T & t. Also, pollen grains of t have a 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 the genotype of the dwarfs was homozygous - tt.

Test Cross

• Crossing of an organism with a 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 a 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.

Select a Topic 👇

Topic 1: Mendel's Experiments Topic 2: Inheritance of One Gene Topic 3: Inheritance of Two Genes Topic 4: Other Patterns of Inheritance Topic 5: Chromosomal Theory, Morgan's Experiment Topic 6: Sex Determination Topic 7: Mutation and Pedigree Analysis Topic 8: Genetic Disorders (Mendelian and Chromosomal)

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