Heredity

Gregor Mendel and the Laws of Inheritance

Gregor Johann Mendel, the "Father of Modern Genetics", conducted hybridization experiments on garden pea plants (Pisum sativum) choosing 7 pairs of contrasting traits (e.g., Tall/Dwarf, Round/Wrinkled seeds, Yellow/Green seeds).

1. Monohybrid Cross (Inheritance of One Gene):

Cross between pure breeding Tall ($TT$) and Dwarf ($tt$) plants:

• $P_1$ Generation: $TT \times tt$
• $F_1$ Generation: All $Tt$ (Phenotypically $100\%$ Tall - Tall is the dominant trait; Dwarf is recessive).
• $F_2$ Generation (Selfing $Tt \times Tt$):
• Phenotypic Ratio: $3\text{ Tall} : 1\text{ Dwarf}$
• Genotypic Ratio: $1\text{ Pure Tall (TT)} : 2\text{ Hybrid Tall (Tt)} : 1\text{ Pure Dwarf (tt)} = 1 : 2 : 1$

Mendel's Laws:

1. Law of Dominance: In a heterozygote, one allele masks the expression of another allele for the same trait.
2. Law of Segregation: Alleles segregate during gamete formation so that each gamete carries only one allele for each gene.

2. Dihybrid Cross (Inheritance of Two Traits)

Cross between plants with Round Yellow seeds ($RRYY$) and Wrinkled Green seeds ($rryy$):

• $F_1$ Generation: All $RrYy$ (Round Yellow).
• $F_2$ Phenotypic Ratio:

$$9\text{ Round Yellow} : 3\text{ Round Green} : 3\text{ Wrinkled Yellow} : 1\text{ Wrinkled Green}$$

• Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair.

Sex Determination in Humans

In human beings, each somatic cell contains $23$ pairs ($46$) of chromosomes:

• $22$ pairs are Autosomes (identical in males and females).
• $1$ pair is Sex Chromosomes:
• Females have a perfect pair of sex chromosomes: $XX$.
• Males have a mismatched pair: $XY$.

          Mother (XX)            Father (XY)
             /    \                /     \
           X        X            X         Y
           │        │            │         │
           ├────────┼────────────┘         │
           │        └──────────────────────┼────────┐
           ↓                               ↓        ↓
      Female (XX)                     Male (XY)  Male (XY)
         (50%)                           (50%)