CBSE · Class 12 · Biology · Principles of Inheritance and VariationExplain Mendel's Law of Independent Assortment with the help of a suitable dihybrid cross. Also, calculate the phenotypic and genotypic ratios of the $F2$ generation.

Mendel's Law of Independent Assortment

\nMendel's Law of Independent Assortment is the third major principle of inheritance formulated by Gregor Johann Mendel. It states that 'when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters at the time of gamete formation.' This means that the alleles of two or more different genes get sorted into gametes independently of one another. The inheritance of one gene does not affect the inheritance of another gene.

Explanation through Dihybrid Cross\nTo understand this law, Mendel crossed a pea plant having yellow and round seeds (YYRR) with one having green and wrinkled seeds (yyrr).

• Parents: Yellow Round (YYRR) $\times$ Green Wrinkled (yyrr)
• Gametes produced by parents: RY (from YYRR) and ry (from yyrr)
• $F_1$ Generation: All offspring were heterozygous for both traits (YyRr) and showed yellow and round seeds, demonstrating the dominance of yellow and round traits. \nWhen selfing of $F_1$ generation occurs (YyRr $\times$ YyRr), the $F_2$ generation is produced. During gamete formation in $F_1$, the alleles for seed color (Y/y) and seed shape (R/r) assort independently. Thus, four types of gametes are produced by each $F_2$ parent in equal proportions: YR, Yr, yR, and yr.

Punnett Square Analysis for $F_2$ Generation\nBy combining these four types of gametes in a $16$-square Punnett square matrix, we obtain the $F_2$ progeny:

• Phenotypic Ratio: Out of 16 individuals:

  • Yellow and Round = 9
  • Yellow and Wrinkled = 3
  • Green and Round = 3
  • Green and Wrinkled = 1

  Therefore, the Phenotypic Ratio is 9:3:3:1.

• Genotypic Ratio: The genotypic breakdown among the 16 individuals is:

  • YYRR (1) : YYRr (2) : YyRR (2) : YyRr (4) : YYrr (1) : Yyrr (2) : yyRR (1) : yyRr (2) : yyrr (1)

  Therefore, the Genotypic Ratio is 1:2:2:4:1:2:1:2:1.

Significance of the Law

1. It helps in introducing new combinations of characters which are useful in plant breeding.
2. It explains genetic variations in sexually reproducing organisms.