Mendel's Laws > The Law of Independent Assortment

Mendel’s Law of Independent Assortment states that, when two pairs of independent alleles are brought together in the hybrid F₁ they show independent dominant effects. In the formation of gametes, the law of segregation operates, but the factors assort themselves independently at random and freely. 

1. When two pairs of traits are considered, alleles of each trait assort independently during gamete formation.
2. The inheritance of one trait does not affect the inheritance of the other.
3. This law is clearly demonstrated in the F₁ generation of a dihybrid cross.

or

When a hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair.

"When two pairs of traits are combined in a hybrid, the segregation of one pair of characters is independent of the other pair of characters."- Gregor Mendel

Chromosomal Basis of the Law:

The Law of Independent Assortment has a clear physical basis in meiosis.

What Happens During Meiosis I

• During Prophase I, homologous chromosomes pair up as bivalents.
• During Metaphase I, bivalent pairs align on the metaphase plate.
• The orientation of each bivalent pair is random and independent of all other bivalent pairs.
• This means maternal and paternal chromosomes (and the genes they carry) are distributed to gametes in every possible combination.

Role of Crossing Over

Recombination (crossing over) during Prophase I further shuffles alleles between homologous chromosomes, increasing genetic diversity beyond what assortment alone produces.

Mendel crossed plants that were true-breeding (homozygous) for two traits:

(R = Round dominant; r = wrinkled recessive; Y = Yellow dominant; y = green recessive)

The Law of Independent Assortment does NOT apply in the following situations:

• Mendel proposed the Law of Independent Assortment based on dihybrid crosses; it states that segregation of one pair of characters is independent of the other pair.
• In dihybrid crosses, the F₂ phenotypic ratio is 9:3:3:1, derived as a combination of 3 yellow:1 green with 3 round:1 wrinkled.
• During meiosis in F₁ (RrYy), each gene pair segregates independently, producing 4 types of gametes - RY, Ry, rY, ry, each with a frequency of 25%.
• A Punnett square of F₁ × F₁ produces 16 combinations, giving 9 different genotypes and 4 different phenotypes in F₂.
• F₂ phenotypic distribution - 4 genotypes give Round yellow, 2 genotypes give Round green, 2 genotypes give Wrinkled yellow, 1 genotype gives Wrinkled green (rryy).
• The genotypic ratio at the F₂ stage is 1:2:1:2:4:2:1:2:1, while the phenotypic ratio is 9:3:3:1.