Mendel's Laws > The Law of Independent Assortment

Mendel proposed the Law of Independent Assortment based on the dihybrid cross.

• Mendel’s Law of Independent Assortment 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’.
• Genes that are located in different chromosomes assort independently during meiosis.
• Many possible combinations of factors can occur in the gametes.
  

  Dihybrid cross – Segregation of gametes

• Independent assortment leads to genetic diversity.
• If an individual produces genetically dissimilar gametes it is the consequence of independent assortment.
• Through independent assortment, the maternal and paternal members of all pairs were distributed to gametes, so all possible chromosomal combinations were produced leading to genetic variation.
• In sexually reproducing plants/organisms, due to independent assortment, genetic variation takes place which is important in the process of evolution.
• The Law of Segregation is concerned with alleles of one gene but the Law of Independent Assortment deals with the relationship between genes.

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.

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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.

• 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.