Refer to Animation: Independent Assortment. In a cross between two individuals who are heterozygous for two traits determined by dominant and recessive alleles (Aa Bb), what is the expected ratio of offspring for the trait determined by the first gene (Aa)?

Phenotypic ratio of 3:1 i.e. 3 offsprings will possess dominant trait while 1 will possess recessive trait.

Explanation:

Gregor Mendel in his experiments discovered a principle that governs heredity which he called the LAW OF INDEPENDENT ASSORTMENT which states that the alleles of a gene will segregate into gametes independently of the alleles of the other gene. This principle applies to DIHYBRID CROSS (a cross involving two different genes/traits).

A cross between two heterozygous individuals with genotype AaBb where A and B are dominant alleles while a and b are recessive alleles. Since, the alleles of each gene A and B will segregate independently i.e without affecting one another, we can look at the expected outcome of the offsprings for the trait determined by the first gene.

Two heterozygous, Aa and Aa will cross and both produce gametes with A and a alleles. This gametes will randomly combine to form four possible offsprings with genotypes: AA, Aa, Aa and aa.

Genotypes AA, Aa, and Aa will all phenotypically express the dominant trait while genotype aa will express the recessive trait. Hence, a phenotypic ratio 3:1 will be produced regarding the trait controlled by the first gene.