Mendel's Law of Independent Assortment
Mendel performed dihybrid crosses (mating of parent plants that differ in two traits) in plants that were true-breeding for two traits. For example, a plant that had green pod color and yellow seed color was cross-pollinated with a plant that had yellow pod color and green seeds. In this cross, the traits for green pod color (GG) and yellow seed color (YY) are dominant. Yellow pod color (gg) and green seed color (yy) are recessive.
The resulting offspring or F1 generation were all heterozygous for green pod color and yellow seeds (GgYy).
(Figure A) Image Credit: Steve Berg, used with permission.
Mendel then allowed all of the F1 plants to self-pollinate. He referred to these offspring as the F2 generation. Mendel noticed a 9:3:3:1 ratio. About 9 of the F2 plants had green pods and yellow seeds, 3 had green pods and green seeds, 3 had yellow pods and yellow seeds and 1 had a yellow pod and green seeds.
(Figure B) Image Credit: Steve Berg, used with permission
Genotype and Phenotype
In Mendel's experiment with pod color and seed color (Figure A) we see that the genotype or genetic makeup of the F1 plants is GgYy. The phenotypes or expressed physical traits are green pod color and yellow seed color. Both of these traits are dominant.
The F2 generation pea plants (Figure B) show two different phenotypes for each trait. Pod color is either green or yellow and seed color is either yellow or green. There are nine different genotypes:
F2 Genotypes (F2 Phenotypes)
GGYY, GGYy, GgYY, GgYy
(Green pod, Yellow seeds)
(Green pod, Green seeds)
(Yellow pod, Yellow seeds)
(Yellow pod, Green seeds)