Mendel's Law of Independent Assortment

The principles that govern heredity were discovered by a monk named Gregor Mendel in the 1860's. One of these principles, now called Mendel's law of segregation, states that the alleles for a trait separate when gametes are formed. These allele pairs are then randomly united at fertilization. Mendel arrived at this conclusion by performing monohybrid crosses. These were cross-pollination experiments with pea plants that differed in one trait, for example pod color.

Mendel began to wonder what would happen if he studied plants that differed in two traits. Would both traits be transmitted to the offspring together or would one trait be transmitted independently of the other? From his experiments Mendel developed the principle now known as Mendel's law of independent assortment.


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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)
GGyy, Ggyy
(Green pod, Green seeds)
ggYY, ggYy
(Yellow pod, Yellow seeds)
ggyy
(Yellow pod, Green seeds)

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