Mendel's Law of Independent Assortment
To this point we have followed the expression of only one gene. Mendel also performed crosses in which he followed the segregation of two genes. These experiments formed the basis of his discovery of his second law, the law of independent assortment. First, a few terms are presented.
Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)
Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)
Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed.
Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed
F1 Generation: All yellow, round
F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled
At this point, let's diagram the cross using specific gene symbols.
| Choose Symbol | Seed Color: Yellow = G; Green = g |
| Seed Shape: Round = W; Wrinkled = w |
The dominance relationship between alleles for each trait was already known to Mendel when he made this cross. The purpose of the dihybrid cross was to determine if any relationship existed between different allelic pairs.
Let's now look at the cross using our gene symbols.
Now set up the Punnett Square for the F2 cross.
| Female Gametes | |||||
|---|---|---|---|---|---|
| GW | Gw | gW | gw | ||
| GW | GGWW (Yellow, round) | GGWw (Yellow, round) | GgWW (Yellow, round) | GgWw (Yellow, round) | |
| Male | Gw | GGWw (Yellow, round) | GGww (Yellow, wrinkled) | GgWw (Yellow, round) | Ggww (Yellow, wrinkled) |
| Gametes | gW | GgWW (Yellow, round) | GgWw (Yellow, round) | ggWW (Green, round) | ggWw (Green, |
| gw | GgWw (Yellow, round) | Ggww (Yellow, wrinkled) | ggWw (Green, round) | ggww (Green, wrinkled) | |
The phenotypes and general genotypes from this cross can be represented in the following manner:
| Phenotype | General Genotype |
| 9 Yellow, Round Seed | G_W_ |
| 3 Yellow, Wrinkled Seed | G_ww |
| 3 Green, Round Seed | ggW_ |
| 1 Green, Wrinkled Seed | ggww |
The results of this experiment led Mendel to formulate his second law.
Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair
As with the monohybrid crosses, Mendel confirmed the results of his second law by performing a backcross - F1dihybrid x recessive parent.
Let's use the example of the yellow, round seeded F1.
Punnett Square for the Backcross
| Female Gametes | |||||
|---|---|---|---|---|---|
GW
|
Gw
|
gW
|
gw
| ||
| Male Gametes | gw |
GgWw
(Yellow, round) |
Ggww
(Yellow, wrinkled) |
ggWw
(Green, round) |
ggww
(Green, wrinkled) |
The phenotypic ratio of the test cross is:
- 1 Yellow, Round Seed
- 1 Yellow, Wrinkled Seed
- 1 Green, Round Seed
- 1 Green, Wrinkled Seed
Mendel's Law of Independent Assortment