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9.2.2.3. Mendel's third law or principle of independent combination

Mendel's third law

Mendel's first two laws are based on the behavior of single-character hereditary factors. To check whether two non-antagonistic characters were transmitted independently, he devised the following experiment.

He crossed purebred pea plants that always gave smooth yellow seeds (AALL) with purebred plants that always gave rough green seeds (aall). Both are homozygous plants for the two characters analyzed.

All the offspring obtained in F1 were homogeneous, yellow and smooth (dihybrids, AaLl).

Afterwards, he selfed F1 obtaining an F2 with phenotypic ratios 9:3:3:1.

Obtaining this proportion explains that the genes are independent of each other. As a gamete is formed, the alleles separate (segregate) independently of the other alleles of a gene for another trait.

For example, Mendel analyzed these two allelic pairs:

            A = yellow seed    a = green seed

            B = smooth seed   b = rough seed.

                                   F 0                    AABB * aabb

                                   Gametes             AB          ab

                                   F 1                    AaBb   *    AaBb

Phenotypic proportions:

          9/16 A_B_          3/16 A_bb                  3/16 aaB_                1/16 aabb

(9 smooth yellow seeds, 3 rough yellow seeds, 3 smooth green seeds, 1 green rough seed).

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Genetics problems solved: Genetics  problems of Mendel's third law  (4th ESO). It contains genetic problems that have come up in EvAU exams from various universities.