The Principle Of Independent Assortment

After Mendel reached his conclusions concerning monohybrid crosses, he continued his research by looking at two traits at a time. Crosses involving true-breeding parents that differ in two traits are called dihybrid crosses. He mated peas that produced smooth yellow seeds with those that produced wrinkled green seeds. As expected, the F1 generation contained only plants that produced smooth yellow (both dominant) seeds. When the F1 plants were self-fertilized, they produced 315 smooth yellow seeds, 108 smooth green seeds, 101 wrinkled yellow seeds, and 32 wrinkled green seeds. He recognized that these results were close to a 9:3:3:1 ratio. This was simply a multiple of the 3:1 ratio produced in the F2 generation of the monohybrid crosses. The pattern is illustrated in Table 2.

From these results, along with further experiments with other combinations of traits, Mendel was able to deduce the principle of independent assortment. This principle states that the segregation of genes for one gene pair is independent of (does not influence) the segregation of genes for any other gene pair.

One of the places in which Mendel was fortunate was that the genes for the seven traits he chose were not in proximity to one another on the chromosomes. Of course, since Mendel was unaware of the concept of the chromosome, he would not have had any way to understand this. The principle of independent assortment does not apply to genes that are near one another on the chromosomes. The degree to which two genes do not obey this principle is affected by their proximity to each other. Failure to abide by the principle is used as the basis for forming genetic maps.

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