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Gregor Mendel discovered the Law of Independent Assortment while working with garden peas in the 1860's. It states that gene pairs separate independently during the formation of sex cells, or gametes. For example, in pea plants, green pod (G) is dominant to yellow pod (g) and yellow seeds (Y) are dominant to green seeds (y). When Mendel crossed two dihybrid parents GgYy x GgYy, he realized that the allele pairs separated during gamete formation. In other words, when sperm and eggs cells were formed, the allele pairs separated resulting in four different types of gametes. If the parent was GgYy, there are four possible combinations of letter g and letter y. They are GY, Gy, gY, gy. These represent the gametes of one of the dihybrid parents. The other parent will make four gametes as well--GY, Gy, gY, gy. When these gametes came together at fertilization, there was a 9:3:3:1 ratio, with 9 offspring containing the Green pod and Yellow seeds(the two dominant traits), 3 with one dominant and one recessive trait, 3 with the other dominant and the other recessive trait, and only 1 plant with both recessive traits--yellow pod and green seeds. This experiment shows that traits are transmitted to offspring independently. It is like shuffling a deck of cards.
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