Independent Assortment and Crossing Over
A chromosome is a structure that contains DNA, the genetic material that is passed from one generation to the next. Meiosis is the process of cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, resulting in the production of gametes. During meiosis, chromosomes (which are found in pairs) swap large portions of their molecules, causing genetic material to be mixed between them. Crossing over increases genetic diversity in offspring, which in turn may increase the offspring's chances for survival in a changing environment because it is more likely that a unique trait may be necessary for an organism to thrive in an environment different from the one its parents lived in.Gregor Mendel, the Austrian monk who discovered the underlying principles of genes and inheritance, did not study only one trait; in total, he examined seven different traits in pea plants. He observed that inheritance of alleles for one trait was independent of inheritance of alleles for a different trait. That is, the genes for different traits did not affect each other. For example, one plant might have round green peas, while another had wrinkled yellow peas. Crossing these plants may yield plants with wrinkled green peas or round yellow ones. The genes for pea shape and pea color are independent of each other and are passed to offspring independently of each other. This is known as the law of independent assortment, which states that genes are sorted independently during gamete (sperm and egg) formation and thus have equal chances of occurring together. Today, scientists know that chromosomal crossing over, a process in which chromosomes swap portions of their arms, resulting in the redistribution of genetic material, also occurs during meiosis and increases genetic diversity.