An individual human has about 35,000 genes, each of which occupies a certain position on a molecule of DNA, called a chromosome, inside the nucleus of every cell. Genes direct the production of proteins that result in all the specific physical traits of the individual. During sexual reproduction, the parent’s chromosomes pair up and join to be passed on to the child, who inherits the DNA of both parents.

What theory best explains the wide variation in physical traits that you see among the thousands of people at a crowded festival or sporting event?


If each pair of corresponding genes—for example, the one that determines hair color—“blended” together, as last choice suggests, every child of the same two parents would appear as a similar “blend” of the parent’s physical traits; and so with each subsequent generation people would appear more and more the same. But what you see at a crowded festival is a broad diversity of physical traits. This suggests that the genes of two parents combine unpredictably, or randomly, to form the child’s unique genetic structure, or DNA.

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