Diploid organisms — those with two sets of genetic information — don't have two copies of one genome. Instead, each locus on the genome is comprised of two sets of alleles.
When a sexually reproducing organism — such as a person — produces offspring, it first must make gametes (in the case of a human, eggs or sperm). Each of the gametes gets one copy of the DNA segments. This gamete combines with a gamete from the organism's mate to produce an individual with two sets of DNA.
When one set comes from Mom and one set comes from Dad, the sets may be slightly different. This situation makes for some interesting genetic questions. One example: How do our bodies read the instructions if the two copies aren't the same? That depends on the particular alleles: what trait or characteristic they're coding for and whether either is dominant.
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