During sexual reproduction, sperm meet with an egg to form a zygote. The sperm contains one strand of DNA from the male, while the egg contains one strand of DNA from the female. As a result, each zygote contains two strands of DNA, one from the mother and one from the father.

Adult animals, including humans, have two strands of DNA in all of their body cells, and one strand of DNA in their sperm or egg cells.

When organisms produce DNA for reproduction, this DNA is "housed" within a sperm if the organism is male or an egg if the organism is female. In humans, males continually create new sperm throughout their lives, so they are able to "grow it back" after sexual reproduction. In contrast, human females are born with a finite number of eggs (usually about 2 million) which are located in the ovaries. When a female is reproductively mature, the ovaries release one of these eggs each month. The egg travels towards the uterus and will become an embryo if it is fertilized by sperm.