A 4.3 In most eukaryotic organisms, sexual reproduction consists of an alternation of haploid (In) and diploid (2n) cells.

I 4.4 Male and female gametes (sperm and egg, respectively) differ in size. In this photograph, a human sperm (with flagellum) penetrates a human egg cell. (Francis Leroy, Biocosmos/Science Photo Library/Photo Researchers.)

There are many ways in which sex differences arise. In some species, both sexes are present in the same individual, a condition termed hermaphroditism; organisms that bear both male and female reproductive structures are said to be monoecious (meaning "one house"). Species in which an individual has either male or female reproductive structures are said to be dioecious (meaning "two houses"). Humans are dioecious. Among dioecious species, the sex of an individual may be determined chromosomally, genetically, or environmentally.

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