In English, as in many other languages, you must keep the gender of a noun in mind when choosing a pronoun to replace or refer to it. Nouns that refer to male human beings (or animals, if the sex is significant) are masculine; nouns that refer to female human beings (or animals, if the sex is significant) are feminine; nouns that refer to inanimate objects or abstract concepts are neuter. (See also Sexist Language.)
Isaac Newton (masculine), Dorothy Mary Crowfoot (feminine), the device, the result (neuter)
When replacing a masculine or feminine noun with a pronoun, choose the pronoun that corresponds in gender to the noun.
Gas bubbles were first implicated in decompression illness by the French physiologist Paul Bert. He proved that the bubbles were composed of nitrogen; because oxygen is metabolized by tissue cells, it is not usually a problem during decompression.
--"The Physiology of Decompression Illness," Scientific American
Mary Leakey first went to Kenya and Tanzania in 1935
and, except for forays to Europe and the U.S., she has made
them her home.
--"Profile: Mary Leakey," Scientific American (modified)