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The notion that sex could be determined by chromosomes was a new idea. Prior to Henking, McClung, and Sutton's reports, sex determination was attributed to factors other than gametes, such as the environment in which egg cells existed.
A baby's sex is determined at fertilisation. A chromosome from the father's sperm determines whether the child is male or female. If an X chromosome is present the baby is a girl; if a Y chromosome is carried by the sperm instead, the baby is a boy.