Slide showing diagram of 'The XY Sex-Determining Mechanism'. Slide shows diagram of 'The XY Sex-Determining Mechanism'. Diagram shows female (XX) and male (XY) and the donation of 1 X chromosome from the female and either 1 X or 1 Y from the male to determine whether a female or male is produced. Title below diagram reads 'Fig. 25 - The XY Sex-Determining Mechanism'. Late 19th to early 20th century