There was no variation in a 729-base pair intron near a gene thought to be associated with sperm and testes development. However, DNA from chimps, orangutans, and gorillas were also sequenced in the same location and much variety was found. It was discovered that not only do male and female differ phenotypically, there is also a chromosomal difference. Furthermore, it was found that the male is the sex-determining person. Males secrete sperm that either carry an X or a Y chromosome.
All females carry only the X chromosome.
The combination of these two chromosomes will decide the sex of the child. It was finally found that the female carries an XX chromosome, and the male carries one of each, having an XY chromosome. Additionally, it was decided that the presence or the absence of the Y chromosome determines the sex. For a diagram comparing the X and Y chromosome, please see diagram . For more information on the history of the X and Y chromosome, see the article Into the Looking Glass .