Answer:
His father
Explanation:
In your genetics, you have two sets of chromosomes. One set comes from your mother, and one set comes from your father.
Your mother is able to pass on an X chromosome, however, your father could pass on either an X or a Y chromosome, and the father's contribution of chromosomes determine the gender of the baby.
Hence, if the investigators need to find the DNA of the Y chromosome, the father would be the best option to test, since the sex chromosomes remain almost identical throughout the family lines.
