What is the thread-like structure containing instructions on the sex of the baby in the zygote known as?

The thread-like structure that contains instructions on the sex of the baby in a fertilized egg or zygote is known as a chromosome. Chromosomes are found inside the nucleus of the cell.

If the zygote has a set of sex chromosomes having one X and one Y-chromosome the sex of the baby would be male whereas, if it has a set of sex chromosomes having a pair of X-chromosomes then the sex of the baby would be female.