What Happens When Someone Has Two Y Chromosomes?

When someone has two Y chromosomes they develop a condition called Jacob's syndrome. In most cases, this condition, which affects men, does not significantly alter a man's way of life, although there are some associated symptoms, like delayed motor skills and low testosterone production.

Jacob's syndrome, also called XYY syndrome, arises when a man inherits an extra Y chromosome. Not everyone displays symptoms, but if they do, these can include:

  • Learning difficulties and speech problems.
  • Developmental delays, including muscle tone, motor skills and speaking.
  • Undescended testicles at birth.
  • Quiet and calm as a baby.
  • An autism diagnosis during childhood.
  • Difficulty growing facial hair.
  • Learning disabilities and problems concentrating.
  • A small penis and undersized testicles.
  • Some men may have difficulty conceiving.
  • Being taller than average, but with weak bones.

There are no risk factors for XYY syndrome, and most people do not receive a diagnosis until they are adults. The diagnostic process may involve screening for low testosterone levels and a chromosome analysis. Although there are no cures for XYY syndrome, there are ways doctors can alleviate the symptoms. This includes testosterone therapy to encourage development during puberty and fertility treatments for men who are struggling to reproduce. Individuals who are struggling to walk may have physical therapy, and speech therapy is available for those with speech delays.