What Are YY Chromosomes?

Medically Reviewed by Minesh Khatri, MD on May 27, 2023
Written by WebMD Editorial Contributors
3 min read

‌Although genetics are hereditary, a phenomenon in genetic alterations occurs when male babies receive an extra Y chromosome in each of their cells, resulting in an XYY combination. The syndrome is commonly known as Jacob’s syndrome, Superman syndrome, XYY karyotype, XYY syndrome, YY syndrome, and 46,XY/47,XYY mosaicism. The genetic alteration occurs in one out of 1,000 male babies and can be traced back 100,000 years to one male ancestor.

The usual number of chromosomes inside every cell of your body is 46 total chromosomes, or 23 pairs. You inherit half of your chromosomes from your biological mother, and the other half from your biological father.

The first 22 pairs of chromosomes are called autosomes. The 23rd pair of chromosomes are known as the sex chromosomes, because they decide if you will be born male or female. Females have two X chromosomes, while males have one X and one Y chromosome. 

The Superman syndrome occurs when males receive an extra Y chromosome at conception, creating an XYY combination. ‌This results in a male baby born with the XYY syndrome, which is a random occurrence due to the sperm cell’s formation before conception occurs.‌

Known as nondisjunction, this is an error in the sperm cell’s division. The resulting child will have an extra Y chromosome in every cell of his body. However, it’s not a hereditary trait — fathers with XYY chromosomes don’t pass on this syndrome to their sons.

The physical characteristics in a person with an extra Y chromosome have broad variations. It’s important to remember that not all boys born with XYY syndrome will demonstrate observable differences. How your physical body functions is a result of the instructions encoded in your genes. You have an inherently unique genetic makeup that creates your genotype.‌

Your genotype combines with environmental factors to determine your physical body structure and function known as the phenotype. The entire embodiment of a full set of chromosomes results in your karyotype.‌

Physical features associated with this syndrome may include:

  • Large testes 
  • ‌Large head 
  • Increased distance between two body parts, typically the eyes
  • Increased belly fat
  • Large teeth
  • Flat feet 
  • Fifth fingers that curve inward
  • Atypical side-to-side curvature of the spine 
  • Tall stature ‌

It’s important to keep in mind that sometimes there’s an absence of any unusual phenotype traits, although above-average height is a common result. Studies reveal that men with the syndrome carry an extra copy of the SHOX gene, which is found on a sex chromosome. The SHOX gene is also responsible for skeleton development, particularly in the growth and maturation of the limbs.‌

A majority of men with XYY syndrome are fertile, with typical testosterone levels and sexual development. However, some boys may develop testicular failure that leads to fertility problems.

The XYY syndrome correlates with certain physical conditions and disabilities in males with the extra Y chromosome. Symptoms range from mild to severe, so the physical symptoms can go undetected:

  • Asthma
  • ‌Autism spectrum disorder
  • Seizures 
  • Hand tremors
  • Involuntary movements
  • Low muscle tone hypotonia
  • Delayed development of motor skills (such as sitting and walking)
  • Speech delay
  • Difficulty with reading and writing
  • Dental problems
  • Autism spectrum disorder (mildly)
  • Attention deficit disorder, ADHD
  • Anxiety 
  • Mood disorders
  • Depression
  • Infertility

‌It’s important to note that intellectual disabilities are not a typical sign. Individuals with XYY syndrome are also able to produce children with typical karyotypes.

Genetic testing for XYY syndrome can be conducted in prenatal tests or anytime throughout the boy’s life. Upon diagnosis, the syndrome can be managed through specialized education and therapies, such as when developmental delays in speech and mobility are apparent. Two commonly ordered tests are:

  1. Karyotype test, which detects all the chromosomes in a cell
  2. Chromosomal microarray test, which detects extra or missing chromosomes

There are also prenatal tests that can confirm a diagnosis of the XYY syndrome, such as an amniocentesis or chorionic villus sampling, which pregnant women can have to detect potential genetic problems with their unborn child. It’s speculated that a portion of the male population who have Jacob’s syndrome never receive diagnosis due to the lack of symptoms.