The syndrome presents in boys, and symptoms generally do not become clear until the onset of puberty. Turner syndrome: this rare disease occurs only in girls and results from a missing X chromosome. [fdna.health]

• Trisomy 21

  Trisomy diseases: Trisomy 21, Down syndrome: perhaps the most well-known and the most common of the chromosome diseases, currently occurring in 1 in every 700 babies born in the US. This disease is caused by the presence of an extra chromosome 21. [fdna.health]