Chromosomal disorders are most commonly the result of aneuploidies and are most often trisomies. Notably, trisomy 21, or the presence of an extra chromosome 21, is the cause of Down syndrome. Down syndrome can present with a variety of signs and symptoms, including brushfield spots, which are small white/grey spots on the edge of the iris, the colored part of the eye; dysmorphic features (e.g., atypically small head or flat face); gastrointestinal problems (e.g., vomiting); cardiovascular problems; neuromuscular problems (e.g., decreased muscle tone); pale skin; fatigue; and shortness of breath.  The other two most common autosomal trisomies are trisomy 18, which results in Edwards syndrome, and trisomy 13, which leads to Patau syndrome. Edwards syndrome often presents with severe mental disability, clenched hands, a large back of the head, a small mouth, low-set ears, and rocker-bottom feet, which are characterized by large heels. Signs of patau syndrome include severe intellectual disability, a small head, small eyes, cleft lips or palate, more than five fingers on a hand (i.e., polydactyly), rocker-bottom feet, and malformation of the forebrain (i.e., holoprosencephaly). Advanced maternal age is a risk factor for all three trisomy disorders and for chromosomal aberrations in general. Aneuploidies of the sex chromosomes can also occur, but they usually present with different symptoms than autosomal disorders. In general, the signs and symptoms of sex chromosome aneuploidies include delayed puberty, absence of menstruation, ambiguous genitalia, and infertility. The most common sex chromosome disorder is Klinefelter syndrome, in which an individual with XY chromosomes has an additional X chromome, or the genotype of XXY, and is often assigned male at birth. The other two sex chromsome trisomies are XYY syndrome and XXX syndrome. Finally, Turner syndrome is the only sex chromosome monosomy, which occurs when a person inherits only one X chromosome and is usually assigned female at birth.