Syndrome, trisomy 13: Condition with three rather than the normal two chromosomes #13. Children born with this syndrome have multiple malformations and mental retardation due to the extra chromosome #13. The congenital malformations (birth defects) commonly include scalp defects, hemangiomas (blood vessel malformations) of the face and nape of the neck, cleft lip and palate, malformations of the heart and abdominal organs, and flexed fingers with extra digits. The mental retardation is profound. The IQ is untestably low. The majority of trisomy 13 babies die soon after birth or in infancy. The condition is also called Patau syndrome after the late geneticist Klaus Patau (at the University of Wisconsin) who discovered the extra chromosome in 1960.