Missing an entire chromosome is rarely tolerated. Monosomy is the absence of one chromosome from a pair, leaving a single copy instead of the normal two. In humans, most monosomies are embryonic lethal.

The only complete monosomy compatible with live birth is monosomy X, which causes Turner syndrome. Partial monosomies, resulting from deletions of chromosome segments, produce a variety of developmental syndromes. The phenotype arises from reduced dosage of the genes on the missing chromosome.

Karyotyping and chromosomal microarray detect monosomies. Prenatal diagnosis identifies affected pregnancies. Understanding why certain monosomies are viable while others are not reveals critical genes that require two copies for normal development.

• Absence of one chromosome from a pair
• Usually embryonic lethal in humans
• Monosomy X causes Turner syndrome
• Detected by cytogenetic methods