Which karyotype would be most frequently seen in liveborn infants (as opposed to spontaneous abortions)?

47,xx,+21-down syndrome is the most common serious chromosomal disorder, seen in about 1/900 newborn infants. since 47,xx,+21, or trisomy 21, is the usual cause of down syndrome (95%), this is the correct answer. most of the other trisomies, including 47,xx,+3, rarely lead to live infants. trisomy 13 (patau syndrome) and trisomy 18 (edwards syndrome) occur in about 1/10,000 births. triploidy, or 69,xxx, is seen in about 20% of spontaneously aborted fetuses but never in newborns. monosomies generally involve the loss of too much genetic material to be compatible with live birth; monosomy is part of 46,xy,-11,+22. the 46,yy karyotype is also not seen in newborns, for similar reasons.