By that is meant Wither one chromosome too many or one too few is present in each body cell . . .The most common chromosomal problem in the population is Down Syndrome, or trisomy 21.2 Third group: Most of the common birth defects, such as congenital heart defects, cleft lip and palate, and open spine defects (neural tube defects) classify under the third group of birth defects called multifacto