Tissue limited mosaicism in a patient with tetrasomy 9p

Cytogenetic analysis of an abnormal newborn girl showed an extra chromosome with the characteristics of an isodicentric 9p chromosome [idic (9)(pter----q12----pter)] in 98% of peripheral lymphocyte metaphases examined. This cytogenetic interpretation was substantiated by quantitative measurement of erythrocyte galactose-1-P-uridyltransferase (GALT) activity, which is consistent with the expression of 4 normal GALT genes. Cytogenetic results from skin fibroblasts showed mosaicism with only 11% of the metaphases having the extra chromosome. Selective genetic pressure based on a functional disadvantage of tetrasomy 9p in the skin is proposed. The in vivo establishment of cytogenetically normal cells in various tissues may be necessary for in utero survival of tetrasomy 9p infants.