Trisomy 16 detected at chorion villus sampling (CVS) may reflect the placental but not the fetal karyotype. We describe a case in which the pregnancy continued until intrauterine death at 37 weeks. Cytogenetic study of two placental samples showed 47, + 16 and 46,XX; the fetus was morphologically grossly normal, but fetal tissue culture was unsuccessful. Conservative management may be appropriate when trisomy 16 is detected at CVS and the pregnancy is normal ultrasonographically.

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The first centric-fusion (or Robertsonian) translocation in domestic sheep was reported by Bruere in 1969 (reviewed in BROAD et al. 1997). Since then, five different Robertsonian translocations have been reported in sheep in New Zealand (reviewed in BROAD et al. 1997). A sheep carrying one of these, designated the t, translocation (rob 5;8), died before passing it on (PEARCE et al. 1994), but the remaining four-designated t, (rob 6;24), t, (rob 9;10), t, (rob 7;25) and t, (rob 8;22) (ANSARI et al. 1993;PEARCE et al. 1994)-have been maintained in a nucleus breeding flock. For about the past two decades, sheep in this nucleus flock have been bred that are homozygous for the t,, t,, and t, translocations (2N = 48). In the 1999 breeding season, a male and female lamb has been generated carrying the t,, t,, t, and t, translocations in a homozygous state (2N=46). This is a unique occurrence. MATERIALS AND METHODSMetaphase chromosomes were prepared from whole blood cultures using standard techniques and banded by conventional Leishman-trypsin treatment. The numbering of the chromosomes followed the recommendations of the 1995 standard sheep karyotype (ANSARI et al. 1999). RESULTS AND DISCUSSIONA metaphase spread from the ram lamb that is homozygous for the four different Robertsonian translocations is shown in Fig. 1.Robertsonian translocations appear to be common in most of the members of the family Bovidae of over 100 species that are related through a series of centric fusions (reviewed by FRANKLIN 1997). For example, three such fusions are proposed to be involved in the Fig. 1. A partial GTG-banded metaphase spread from a ram lam showing four different homozygous Robertsonian translocations, t, (rob 6;24), t, (rob 9;10), t, (rob 7;25) and t, (rob 8;22).

Trisomy 7, in mosaic state, was identified at chorionic villus sampling. The pregnancy was closely followed, and proceeded uneventfully. Mosaic trisomy 7 was confirmed in the term placenta, the organ having no structural abnormalities; the karyotype of the phenotypically normal baby was 46,XY. Trisomy 7, mosaic or nonmosaic, detected at chorionic villus sampling in an ultrasonographically normal pregnancy, appears typically to be associated with a normal fetal karyotype, and placental growth, structure, and function are not discernibly compromised.