Intercrosses of mice doubly heterozygous for Rb(6.15)lAld and Rb(4.15)4Rma (thus are characterized by monobrachial homology for chromosome 15) produced embryos with zero to four copies of chromosome 15 in their expected frequencies at the first cleavage division. By 3½ days’ gestation, nullisomy 15 embryos were missing. At 8½ to 9½ days, no monosomy 15 embryos were found, although trisomy 15 and tetrasomy 15 embryos were still present in their expected numbers. Tetrasomics were more severely affected than trisomics at this gestational age; the former were severely retarded “streak” embryos, while the latter had open neural tubes and were 2/3 the size of euploid embryos. The functional activity of chromosomes during the embryonic development of autosomal aneuploids is discussed in light of these
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