Fig. 1. Mating scheme to generate BMP type 1 receptor (Bmpr) double knockouts. (A) Parental genotypes required to generate mutant embryos with a BMP type 1 receptor double knockout. Parent 1 expresses the Bcre32 transgene and is heterozygous for the Bmpr1b knockout allele (Bmpr1b^KO). Bmpr1a^KO is a Bmpr1a receptor null allele produced by classical knockout technology (Mishina et al., 1995). Parent 2 is homozygous for the floxed Bmpr1a allele (Bmpr1a^flox) and heterozygous at the Bmpr1b locus. (B) Four classes of embryos are generated by the parents in A. The genotypes of animals is depicted below the names of each classes, and refer to the genetic composition of the neural tube. Normal embryos have at least one functional allele of Bmpr1a and Bmpr1b genes in all tissues and do not show any phenotype. The top row of the table depicts the status of the Bmpr1a gene in each class. The middle row depicts the status of the Bmpr1b gene. The bottom row depicts the expected Mendelian ratios of each phenotypes.