ζ- and α-globin mRNAs in ζ- and α1-heterozygote and null embryos. (A) Total RNA analyses of 10-day pc littermate embryos that are heterozygous (+/−) and homozygous (−/−) for the ζ-null mutation. The blot was hybridized sequentially with three different probes: ζ- 1 Kb genomic Xba I-EcoRI fragment containing exon-1 plus a portion of the 5′ untranslated region of the ζ-globin gene), α1- 600 bp genomic BamHI-EcoRI fragment containing exon-3 and a portion of the 3′ untranslated region of the α1-globin gene), and a murine actin probe used to quantitate RNA loading. (B) Total RNA analyses of 12-day pc littermate embryos that are wild type (+/+), heterozygous (+/−), and homozygous (−/−) for the α1-null mutation. The filter was hybridized sequentially with the three different probes as described above in A. The relative amount of mRNA loaded on the gel was determined by measuring the relative density of the actin mRNA band detected with the actin probe in each sample and is indicated by the relative “load factor” shown in the figure.