The difference between rescuable and nonrescuable maternal-effect lethal mutations is as follows:
The rescuable mutations are mutant embryos produced by heterozygous females. The non-rescuable mutations are embryos produced by homozygous females
When homozygous mutant females are mated with mutant males, all the embryos die. When homozygous mutant females are mated with normal males, female offspring are produced.
Recessive maternal-effect mutations are grouped according to the effect that can overcome by a wild-type allele contributed by the sperm.
Similarly, rescue of embryos by the paternal wild-type allele shows that the gene required, for the particular development in the embryo can be provided by maternal alleles. During oogenesis own alleles functions after the fertilization.
In contrast, non-rescuable maternal effect mutations reflect a strict requirement, for only maternal gene expression without the participation of the embryo's alleles. Otherwise, it may reflect requirement, for maternal as well as embryonic gene expression in which case both of maternal and embryonic gene expression are insufficient, for normal activity.