Embryonic lethal mutations

Embryonic lethal mutations can be identified
by appropriate crosses. One-fourth of the progeny
of heterozygous flies (A/a) for an embryonic
mutant gene (a) are homozygotes (aa) for the
mutant allele (1). If a mutation involves maternal
genes only (maternal effect), progeny of
female homozygotes (bb) are lethally affected
(2). Maternal effect genes code for early gene
products that determine the polarity of the embryo;
see below (C).