This was the first estimate of the number of genes by a geneticist. The molecular biologists did not accept this number at first. Then Dr. Wiechshaus set up a study of 20,000 lethal mutations to Drosophila to determine how many were lethal in the embryo. He did complementation tests on these genes and determined that there were 139 sets of genes responsible for embryonic development. The essential problem was that there are two sources of genetic products in the embryo.
Some proteins come from the mother, stored in the egg.
Some are made by the embryo. So it was necessary to isolate only those which were produced by the embryo. This discovery showed the relationship between genetics, gene expression and embryological development. But more proteins are supplied by the embryo because it develops for nine months.