Genetic screen

Genetic screening has identified certain cytoplasmic determinants. Genetic screening is made difficult by diploidy, and that most mutations are recessive. Thus, it is very time-consuming to induce thousands of mutations, make each mutation homozygous and then examine individual mutant embryo phenotypes. Newly induced mutations relevant to development can be classified to specific genes by complementation analysis, then roughly mapped via recombination analysis. Once the precise map position of the relevant gene is known, it can be cloned, sequenced and characterized. Nusslein-Volhard & Wieschaus developed a genetic screen for mutations which are lethal only for zygotes, meaning these mutations are related to development. Their procedure, described below, is useful for understanding genetic screens as a whole.

Step	Overview
Step 1	Mutagenized F0 males are mated with non-mutagenized F0 females.
Step 2	The resultant F1 offspring are heterozygous (m^*/+) for the resultant mutations.
Step 3	F1 progeny are mated together; of the resultant F2, 75% (m^/+ or +/+) are normal and 25% (m^/m^*) carry a lethal phenotype.
Step 4	Screen m^/m^ chromosomes to identify zygotic lethal genes: 20 controlling A/P axis and 12 controlling D/V axis.
End	Only a small number of genes is speciﬁc to forming pattern in early embryo. Most such genes were identiﬁed in the screen (since found many alleles in each gene), thus making this a saturating experimental procedure.