Lethal Factors in Development

Abstract

In this review the factors analyzed may be genes in the conventional sense or chromosomal changes such as rearrangements and deficiencies which are transmitted by hereditary mechanism and in which the effects have been studied from an embryological approach. Many of these are listed for Drosophila. namely. "White deficiencies", "Notch" facet deficiencies, "Kruppel", "1-7", "1-me" "1-gl", "1-tr". Information is available for many lethals responsible for death of the larvae, some for prepupal, and for pupal lethals. Attempts have also been made to gain insight into the mechanisms of lethal gene effects by the production and study of phenocopies. For the vertebrates such studies are available in limited number, for amphibia, many for the chick, and some for guinea pigs and mice. A study of the actual number of lethal factors affecting early stages of development reveals this number to be small in organisms considered in this review. The reasons accounting for this may be greater difficulty of detection in early stages, but this is not considered likely. Primarily it appears to be the regulatory power of the embryos enabling them to repair damages of external or internal injuries during development. Damages of early development stand better chance of repair than those of later stages because of increasing restriction of regulatory capacity. A mechanism of recovery may be operative in the effect of gene suppressors, the steps of which if known should eventually reveal the individual biochemical steps and the interaction of biochemical processes by which the suppressor''s effects may operate.