
Hereditary exencephaly was reported by Mellen’ in 1954 
as a recessive facultative lethal which produces a wide range of 
effects from slight hyperencephalocele to a massive herniation 
of the brain, 
Cyclocephaly and cyclopia lethals were reported by 
pendeuer in eee Wetherbee (1958) described microphthalmia in 
ulscalus guiscula and synophthalmia in Mimus Doet enc tted Other 
lethals recently described aré “dehydrated” by Taylor an ; 
Stinnett (1956); Second Diplopod by Landauer (1956); Paroxysm 
by Cole (1961); and Congenital loco by Cole (1956). 
The same syndrome of effects may be produced in a 
given species by entirely inda@pendent recessive or dominant muta- 
tions. An example of this is the various forms of micromelia 
or rumplessness reported as a dominant by Dunn (1925) and as a 
recessive by Landauer, 1945. Extragenetic factors such as 
season and environment may act to modify the gene expression. 
Then, too, penetrance (incidence of affected individuals) and 
expressivity (the degree of the effect) may be altered by 
genetic and environmental factors. 
In summary, then, the lethal and teratogenetie effects 
of hereditary factors are many and varied. Not all are the 
result of a dominant mutations, but may be dominant or recessive 
and facultative or obligate. All act to inhibit embryogenesis 
in some waye 
The discovery of genetic lethals has been accompanied 
by a corresponding study in the mechanisms of gene action. Exe 
perimentally, chemical teratogens have been employed that have 
effects similar to genetic teratogens,. 
3e Phenocopies. 
A normal embryo can be modified in its development so 
that the end product "duplicates" or closely resembles known 
mutant traits of the same species. This phenomenon has been 
termed phenocopy by Goldsmidt, (1935). Landauer (1959) warns 
that this term should be limited "to external stimuli or sources 
of modification which exert or cause identical action on the 
embryo (similar phenotypic expression) without vital invasion 
and destruction of certain embryonic organs and tissues." 
One of the most frequently studied abnormalities is 
that of micromelia. During the interval from 4 to 6 daya of 
age, the embryo is especially sensitive to a host of compounds 
which can induce this response. Landauer (1954) has summarized 
the typical symptoms produced in the chick embryos by these 
teratogenic substances and his summary is reproduced here in a 
somewhat modified table form. (See table #1). 
38 
