Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
A type of factor or gene which produces so drastic an effect as to cause the death of the individual, usually during gestation or shortly following parturition is known as a lethal factor. Such factors produce their lethal effect only in the homozygous condition. Some lethal factors produce a visible but non-lethal effect when in the heterozygous condition, in addition to their lethal effect when in a homozygous state. Other lethal factors produce no visible effect in the heterozygous condition, their only visible effect being the killing of the animal when in the homozygous state.
Venzke, W. G.
"Inherited Lethal Genes,"
Iowa State University Veterinarian: Vol. 5
, Article 5.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol5/iss1/5