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.