ABSTRACT
Extra-toes (Xt) is a new semi-dominant gene in the mouse. Xt/Xt animals die in utero or at birth. They have multiple abnormalities including paddle-shaped feet with up to eight or nine digits, hemimelia, disturbed spinal cord and brain, nose, eye and ear, and oedema. The heterozygote has pre- and post-axial polydactylism of all four feet.
The Extra-toes homozygote can first be recognized as a 9-day embryo by excessively large pharyngeal arches and persistently open neural tube.
Many abnormalities of the brain and sense organs can be traced back to malformation of the neural tube.
Malformations of the upper jaw region and limbs can be traced back to overgrowth from the 9-day stage onwards.
A third series of malformations due to oedema (first seen at 13 days) is described.
The possible interrelations of these three groups of abnormalities to each other, and the relationship of the Xt syndrome to other gene effects in the mouse, chicken and guinea-pig are discussed.
