A comparison of the anatomical variations found in trisomies 13, 18 and 21

Little or nothing is known about how genetic information translates into body form. However, it is recognized that small changes in the genome can result in serious malformations and that environmental factors can cause various defects depending on the timing of the interference. It is a difficult task to unravel the mechanisms involved in such cases and most attempts to explain the results are based on descriptions of the defects in question, a knowledge of how the body parts develop normally and inferences drawn from embryolog-ical experiments on animal models which provide clues that can be used to explain how the developing tissues interact.