inheritance of abnormal genes, e.g. polydactyly, ectrodactyly or brachydactyly, symptoms of deformed limbs then often occur in combination with other symptoms (syndromes) external causes during pregnancy (thus not inherited), e.g. via amniotic band syndrome teratogenic drugs (e.g. thalidomide, which causes phocomelia) or environmental chemicals ionizing radiation (nuclear weapons, radioiodine, radiation therapy) infections metabolic imbalance