Besides Mendelian patterns of inheritance, birth defects or genetic disorders may be caused by multiple genes or polygenic inheritance; or occurs because of interaction between gene and environment known as Multifactorial inheritance. Examples being neural tube defects, congenital heart defects, cleft lip and palate. Many familial traits and defects appear to be genetic in origin. Prevention, recurrence risk estimation and prenatal diagnosis and counseling takes into account these modes of inheritance for disorders that are known to be caused by multifactorial modes or with polygenic inheritance.