You'll find hundreds of files on cleft lip, cleft palate here on widesmiles.org . It occurs in approximately one in 700 births. It is not evenly distributed racially. Cleft occurs more often proportionally among the Asian populations and less often among Blacks. However, it does occur among peoples of every race.
Many factors contribute to cleft conditions.
Among them are heredity, pre-natal nutrition, drug exposure, and other environmental factors, etc. A cleft occurs when something interrupts the normal development of the face. In the overwhelming majority of cases, when a cleft occurs, it is considered an isolated cleft. That is, it is a cleft in the absence of a syndrome. The alternative to an isolated cleft is a syndromic cleft, or a cleft associated with a syndrome.