Rh+ is dominant. a female with Rh+ can have a foetus of Rh- as the factor Rh comes from male parent. the disease erythroblastosis foetalis occurs because female being Rh+ produces antibodies against Rh- which enters the female body from the foetus during the birth of child. hence during 2nd pregnancy the foetus which is Rh - is killed because the antibodies towards Rh- are present in the mother's body. hence lead to miscarriage.