If identical twins marry identical twins...

What happens, genetically, when a black and a white human partner have children? And if a pair of genetically identical twins marries another pair of identical twins, genetically speaking, are their kids effectively brothers and sisters rather than cousins? Meanwhile, do birds have ears and how does directional hearing work? If you lose one of your special senses, do other senses strengthen to compensate? What's the reason for human handedness? Does a blood transfusion recipient carry the genes of the blood donor? Dr Chris has the answers...