-They lead to death when expressed in an individual in homozygous condition
-They are dominant and are expressed in heterozygous condition
-They  are most active in homozygous condition
-This effect is clearly illustrated by the inheritance of colour in mice. Wild mice have grey colour fur (Agants). Some mice have yellow coloured fur. A cross breeding between the two produces offspring  with yellow and agant fur in ratio 2 to 1 (2:1). The mice with yellow fur are heterozygous and the homozygous ones will die.