Heredity is transmission of traits from parents to their offsprings. The heredity information is carried by zygote or fertilized egg, which develops only into certain type of offspring. Therefore, only sparrows are hatched from eggs laid by sparrow, and only puppies born by a dog.
Some individual differences in an organism that distinguish it from the other generation are called variations. They are passed through generations. The similarities and the variations of the offspring of the same parents are studied in sexual reproduction. It is the variations that cause two persons barely have absolute similarity.
