How is the law of karma related to physical heredity? Physical heredity plays a great role; we know that some of the characteristics of a father and his ancestors may be found again in the son. In the Bach family, for instance, there were twenty-eight highly gifted musicians in a period of 250 years. Again, Bernoulli was a great mathematician, and eight other gifted mathematicians came after him in his family. This is all a matter of heredity, we are told; but that is only partially true. In order to be a good musician you need more than a musical predisposition in your soul; you need also a good ear in the physical sense. This good ear is a physical quality to be found in a family of musicians, and is passed on from one generation to the next. In a family, then, where a great deal of music is performed, you will find good musical ears, and so when a soul with a strongly developed musical talent is to be incarnated, it will naturally not choose a family with no interest in music — where it would languish — but one which has suitable physical organs. This fits in very well with the law of karma.
The same thing applies to moral courage. If a soul with that predisposition cannot find a suitable heredity, the characteristic will fade out. You can see that you have to be very careful in your choice of parents! The fact is not that the child resembles his parents, but that he is born into a family where the parents most resemble him.
Source: Rudolf Steiner – GA 95 – At the Gates of Spiritual Science: Lecture 7: Workings of the law of karma in human life – Stuttgart, 28th August 1906