Bone marrow can be taken pretty much from any individual, including an infant with a genetic disease such as Tay Sachs. It's a simple (but painful) procedure that uses a needle to remove (aspirate) the bone marrow cells.

Tay Sachs is a genetic disease - it's pretty rare, but can be very serious. It causes deterioration of nerve cells. It's due to a mutation in one gene (called HEXA) on chromosome 15. Both copies (alleles) of the gene have to be mutated to get the disease. Some populations of people have a higher incidence of this mutation. Because we know exactly what gene to analyze, it's possible to do a quick genetic test to determine if a newborn has the mutation in one or both alleles. So, it's pretty unlikely that a Tay Sachs carrier would be able to donate bone marrow, especially if they were in a high risk population.

I'm wondering if you are thinking about how to potentially treat an infant with Tay Sachs, perhaps by isolating cells, repairing the mutation and returning the "fixed" cells to the infant? That's a great idea, but in this case, the entire nervous system would already be affected in an infant - you pretty much need a way to replace all of the nervous system cells or fix the genome in the newly fertilized egg. Researchers in China have just reported such an effort, to edit human fertilized eggs and repair the mutation that causes b-thalassemia. In the United States, the National Academy of Sciences and National Academy of Medicine just released a "Guidelines for Human Gene Editing." All of this is happening very quickly, largely due to a new editing tool called CRSPR-CAS9.