My partner has red hair, and I have blonde. Our son is blonde but our 4 week old daughter has black hair. Can you tell me how this can happen please?

The determinants of hair colour are complex and it is controlled by a mixture of many genes (probably 15-20), rather like eye colour. It is not surprising therefore that in one family many different colours can occur. The one hair colour that is controlled by a single gene is red hair - that is due to an inactivating mutation in the MC1 receptor.