Q. As we spoke by phone, I was wondering if one of my children does not look at all like his siblings but rather is very similar to a neighbor. do I have to be concerned? Should I do a DNA testing on him?
A. Maharsham (3: 160) rules that similar looks do not prove at all parental relationship. He quotes a story told by Medresh Rabba (Vayetze) of a dark couple that gave birth to a white child.  When the very surprised parents consulted Rebi, he said it was the result of the environment in their home. A similar ruling is to be found in Chassam Soffer (E.H. 1: 9).
See also Talmud (Berochos 20a) Rabi Yochanan would go and sit by the entrance to the Mikva. He said to himself: When Jewish women come up from their immersion [after their menstruation,] they should see me first so that they have beautiful children like me.
Horav Shlomo Miller’s Shlit’a opinion is similar.
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a