Princeton bioethicist Peter Singer is famous for two primary reasons: First, he jump started the animal rights/liberation movement with his 1975 book Animal Liberation. Second, he is the world’s foremost proponent of the legitimacy of infanticide. Thus, writing on page 186 in Practical Ethics, he supported the right of parents to kill a newborn with hemophilia in order to make life easier for a hypothetical, yet-to-be-born sibling:

When the death of a disabled infant will lead to the birth of another infant with better prospects of a happy life, the total amount of happiness will be higher if the disabled infant is killed. The loss of happy life for the first infant is outweighed by the gain of a happier life for the second. Therefore, if killing the hemophiliac infant has no adverse effect on others, it would, according to the total view, be right to kill him.It should be noted that the disability of the infant isn’t why he can be killed, but rather, his view that infants are not persons.

http://www.wesleyjsmith.com/blog/labels/Peter%20Singer.%20%20Utilitarianism..html