Aileeny's answer is correct, but of course there is also the common additional factor that your biological father may not be who your mother says he is...
Sexually Produced Offspring Often Resemble, But Are Not Identical To, Either Of Their Parents. Explain Why They Resemble Their Parents Butare Not Identical To Either Parent?
A persons genome is unique ( with the exception of identicals) .This is due to a special type of cell division called meiosis which causes a random recombination of the parental chromosomes ensuring uniqueness. We all inherit 50 per cent each of both parents genepools and sometimes a sibling will inherit more "like" parental genes than another sibling will. It is all a matter of random chance. Just like by chance if some siblings picked randomly a certain number of cards from a pack, some would share more of the same cards than others would. . But sexual reproduction or having two parents ensures that the offspring are not identical to either parent. Hope this helps.
