Just remember another related *no pun intended* assumption.
BG: When DH was 7mos old, his bio-father had enough of fatherhood. Just left MIL and Dh, and filed for divorce, leaving MIL with a lot of credit card debt that he'd racked up. When DH was about 3 or 4, MIL remarried, and her new husband (FIL) adopted DH and is "Dad" to DH. /BG
When DH and I first started dating
, I didn't know this and asked him if he looked like his mom or his dad. He said "Oh, I look like my father, but my dad and I look nothing alike." He explained the difference and went on to tell me what happened when he forgot to tell his ex this information.
Now, since DH and FIL do not share any DNA, of course they wouldn't resemble each other physically, though personality wise they make a good argument for Nurture vs. Nature. DH and MIL are of stocky build, average height, overweight, and blue eyed, and DH is blonde. From pictures I've seen, he bears very strong resemblance to his bio-father. Now, FIL is rail thin (buck 40 soaking wet and holding a brick-maybe), avg height, thinning dark hair and dark eyes, and an olive complexion.
Not knowing any of this, not even that FIL was not DH's biological father, when the ex girlfriend first met them, she took one look at FIL and said to DH, "Wow, you don't look ANYTHING like your Dad!!" I think it might have been the only time I felt bad for her, or was annoyed with DH on her behalf, since he hadn't warned her and let her walk into it. He said "Well, I thought she'd be able to tell just by looking at him that I don't share DNA with him!!!"
He did learn from his mistake though, and I'm so glad! LOL!