Mom and Dad in one single parent?

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Many times I've heard a scorned single mom say 'I think I've done a pretty good job of being Mom and Dad to my children.' (I'll admit as a single mom with the ex more often out of the picture then in i've said this before) and I've heard a response to this, that it's not possible to be Mom and Dad in one. That you can be an amazing Mother, but you can totally take over the position of Dad. I'm curious, what other people's opinion on this. Can a single mom (or single dad, not trying to be sexist) be both parents to a child? (figurativly of course, lets not use the transexual arguement)