does the fact that birds are the modern day descendants of dinosaurs mean that temperature didn't determine the gender of dino young?

If you go back far enough in the evolution, yes, birds did evolve from some form of dinosaurs (theropods?). They are, somewhat remotely, related to alligators. But that does require us to go WAAAY back.

It is already known that, for some species, temperature does have some (statistical) effect on the performance of spermatozoids, thus causing a (small) preference for one gender rather than the other (depending on being too hot or too cold). However, this does not prevent either gender-specific spermatozoid to reach the ovum.

However, the effect of temperature (by itself) is small, relative to other effects. Also, it does not affect all members of the species equally (and not all subspecies).

What the presence of birds "prove" is that temperature, by itself, is not a sufficient explanation to explain gender disparity.

WOW,,you know something I never heard of, and can't find reference to.

"gender" does not apply to birds or dinosaurs.

In some species temperature of the egg determines the SEXE of the young animal. In birds it does not.

and just because your grandparents had some genetic trait doesn't necessarily mean that you or your offspring have it too

1.The theory that birds evolved from a dinosaur is pure BS based on clado-phenetics. Besides we simply do not know how gender is determined in dinosaurs. Cladists  are religious zealots and they treat the dinosaurian origin of birds as gospel truth that cannot be falsified. They say that only a cladogram can falsify another cladogram and that is just purely unscientific and insane. 

2. Birds have sex chromosomes and gender is not determined by incubation temperature but by which chromosomes an egg has. Birds have a ZW system of sex chromosomes. An individual with 1 Z and 1W chromosome is a female, and individuals with ZZ but no W chromosome is a male. Dinosaur chromosomes are unknown and therefore we do not know if they have sex chromosomes or not. 

What do you mean, "fact"? That's only a hypothesis and a dubious one at that, except among laymen.

I'm going with a "We don't know" thing.

Birds and dinosaurs and crocodilians are all archosaurs.

Birds use the ZW genetic system.

Crocodilians use a temperature-dependent system.

I'm going with "We don't know about dinosaurs, and maybe not all dinosaurs used the same system anyway."

What you cite is not a fact.  Birds did not descend from dinosaurs.