i've made babies that have had up to 3 points on a couple of stats, that was from breeding parents that have 6's on multiple stats. although i suspect it does not matter.
however, in one breeding session i had a guy with 7 luck impregnate 6 women, while rothchild 7 charisma impregnated 4 and most of the babies came out with 3 luck or 3 charisma, i can't tell if that was genetics or just RNG.
i'm currently training up some guys, i want to make at least 1 "perfect dweller" with 10 in each stat in the hopes of creating a "master race" of super babies
and the "rare dwellers" have higher stats and some rare equipment and pop out at a higher level..but. you can build training rooms to raise the your stats, so with enough patience, you can train any common dweller to have better stats than a "rare" dweller.
it's probably higher than .001% to get a rare dweller..i've earned about 12 free lunchboxes and got scribe rothchild, he appears in my collection screen and came with scribe rothchilds robe a legendary rarity outfit.
and in another box i got a rare dweller, i forgot his name, he does not appear in my collection but he popped out with 7 luck and 4-5 on all the other stats and i think he came with the lone wanderer handgun 2-7 damage...not great...
and i got another rare guy that came with a horror fan outfit high strength and endurance and all the other stats were low and i changed his name to Jason Vorhees(i know, not creative but meh...)