Yes they can cross,but it doesn't happen too often.In the western part of South Texas it's really desert type country.The whitetail deer are among the largest of the state.There are no mule deer in that area.The farther north and west of the South Texas brush country,you start getting into a mix of whitetail and mule deer.One thing I've notice about the deer in the western part of South Texas is they carry an unusually trait.They commonly have a darker or black tipped tail like the one pictured in the article below.Maybe they have cross genetics that make them carry that trait,or maybe they are just one of the many recognized sub-species.Whatever it is,it happened many years ago and the percentage of mule deer genes would be nearly none existent.Most people will find this hard to believe,but South Texas wasn't always the vast brush country it is today.When cattle were brought in from Mexico in the 1800's they brought the brush seeds with them in their bellies.This now brush country really didn't have many deer back then because it was once a vast grassland and desert.There may have been mule deer then in the western part of South Texas and maybe they could have crossed with the few whitetail that were around.As the brush thrived and grew taller it created great habitat for whitetail,not so much for mule deer.I do know the last known pronghorn antelope was killed on the King Ranch in the late 1950's or very early 1960's.It was the last one,so no chance to reproduce,so it was killed.Just shows how habitats can change over the years.

https://www.deeranddeerhunting.com/...-unique-tail-of-whitetails-and-mule-deer

Here is a picture of a buck's tail from far west South Texas near Eagle Pass.Notice the black tipped tail
[Linked Image from i.postimg.cc]

~Molɔ̀ːn Labé Skýla~
As Bob Hagel would say"You should not use a rifle that will kill an animal when everything goes right; you should use one that will do the job when everything goes wrong."Good words of wisdom...............