Now I'm no expert in this area of muscles fibers and the composition of either in a racial paradigm but..
I started my freshman year running a 5.29 forty yard time.
I had no other formal training in speed and never really considered myself fast by any stretch of the imagination. I did however, consider myself quick. I always felt like I had fast feet and my agility was always excellent as I had done gymnastics for 10 years. So, when it came time to play football in my freshman year I chose to play runningback because I felt I was fast enough for the job. Well, in my JV years I found that I could hit the hole FAST but after 10 yards my speed capped and I was stuck in 3rd gear it seemed. Well I was lucky that my school had a strength and speed coach and decided to take me under his wings and show me how to "train" for speed. He said I clearly had quickness and through training my muscles and running techniques he could significantly increase my speed. Well it took 2 years but by the middle of my junior season I lined up on the track and clocked a 4.42 forty time. My vertical jump had increased 15 some odd inches and I finally had that top gear that was missing. So I must say that YES you can train fast twitch muscles you do have but you have to be diligent and work with someone who knows what they are doing. I remeber instead of lifting heavy weights like all the cool kids were doing much to my behest my coach had me doing much lighter weights at blazing speeds. Very high rep very hig speed sets. Running increasingly longer distances with a weighted vest. Start at 10 yards, then 20, then 30. I had to do these at peak performance nearly every day. If you are white you can do it, it may not be as natural as black athletes but the capacity is there. you may never have the potential to be an olympic sprinter but you can bet your ass unlike the genius above that 4.4 -4.5 speed is NOT out of the realm of possibility.