So far the Fresno seedlings available around here are reliable for Fresno heat and color. Serranos are too, when I can find them, but I grow those, and various Poblano cultivars from seed. I think they are all of “Anum” species, and sprout fairly easily, making them easier to grow from seed.
Here is one opinion about the jalapeño situation.
There’s lots of words, but I focused on this bit;
" The standardization of the jalapeño was rapidly accelerated by the debut, about 20 years ago, of the TAM II jalapeño line, a reliably big, shiny, fleshy pepper that can grow up to six inches long—with little to no heat. TAM II peppers have become some of the most popular in the processing business. The 2002 paper in HortScience trumpeted TAM II’s benefits: virus resistance, absence of dark spots, longer fruit with thicker flesh, earlier maturation, and, compared to a variety of jalapeño called Grande, less than 10 percent of the spiciness. TAMs grown in one location measured in at 1620 Scoville units, while those at another came in at just 1080, which is milder than a poblano.
In conclusion, the paper’s authors wrote, “The large, low-pungency fruit of ‘TMJ II’ will make it equally suited for fresh-market and processing uses.”