When you buy 'white leghorns', or brown sexlinks from most of the major hatcheries you are likely getting Hy-line Whites and Hy-line Browns. In a former career I was the Breeder Manager for Hy-line International's largest hatchery and we sold hatching eggs to the same hatcheries you all deal with for their white leghorns and their brown sex-links. If you buy from a hatchery in Texas, Iowa, Ohio, Missouri, Minnesota, Pennsylvania, California or New Mexico you are most likely buying Hy-lines. These businesses do not keep their own breeding flocks of these particular varieties - they buy the hatching eggs. They also likely buy their broiler hatching eggs from one of the broiler hatching egg suppliers. There are very few of the hatcheries that actually breed their own chickens any more. Nearly all of the new on-line 'hatcheries' have no chickens - they are just drop-shipping from Ideal, Privett, Cackle or Meyer. If they are using poultry pictures from one of these companies in their advertising, then they do not have their own chickens.
There is a pretty good chance that eggs hatched from grocery store dozens are Hy-lines - since they hold about 90%+ of the layer genetics used in egg produciton in this country.
Matt