Ponderosa lemon
The ponderosa lemon originated in roughly 1887, and is believed to come from a chance seedling grown in Hagerstown, Maryland. It was later named and introduced to the nursery trade in 1900. Though often referred to as a cross between the lemon and citron, a recent genomic analysis showed it to solely contain pomelo ("Citrus maxima") and citron ("Citrus medica") DNA, perhaps being an F2 hybrid. Thus they cannot derive from true lemons, which have a mandarin orange component.
Mandarin orange
According to molecular studies, the mandarin, the citron, the pomelo, and to a lesser extent the papedas and kumquat, were the ancestors of most other commercial citrus varieties, through breeding or natural hybridization; mandarins are therefore important as the only sweet fruit among the parental species. Though some mandarin cultivars remain pure, most have some degree of pomelo hybridization, while in some cases the amount of pomelo is substantial.