An F1 is a first generation hybrid between different strains or species. F1 refers to first “filial” generation, Latin for first “offspring” generation. Nuclear genes tell us if the hybrid is an F1. Most organisms carry two copies of each gene, one from their mother and one from their father. We know a hybrid is an F1 if it carries different alleles for all nuclear genes that differ between its mother and father species.
Mitochondrial genes come only from the mother through the cytoplasm in her eggs. Thus, F1 hybrid waterfowl will carry the mitochondrial genes of the mother species; then, if the plumage of the hybrid clearly identifies the father species, we can infer the siring species. Surprisingly, some F1 hybrids do not look like blends of their parents; in these cases genes on the sex chromosome can identify the father species with certainty.