Backcross hybrids are progeny derived from an F1 hybrid mating with a member of one of its parental species. At each genetic locus F1 hybrids carry an allele from their mother species and an allele from their father species. Thus, for all genes that show fixed differences between the parental species, F1 hybrids will carry one gene from each parental species.
Note that the rarity of F1
hybrid waterfowl means that F1 hybrids will not find other F1s as mates; instead, they will mate with one of the parental species, forming F2 backcross progeny. Three
quarters of the genes in F2 backcross hybrids come from their backcross
parental, and ¼ genes from the other parent that produced their F1 mother or
father. Backcross hybrids are important because they transfer genes laterally
between the parental species. When F1 hybrids mate with only one of the
parental species, then gene transfer is unidirectional, increasing genetic
diversity only in the backcross parental.