Answer: F or f
Explanation:
The cross between the two parent is a monohybrid cross and since the parents are heterozygous the genotype will be Ff for both parents and either of the F or f could be the gametes for both the male and female parent .
A cross between this two parents will give -three genotype in ratio 1:2:1 (FF, Ff Ff, ff) and two phenotype this include a six fingered person and five fingered person.
Ff× Ff
F or f - gametes
FF Ff Ff ff- offspring