Father is short finger, and male short finger gene TS is dominant, so the probability of father having 1/2 is TSTS, and the probability of having 1/2 is TSTL.

Mother's short fingers are recessive in women, so the mother's genotype can only be TSTS.

Because the index finger of offspring is known to be long and short, the possibility of father TSTS is ruled out.

Finally, the mother genotype was TSTS and the father genotype was TSTL.

The genotype of 1/2 is TSTS, and 1/2 is TSTL.

All TSTS people have short fingers,

According to TSTL, it is related to gender, boys have short fingers and girls have long fingers.

So if the child has long fingers, it can only be a girl with TSTL genotype, and the probability is1/2×1/2 =1/4.