Hitchhicker's thumb is a condition in which a person is able to bend his thumb backwards. This trait can be seen in many people but the condition is recessive, which means a person needs two alleles for hitchhiker's thumb to have this particular characteristics. The gene for straight thumb is commonly indicated by T while that for hitch hicker's thumb is indicated by t.
Let's see what will happen if a person heterozygous for hitchhiker's thumb marries a woman who is also heterozygous for hitchhiker's thumb.
P1: Tt X Tt
Gametes: t T : t T
Offsping : TT : Tt : tT : tt
TT : 25 % : Straight thumb
Tt + tT : 50% : Straight thumb
tt : 25% : Hitch hicker's thumb
Therefore, there are 25 % chances that their offspring will have Hitch hicker's thumb and 75 % chances that they will have straight thumb.
Hope it help!