How can two parents without hitch hiker's thumb have a child who does have hitch hiker's thumb?

Answer 1

No. strickly speaking hitchhikers thumb is autosomal recessive. that means each parent has two copies of the gene if they are true "hitchhikers". That means the only possible outcome is that all offspring will also have two copies. Of course if one of the parents is just hyperextendible and does not have the genes that's different. The child could also just not be very extendible but have the genes. But genetically there is no other options.