Being an autosomal recessive condition means that a child has to inherit two copies of the gene for hitchhiker's thumb - one from each parent.
Each gene generally comes in two versions, known as alleles, and an allele can either be dominant or recessive. And an inherited characteristic or phenotype (such as hitchhiker's thumb) is known as a trait.
In genetics a dominant allele is expressed with a capital letter and a recessive allele is represented by lower case letters.
So let's consider the body contortion hitchhiker's thumb. First of all in terms of thumb position, a straight thumb is the dominant allele and expressed with the upper case S. Hitchhiker's thumb is the recessive allele and expressed by the lower case s.
Then, you have to bear in mind that a genotype (which creates the phenotype) is made up of either - two dominant alleles, two recessive alleles, or one dominant and one recessive allele.
A person with ss genotype will have hitchhiker's thumb - there are two recessive alleles
A person with Ss genotype will have a straight thumb, but will also be a carrier of hitchhiker's thumb
A person with SS genotype will have a straight thumb, and will not be a carrier of the condition.