Asection of a dna molecule that controls the shape of a human thums is called

A section of DNA molecule that controls the shape of thumb is called genes.  

Explanation:

DNA is the molecule, contains genetic information, and is further broken into strands called chromosomes. These chromosomes further comprise of small segments known as genes.  

These genes comprise different alleles that provide people different shapes of a thumb. An organism with two alleles of each gene is called a diploid organism as they get one allele from each parent. Whereas, in haploid organism, one allele of each gene is only present.  

this is true, assuming that the allele is completely recessive and incomplete dominance is not a factor.