Adoctor finds that the mammary glands of a woman are not functioning due to a genetic abnormality that influences the structural design of the thoracic cavity. this is likely the result of a mutation:

The result of a mutation on a Hox gene.

Explanation:

The mammary gland of a women is not functioning properly due to the genetic abnormality which is caused due to mutation in the hox gene.

The hox gene helps in the thoracic development which later on provides support to the mammary gland.

The gene is responsible for the development of the structure and and mutation in the gene is responsible for genetic abnormality that influences the  mammary glands of human beings.