Answer: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.