The Pattern loss is genetic hair loss where the hairs at the top central and horse shoe shaped are is where the hairs gradually miniaturize and die because these hairs are Genetically Hormone (5DHT) sensitive. This is the reason this hair loss is also called Andro Gentic Alopecia (AGA).
When the head is developed from embryo the frontal and parietal bone area of the head has migrated Neural crest tissue but the occipital and temporal bone area ( back and side of the head ) does not have this tissue of origin. It is believed that certain genes responsible for AGA affect only these hairs which are having Neural crest origin and the area at the back and sides escape the effect of these genes.
