The concept of whole sign houses comes from the foundational astrological belief that houses were not different entities from whole signs.
The word house is another word for "place" And both are other words for "sign". The 1st "house" is the 1st "place" to rise at birth. Aka., the 1st sign to rise at birth. The 2nd house is the 2nd sign to rise after birth. The 3rd sign to rise after birth is the 3rd place or 3rd house, etc all the way around to the 12th sign to rise after birth, which is the 12th "house".
With whole sign houses, the ascendant degree and descendant degree are still very important points in the 1st, and 7th houses but they do not mark the beginning of the 1st or 7th house. The beginning of the sign does. The MC and IC are both also very important points in the houses but do not mark the beginning of the 4th and 10th houses.