The nature of Brahmanas, Ksatriyas, Vaisyas, and Sudras are due to their respective inherent dispositions. The meaning is that their past Karma has been the cause of determining births as Brahmanas etc. The Sattva and other Gunas are the result of such Karma. The Sattva-guna is born from the inherent nature of the Brahmana becoming dominant by suppressing the alities of Rajas and Tamas. The ality of Rajas originates from the inherent nature of the Ksatriyas becoming dominant by suppressing alities of Sattva and Tamas. Tamoguna arises from the inherent nature of the Vaisya, becoming dominant in a little way by suppressing Sattva and Rajas. The duties and works assigned to them according to the Gunas constituting their inherent nature, are expounded and allotted by the Sastras in the order described. For the Sastras analyse that the Brahmanas etc., possess such and such attributes and such and such are their duties and occupations.
These lines clarify two points
Proportion of gunas in an individual, which are obtained by birth, are result of her karmas in previous lives.
Varna obtained by birth solely depends on the proportion of gunas obtained by the individual during birth.
I can roughly present it as below
Brahmana: Sattva >> Rajas, Tamas
Ksatriya : Rajas >> Sattva, Tamas
Vaisya : Tamoguna > Sattva, Rajas
Proportion of gunas in Sudra are absent in those lines.
What is the proportion of gunas in a Sudra? Does any scripture other than commentaries contain details about all these proportions of varnas?