This part looks at the concept of breeding and, to an extent, the bloodlines, generally with the help of Beatrice Tarleton, a paramount character, despite being a moderately-minor one. One of the novel’s repeating themes is how a few contrasts between individuals, specifically unfavorable ones, are “in the blood”. The Wilkes family is “unique” because of their propensity for getting cousins as wives.
Beatrice Tarleton portrays the Wilkes group as lacking in quality and stamina. Since readers definitely think about Scarlett's ability and quality of will, it appears to be clear a marriage amongst Scarlett and Ashley would be troublesome, at best. Their contracts are “in the blood”.