Picture a circle that has been closed for a long time. Whatever is inside it came from inside it, and nothing has arrived from outside for generations. The same features keep coming round, the same weaknesses too, and after a while nobody in the circle can see what is missing, because there is nothing there to compare with. That closed circle is the one picture behind all three uses — the only thing that changes is what is being kept inside it.
From the word’s history
The packet gives the make-up directly: inbred is the past participle of inbreed, which is in- plus bred. So the word literally says bred within, and the source's own first gloss for the character sense is 'bred within; innate'.
來源把組成寫得很清楚：inbred 是動詞 inbreed 的過去分詞，也就是 in-（在裡面）加上 bred（繁殖、長出來的）。字面上就是「在裡面長出來的」。來源在講性格那個意思時，第一句解釋正好也是 bred within（在裡面長出來的），所以兩個用法的關係在字典裡就看得到，不用另外推測。
Wiktionary, "inbred"
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