mamluks weren't generally eunuchs, the mamluks were a special class removed from dynasty and family structures in order to (theoretically) make them a loyal army for a ruler. when they seized power they generally did not have legal offspring and the martial culture of mamluk corps were less concerned with heredity, ersatz familial patronage between senior and junior mamluks was just as likely the path of succession in the first or second generation until one of them had a heir who reaches majority before the father dies and can perpetuate a real hereditary line. Sebuktigin of Ghazna took over from his commander/mentor Alptegin. The mamluk sultanate of Delhi didn't settle into a hereditary succession til the third, Iltutmish.