Hypothetical case: 2 half brothers with a common father. One brother is kosher and the other brother is a mamzer. The mamzer marries a mamzerres and they have no offspring. The mamzer dies and the mamzerres is now "zakuk leyibbum" bound by her brother in law and cannot marry any one else until he does yibbum or chalitza.
Does the kosher brother in law have to do chalitza as he is prohibited to marry a mamzeres?
Or maybe she can do yibbum as the asei (positive mitzva), "Yevama yavo aleha" overrides the lo taaseh (negative mitzva) of "lo yavo mamzer bikhal hashem" ?
The question would likewise apply if the kosher brother dies childless leaving his kosher wife to be zakuk (bound) to his brother who is a mamzer.
The other cases where this question would be applicable Would be in cases of Petzua daka (a man with crushed reproductive organ), Kerus shofcho (a man with a cut off reproductive organ), Almono Lecohen Gadol (women that had been betrothed and become a widow, to do yibbum to a high priest), Gerusha lecohen Hediot (divorcee to any Priest),