DNA harvest and black goo by product

I started a thread a wee while ago about the xeno being designed as a DNA harvestor . After going through all is cycles eg egg----face hugger----chestbuster----xeno--------then victims being transformed into eggs. After a while the all the 'resources' are used up the xenos would all die out leaving only the eggs. At a later date the eggs would be harvested by the engineers . By some method the DNA is extracted from the eggs . I think there is a process where the extracted DNA is processed . All the good DNA eg; longevity, characteristic of strength, intelligence and so on ( i hope you understand what i mean) leaving behind the bad DNA eg diseases, weakness and so on. This bad DNA is the black goo which is in the cylinders. your thoughts ( i have my head gear on , so go for it) lol