Book about genetic engineering to defeat disease that turns children into hermaphrodites

Question

The initial plot was about genetic engineering being used to defeat disease, but something went awry that caused a massive social upheaval, where people in social settings would have a lapel pin or small badge that showed their blood type, so they could match up with someone compatible ( and not catch/cause a worse disease ).

An unknown effect of this genetic engineering was that newer children born turned into a sort-of hermaphrodite where they would physiologically switch genders under certain circumstances ( either puberty or pregnancy, I don't remember ).

The protagonist eventually adopted the surname Dos Caras ( meaning two-face or two-faced? ) and ended the story with a compatible mate.

All I remember of the antagonist was that he was a real ass, and regardless of the gender he/she was at the time, would flaunt the social custom of showing blood type in a gathering ( by wearing whatever blood type corresponding to the person he/she was interested in, and not his/her own ) just so he could 'sow his seed far and wide' to spread the genetic mutation that causes children to switch physiological genders.

The antagonist might have even died, by the end of the story, but that's one of the more fuzzier parts.

Answer 1

This is The Breeds of Man by F. M. Busby.

The genetic treatment was an attempt at a cure for AIDS but it went wrong and created a new race of humans informally referred to as the Mark Twos.

The Mark Twos do change gender, and later in the book a drug is found that allows them to do so at will rather than under stress:

The establishment of new IDs had been in the mill all along; Dr. Gill's developments put some new twists to the plans. For instance, any M-2 with supplies of Gill's period-inducing hormone and stabilized menstrual pheromone could pretty much change gender at will, and carry IDs for both modes; the only real danger would be during the two-to-three-day transition period.

The blood type problem is because Mark Two women could only bear one child from a father of any given blood type, so at singles bars they would wear a badge to indicate their blood type and what blood type they were looking for. For example:

Alvin had agreed she could have a second child, so here she was at a matchup bar. Wearing a green triangular pin. Strictly speaking, green meant a woman was looking for blood type AB, but it could be used to include A and B as well: simpler than also bedecking oneself with yellow for A and blue for B. For a new fad, she thought, the pins had caught on fast.

The protagonist is Troy Hagen:

My name then was Troy Hagen. It was only later, when we began to move outside the compound and live among the Mark Ones, that Rome and I took the surname of dos Caras. There's a joke to that; in Spanish it means "Two Faces." Somehow my brother and I thought it most appropriate; I have no idea what the M-ls think.