yea there are cats out there born with 1 testicle.. it is genetic from the whole famly.

so basically that's consider "cacat".

the male cat will also produce abnormal sperm. healthy sperm will have 1 head and 1 tail... abnormal sperm may have 2 heads 1 tail, 4 heads 2 tails, 1 head 4 tails, etc.

potential, this sperm will produce abnormal kitten, alive, short time age, or died during pregnant.

abnormal sperm not only come from 1 testicle male, but also if a father cat mate with his own daughter, mother mate with son, brother mating with sister = this will produce abnormal sperm as well...

you may see the kitten born healthy from outside, but higher % they have problem inside.

so your male cat is recommended to be neuter. you might not want abnormal kitten.

abnormal kitten during pregnancy will harm the mother.. you may not want anything happen such as miscarriage.