atavistic

Of the recurrence of a trait reappearing after an absence of one or more generations due to a chance recombination of genes.

Although the heterozygote gives it an atavistic appearance, the gene is not atavistic.

Thus the gene which produced atavistic digits in the vigorous heterozygous pentadactyl condition is a lethal monster in the homozygous condition.

Of a throwback or exhibiting primitivism.

They made me feel that I was alive in the nineteenth century, a sort of atavistic remnant, a romantic shred […]

The true perversion took place only in the privacy of her mind — the way she imagined an atavistic macho atop her when engaged in a mandatory contribution to the fetus-banks with some cretinous inept breeder…

Relating to earlier, more primitive behavior that returns after an absence.