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.