Not necessarily. There is a slew of complication that exists atop simple mendelian genetics that occur quite frequently, not to mention gametes themselves can u

Not necessarily. There is a slew of complication that exists atop simple mendelian genetics that occur quite frequently, not to mention gametes themselves can undergo mutations. And that all assumes fertility of the select couple in the first place. And the real key here is generation times. A human takes 9 months to gestate, our natural replacement rate is 5, even now about 50% of all conceptions result in spontaneous abortion. Then 20-25 years to fully mature, and some genetic conditions only show later in life. Other animals mature far faster, mice are considered aged at 18 months, dogs are mature at a year. Teosinte apparently matures in only 180 days. Each of these has in common that within those generations there are many progeny on top of the short generation time so you can very rapidly select, even inadvertently, by picking the best and coincidentally growing them in middens. From there I think its safe to say that our ancestors weren't the dullards they are often made out to be.