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One study found that 40 percent of children whose parents were first-degree relatives were born with autosomal recessive disorders, congenital physical malformations, or severe intellectual deficits.According to the study, when first-degree relatives mate, the risk of their child suffering from an early death, serious birth defect, or mental disability increases to almost 50 percent.
It’s fairly common for dog breeders to preserve desirable traits within their litters through inbreeding, but frequent inbreeding over many generations causes serious problems.
Female lemurs, for instance, can tell if a male’s genes are too similar to her own by smelling the pheromones that emit from his genitals.
Mice also use smell to identify ideal potential mates, much like the human females in the smelly t-shirt test.
Fortunately, Mother Nature has developed her own strategies to deter us from mating with close relatives.
According to evolutionary psychologists, just observing your mother care for another baby is a cue that that other child is a sibling and discourages sexual attraction.
We all seem to know that incest is wrong, either through cultural conditioning or what appear to be innate evolutionary cues, yet some humans can’t seem to resist the urge to bed their relatives.
In fact, since the DNA of every living human is 99.9 percent the same, it may be much more common than we think. As we will see, breeding with a close relative can result in some dire consequences.Many times, those genes remain dormant, but when people who share a large number of genes breed, the chance of passing on conditions known as autosomal recessive disorders—conditions that are inherited through recessive genes, including cystic fibrosis, sickle cell anemia, and albinism—increases significantly.Other side effects of inbreeding include the increased risk of infertility, birth defects like cleft palates, heart conditions, facial asymmetry, low birth weight, slow growth rate, and neonatal mortality.If the female has already been impregnated by her brother, she can even abort the pregnancy before mating with the unrelated male.Because hyenas don’t use scent or other cues to identify relatives, females only mate with males that are new to their group.Experiments have shown that keeping a brother and sister mouse isolated in a cage will often result in their mating, but only if there are no other viable partners.