Inbreeding and it's General Effects
Mammals, most other animals, and higher plants as well, have evolved mechanisms
to avoid inbreeding of any sort. Some, like sweet cherries, have even evolved
elaborate biochemical mechanism to ensure that their flowers can not be
fertilized by themselves or by very genetically similar individuals.
Most pack animals (like lions, primates, and dogs),kick young males out of the pack so as to prevent them from mating with female relatives. Humans have very strong taboos against mating with relatives. Even fruit-flies apparently have a sensing mechanism to avoid too close of inbreeding, even in a closed population they maintain more genetic diversity than they ought to by random mating.
Why do living things avoid inbreeding? Because in general, it is quite bad for a population or an organism to be very inbred. There is a well studied, although only partially understood phenomenon called inbreeding depression.
Inbreeding depression is thought to be caused primarily by the collection of a multitude of deleterious mutations, few in themselves fatal, but all diminishing fitness. Normally, in an outbreeding population these alleles would be selected against, hidden, or corrected by the presence of good alleles (versions of genes) in the population.
We generally think that mutations arise only once in a great while. That is certainly true for individual genes and specific kinds of mutations. However scientists have measured mutation rates in humans, chimpanzees and gorillas and discovered that there are roughly 4.2 mutations/individual/generation that affect the actual final proteins encoded by the genes. (A, Eyre-Walker, P. D. Keightly, Nature 397:344-347. 1999). Of these mutations about 1.5 are deleterious, in other words would cause harm to the animal if they were homozygous. The scientists who performed this analysis suspect that their numbers are actually artificially low for a variety of valid reasons, and estimate that the actual number may be closer to 3 deleterious mutations per individual per generation.
So how come we don't all have tons of genetic diseases? The answer to that is fairly simple, sexual reproduction, and the shuffling of alleles of genes that occurs when two unrelated individuals mate.
When that shuffling can't happen because both parents already have mostly the same alleles, the result will be inbreeding depression, if not in a given litter, then in a few more generations of such breedings.
Inbreeding depression encompasses a wide variety of physical and health defects. Any given inbred animal generally has several, but not all, of these defects. These defects include:
Elevated incidence of recessive genetic diseases
Reduced fertility both in litter size and in sperm viability
Increased congenital defects such as cryptorchidism, heart defects, cleft palates.
Fluctuating assymetry (such as crooked faces, or uneven eye placement and size).
Higher neonatal mortality
Slower growth rate
Smaller adult size, and
Loss of immune system function.
Can you at least eliminate specific genetic defects through inbreeding? (next)
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@Heather E. Lorimer, Ph.D.