I have recently got into watching Game of Thrones so I
thought there is no topic more appropriate than the wonders of inbreeding. Many
people have heard the rumours that having offspring with blood relatives
results in children with physical and mental defects, however there has never
really been an explanation as to why that is the case. This article will
outline the genetic reasoning behind these abnormalities (despite my hatred of
studying genetics).
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What is inbreeding?
Inbreeding is defined as the mating or breeding of individuals
with a close genetic relationship (brother and sister, father and daughter etc).
The problem lies with recessive genes. Whilst we all carry either beneficial or
neutral genes, we also possess a selection of deleterious, recessive genes that
have the potential to negatively impact our health. These recessive genes
generally remain inactive as they are the recessive form of the gene and are
never expressed in the presence of the harmless, dominant gene. If the
recessive gene is not expressed, an individual will be a carrier of a
potentially harmful genetic condition rather than a sufferer. Family members
have a higher probability of carrying the same recessive, gene leading to its
expression. The homozygosity of intrafamily-produced offspring means that they
are at increased risk of being affected by recessive or deleterious traits
which in turn leads to a decreased biological fitness.
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| Here is an example of a recessive gene being passed from parents to offspring. The second generation are only carrying the gene without experiencing any decrease in biological fitness. When the brother and sister snake then have a baby, the 3rd generation was passed matching recessive alleles which expresses the maladaptive gene. That red snake probably has an extra head, spinal bifida, cystic fibrosis or perhaps it is just red despite being from a black family. |
The probability and severity of developing such disorders is
increased the closer the genetic relationship between the biological parents
(i.e. a brother and sister’s offspring has a much higher likelihood of
congenital birth defects than a girl and her mother’s brother). This is because
the closer the biological relationship, there is an increase of pairings of
deleterious alleles which produce disorders. As the predominance of these
deleterious, recessive alleles are rare in populations, it is highly unlikely
that two unrelated parents will carry the same allele but close relatives share
a large number of alleles, which increases the chance of one becoming dominant
and therefore detrimental.
Is it exclusive to
humans?
In the animal kingdom, inbreeding is more commonplace.
Firstly there isn’t as much stigma to mating with a cousin when you are an
elephant seal but also because often when a population becomes isolated, they
do not have an option to breed with non-family. Animals also experience the
negative impact of maladaptive homozygosity such is the example of the Isle
Royale Wolves. As the population of wolves is isolated on an island,
malformations are “stuck” in the population. In the recently published, annual
report of the wolves, it was noted that the predation rate on the island has
decreased significantly. It has been suggested that this is a result of ongoing
inbreeding of the pack (Isle Royale is currently the longest running study of
predator/prey interaction due to the observation of wolves and moose that
crossed onto the island via an ice bridge. The study is currently in its 57th
year). Abnormalities such as too many bones in the vertebral column and
vulnerable immune systems have been noted and are showing signs of spreading
and unfortunately, there is no opportunity for genetic relief as “foreign”
wolves cannot be naturally introduced to the island.
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| This is a comparison of a normal wolf vertebrae compared to one taken from an Isle Royale wolf. Note the slight size difference as well as the obvious deformity. |
Benefits of Inbreeding
Of course, eliminating genetic diversity can be beneficial.
Inbreeding produces uniform and predictable offspring. This is commonly seen in
the pure-breeding of animals to ensure desirable traits are maintained and when
an undesirable trait appears, selective mating is used to eliminate that
particular mutation. Also, the primary issue with inbreeding is the doubling up
of deleterious recessive alleles. On the contrary, inbreeding also doubles up advantageous
“good genes”, keeping them within a population.
