Inbreeding

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).

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.

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 57^th 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.

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.