Thursday, January 29, 2009

Inbreeding Depression Got You Down?

When I was researching last month's post on naked mole-rats, I learned that their colonies are inbred: as the National Zoo's article puts it, "because only a few members of the colony produce all the young, and the queen typically mates with close relatives selected from within the colony, naked mole-rat colonies are highly inbred." This made me think about inbreeding depression, a subject which has intrigued me for awhile (you all know I think about some freaky shit). So why don't we explore inbreeding depression and its effect on animals a bit before we go off in search of examples of human beings messed up by inbreeding.

What is Inbreeding Depression?

Inbreeding depression refers to the decreased biological fitness of individuals and populations due to mating between related individuals.

First off, there are two evolutionary disadvantages to inbreeding:

(1) the more closely related two individuals are, the more likely it is that they are both carriers of the same recessive deleterious or lethal gene, which means that if they mate their offspring may be born with the undesirable genetic condition. All human have a number of these recessive genes which are either inherited from a parent or produced by mutation, but the occurrence of each of these variants in the global population is rare. Thus, the chances of non-related individuals both being carriers of the same "bad gene" are small, and since the trait is recessive you're OK as long as you have one good copy, but when related individuals mate the chances increase that they might both have bad copies of the same gene to pass on to their children.

(2) the smaller the gene pool in a given population, the less genetic variation and the more likely it is that the population could be wiped out by an epidemic. In other words, a disease might arrive which nobody is immune to and kill everybody off. This makes me think of the Black Death which wiped out a third the population of Europe in the 1300s.

In addition to these concerns, there are a number of vague symptoms which may manifest themselves in excessively inbred individuals. First, inherited physical characteristics may become exaggerated to the point of deformity. Individuals suffering from inbreeding depression may also be weak and sickly; this can mean having a substandard immune system and being susceptible to every "bug" that goes around. Inbred individuals may also reproduce less frequently: males may have reduced libido and low sperm count, and females might be less fertile and show diminished maternal instinct towards their young.

Inbred animals

Inbreeding can sometimes be the result of animal husbandry by humans. Lab mice, for example, are notoriously inbred brother to sister, and they have been shown to suffer from reduced fertility, life expectancy and vitality and weakened immune systems. It is theorized that a lot of these inbred mice are only biologically viable in laboratories where they are given ample food and sheltered from predators and that they would not be successful in the wild.

Inbreeding depression can also be a problem for purebred dogs and cats where, for example, physical traits which are prized as being characteristic of the breed can be exaggerated to the point of becoming deformities. An example of this is overly purebred Persian cats whose faces are so flat that they are prone to tear duct overflow, breathing and sinus problems. And as a result of inbred stock, purebred German shepherds often suffer from hip dysplasia which can cause arthritis and lameness.

In the wild, in addition to naked-mole rats, the population of some endangered species such as cheetahs are very inbred. Even when species like these seem perfectly vigorous with no signs of fertility problems or sickliness, there is still an increased risk of population extinction in the face of infectious disease because of a lack of variation in individuals' immunities.

Human inbreeding

In relation to genetic disorders, one should note that some specific birth defects are more common in certain relatively insular populations – for example, Tay-Sachs disease among Ashkenazi Jews and Usher Syndrome Type I among Louisianians of Acadian descent (cf. founder effect). These are both autosomal recessive traits meaning both parents must carry a bad copy of the gene in order for their child to be born with the birth defect.

A more dramatic example of a population with a small gene pool where a rare genetic disorder has grown widespread would be the Vadoma people of Western Zimbabwe. In this isolated tribe, one in four children is born with a birth defect called ectrodactyly (also known as "lobster claw syndrome"), and as a result they have only two large toes on each foot. This earned the Vadoma the nickname "Ostrich people." Ectrodactyly is actually an autosomal dominant disorder; it is thought to have developed among the Vadoma due to a mutation and to have become so prevalent because of inbreeding among members of the small tribe. One should also note that, while this condition is unusual, it is not very debilitating and the Vadoma actually claim it aids them in climbing trees.

NEXT TIME: Our search continues with Inbred Royals.

Images: Photo of white lab mice by flickr user Rick Eh? used under Creative Common non-commercial license; photo of Persian cat found on cat chitchat from Stockxpert

1 comment:

Cyndy said...

Very interesting stuff here. I'm looking forward to your post on inbred royals.