Does Inbreeding Give You Six Fingers?

It is a common joke amongst the youth where I live that people who live in a town called Coalville have six fingers on each of their hand, and this is put down to supposedly high levels of inbreeding, usually accompanied with a made-up-on-the-spot statistic that claims Coalville to have the highest rate of incest in the UK (although this is most likely untrue and has never been supported by figures by any national survey!). Other tasteless jokes also claim that those who live in Coalville are less intelligent, but of course, this is yet to be proven!

The condition of having extra fingers or toes is known as polydactyly. It is a congenital disorder, which means that a baby would be born with polydactyly, and did not develop the disorder after he or she was born, and there are several reasons why people may have supernumerary digits (extra fingers).

Polydactyly does occur in other animals too, including dogs, chickens, mice and cats, usually as a result of mutations to the LMBR1 gene. This gene encodes for a protein that is involved in the development of limbs and as a result mutations (which are changes to the sequence of bases in the DNA) can alter how limbs develop, and therefore cause polydactyly.

Some Statistics

Here are some reliable statistics taken from a scientific study by Helen Dolk and Martine Vrijheid at the University of Ulster, published by the British Medical Bulletin:

  • In 2002, only eight infants were born alive with polydactyly
  • There were three cases of abortions that occurred after the foetus was diagnosed with polydactyly
  • There were no stillbirths or foetal deaths due to polydactyly
  • In 2002, the condition occurred at a rate of 3.74 affected per 10,000 births (around one affected per 2,700 births)

The condition is actually uncommon in humans, with a world average of one affected with polydactyly per 500 live births, so in the UK, there are far less "polydactyls."

There are many causes of the condition.

Most of the time, it occurs on its own, and not as a part of a genetic disease. In these cases, they occur as a result of a mutation on a gene, such as the LMBR1 gene. Mutations are random changes to the DNA base sequence and can happen randomly. Sometimes, mutagens like radiation, UV light and cigarettes can cause mutations, but not all mutations have an outside cause. It may be due to spontaneous breaking of chemical bonds in the DNA or errors in the way in which DNA is copied.

In spontaneous cases, when polydactyly occurs, it is usually a dominant trait, which means that any children of a polydactyl would have a 50% chance of inheriting the trait.

Fortunately, in most cases of polydactyly, the extra digit is just an extra finger sticking out of the thumb or the little finger with no bone which can be easily treated at birth by tying a suture around it, causing the blood circulation to be cut off and the supernumerary digit would fall off after a while; the hand or foot would then heal and look completely normal.

In other cases, polydactyly is more complex, involving bones in the extra fingers, and x-rays would need to be done to see if the digit should be removed and how it would be done. Any treatment would most likely be done more than a year after birth.

Polydactyly does not occur any more or less in either males or females as the LMBR1 gene is not located in the sex chromosomes (X or Y). It is located in chromosome 7 in humans, and chromosome 5 in mice.

Inbreeding, which is when a pair that is genetically related mates with each other, does increase the likelihood of genetic diseases, but most especially those that are recessive.

Recessive alleles mean that it takes two parents that are carriers (but not affected) of the same gene to produce an offspring with a 25% chance of being affected and 50% chance of being a carrier.

As polydactyly is usually caused by a random mutation, only one of the parents would be affected and so inbreeding does not increase the likelihood of a child developing polydactyly.

If both parents were polydactyls, then the likelihood of the child being affected is 75% but this is not increased by inbreeding.

There are, however, other causes of polydactyly that are not due to spontaneous mutations....

Other Causes of Polydactyly
Although rare, polydactyly can also occur as a part of a genetic disease. Laurence-Moon-Bardet-Biedl Syndrome (LMBBS), whose clinical presentation can involve obesity, mental retardation, renal failure, heart problems, speech disorder hypogonadism and partial-sightedness, can also cause supernumerary fingers. There are many genes that are involved with this syndrome, and is not a condition that is caused by one gene (monogenetic disorder).

This baby has Ellis-van Creveld syndrome
and has six fingers on each hand
Another cause of polydactyly is acrocallosal syndrome is a recessive genetic disease that manifests as the partial or complete absence of a part of the brain called the corpus callosum. But along with it, other symptoms include polydactyly and motor and mental retardation (impaired movement and cognitive functions).

Yet another cause of polydactyly is Ellis–van Creveld syndrome, which causes heart defects, babies born with teeth, abnormal fingernail development, a type of dwarfism, cleft palate and polydactyly.

The above three conditions are recessive, and these would be increased by inbreeding, however these diseases are so rare that the chance of two partners with these conditions inbreeding is infinitesimal.

So in conclusion - in general, polydactyly on its own is not caused by inbreeding!

Incidentally, my co-blogger Brenda lives in Coalville and we are yet to count the number of fingers on her hands! ;)


Careless said...

"As polydactyly is usually caused by a random mutation, only one of the parents would be affected and so inbreeding does not increase the likelihood of a child developing polydactyly."

This is obvious nonsense.

G Thunder said...

The last poster (Anyonymous) is correct, this is not a birth defect, it is a recessive genetic trait (linked to Haplogroup X) passed down from our Ancestors. Wisdom teeth/supernumerary teeth are also a common trait, as those people were much larger and thus the skeleton could accommodate more teeth and fingers/toes. Only the most isolated Aboriginal genetics are virtually devoid of these traits.