What causes "butt chin?"
(my students' wording, not mine)
Back when my Anatomy students studied the mandible, folks became curious about chin clefts (referred to as "butt chin" by the student who finally asked about them). A fine example of this may be viewed on one of my childhood heroes, the Muppet Show's own Statler:
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| Waldorf (left) and Statler (right, rockin' the chin cleft) |
There are several things going on with Statler's chin, but let's focus on the cleft running along the midline.
It's worth noting that chins do not exist within a smooth/clefted binary - there is instead a variety of chins that can exist along the spectrum between smooth and clefted. For an example of this spectrum, check out John McDonald's photographic survey of congressmans' chins here.
Conventional wisdom has told us that chin clefts were caused by a single gene. More recent information suggests, however, that this is either a polygenic trait (McDonald, 2011) or a trait caused by a single gene with variable penetrance (Starr, 2004). Let's translate that out of fancy-pants science-speak, shall we?
"... this is either a polygenic trait ..."
Many folks are familiar with how genes (the functional sections of DNA in each cell of the body) influence traits (the physical characteristics of the body). If you're not one of these folks, I recommend checking-out this resource and/or watching this tutorial before continuing. The simplest way to think of how genes lead to physical characteristics is with Mendelian traits, those that are caused by a single gene location. For a Mendelian trait (for example, albinism), a person will have just 2 copies of the responsible gene - one from the biological mother and one from the biological father. These 2 copies (alleles) of the gene will duke it out to determine the physical characteristic that manifests in the body. If a trait is a Mendelian trait, it generally follows particular patterns of inheritance and manifests in a binary fashion - in the case of albinism, folks are generally albino or not albino. If you've learned about physical characteristics that are caused by genes from a textbook or in a class, these were most likely Mendelian traits as they're the most straightforward examples to use for folks just learning about genetics.
Not all physical characteristics are Mendelian traits - that is, not all physical characteristics are caused by a single gene location and follow a predictable inheritance pattern. Traits that are caused by multiple genes are referred to as polygenic traits. Many of a person's most noticeable features - height, hair color, eye color - are examples of polygenic traits. What this means is, for example, a person's eye color is the result of input from several genes. Polygenic traits are also referred to as "continuous traits," or traits where there is a small grade of change between individuals. As opposed to Mendelian traits which are generally binary (individuals in a population either have a particular trait or they don't), polygenic/continuous traits exist along a continuum. Again, consider human height as an example. If height were a Mendelian trait we might expect to see 2 possible characteristics within the human population - "tall" (all the individuals of the same relatively tall height, say 6 feet) and "not tall" (all the individuals of the same relatively shorter height, say 5 feet). If we actually look at the human population this is not what we see - human height exist as a continuum where a person might be 5 feet 2 inches, or 6 feet 3 inches, or 4 feet 11 inches or anything in between (depending upon what that person's genes say). For further explanation/an example of this, go here.
Okay, so what does this have to do with chin clefts? Well, I stated earlier that current thinking is that chin clefting seems to be a continuous trait - this physical characteristic doesn't appear to be one that individuals in the population either have (to an equal degree) or don't have at all, but rather one that exists on a continuum from "no cleft" to "very clefted." This may point to it being a polygenic trait.
Another way to have a physical characteristic that is continuous is variable penetrance.
"... or a trait caused by a single gene with variable penetrance."
In genetics-speak, penetrance refers to the proportion of individuals carrying a particular copy/allele of a gene who actually display the physical characteristics that it causes. If a gene is said to have "complete penetrance" that means that whenever a particular allele of that gene is present, the same physical characteristics are present throughout the population (so any individual with that allele will have the characteristic that goes with it). If a gene is said to have "incomplete penetrance" that means that individuals who carry a particular allele of a gene don't always display the physical characteristic that it causes, or display them to the same degree. This would allow for a trait that is controlled by only 1 gene location to also be continuous. And again, since current information suggest that chin clefting is a continuous trait, this may be what is causing the continuum of chins that we see in the world.
The Upshot
Overall, the answer to what causes chin cleft seems to solidly be, "We don't know!" but current thinking points to it not being a Mendelian trait.
"... this is either a polygenic trait ..."
Many folks are familiar with how genes (the functional sections of DNA in each cell of the body) influence traits (the physical characteristics of the body). If you're not one of these folks, I recommend checking-out this resource and/or watching this tutorial before continuing. The simplest way to think of how genes lead to physical characteristics is with Mendelian traits, those that are caused by a single gene location. For a Mendelian trait (for example, albinism), a person will have just 2 copies of the responsible gene - one from the biological mother and one from the biological father. These 2 copies (alleles) of the gene will duke it out to determine the physical characteristic that manifests in the body. If a trait is a Mendelian trait, it generally follows particular patterns of inheritance and manifests in a binary fashion - in the case of albinism, folks are generally albino or not albino. If you've learned about physical characteristics that are caused by genes from a textbook or in a class, these were most likely Mendelian traits as they're the most straightforward examples to use for folks just learning about genetics.
Not all physical characteristics are Mendelian traits - that is, not all physical characteristics are caused by a single gene location and follow a predictable inheritance pattern. Traits that are caused by multiple genes are referred to as polygenic traits. Many of a person's most noticeable features - height, hair color, eye color - are examples of polygenic traits. What this means is, for example, a person's eye color is the result of input from several genes. Polygenic traits are also referred to as "continuous traits," or traits where there is a small grade of change between individuals. As opposed to Mendelian traits which are generally binary (individuals in a population either have a particular trait or they don't), polygenic/continuous traits exist along a continuum. Again, consider human height as an example. If height were a Mendelian trait we might expect to see 2 possible characteristics within the human population - "tall" (all the individuals of the same relatively tall height, say 6 feet) and "not tall" (all the individuals of the same relatively shorter height, say 5 feet). If we actually look at the human population this is not what we see - human height exist as a continuum where a person might be 5 feet 2 inches, or 6 feet 3 inches, or 4 feet 11 inches or anything in between (depending upon what that person's genes say). For further explanation/an example of this, go here.
Okay, so what does this have to do with chin clefts? Well, I stated earlier that current thinking is that chin clefting seems to be a continuous trait - this physical characteristic doesn't appear to be one that individuals in the population either have (to an equal degree) or don't have at all, but rather one that exists on a continuum from "no cleft" to "very clefted." This may point to it being a polygenic trait.
Another way to have a physical characteristic that is continuous is variable penetrance.
"... or a trait caused by a single gene with variable penetrance."
In genetics-speak, penetrance refers to the proportion of individuals carrying a particular copy/allele of a gene who actually display the physical characteristics that it causes. If a gene is said to have "complete penetrance" that means that whenever a particular allele of that gene is present, the same physical characteristics are present throughout the population (so any individual with that allele will have the characteristic that goes with it). If a gene is said to have "incomplete penetrance" that means that individuals who carry a particular allele of a gene don't always display the physical characteristic that it causes, or display them to the same degree. This would allow for a trait that is controlled by only 1 gene location to also be continuous. And again, since current information suggest that chin clefting is a continuous trait, this may be what is causing the continuum of chins that we see in the world.
The Upshot
Overall, the answer to what causes chin cleft seems to solidly be, "We don't know!" but current thinking points to it not being a Mendelian trait.
References
Beckman, L., Book, J.A., Lander, E. (1960) An evaluation of some anthropological traits used in paternity tests. Hereditas. 46(3-4): 543-569.DOI: 10.1111/j.1601-5223.1960.tb03100.x
McDonald, J. (2011) "Cleft chin: The myth." Myths of Human Genetics. University of Delaware. Retrieved on 14 August 2013 from
Gray, H. (1918) "5b. 8. The Mandible (Lower Jaw)" in Anatomy of the Human Body. Retrieved on 23 February 2014 from http://www.bartleby.com/107/44.html
McDonald, J. (2011) "Cleft chin: The myth." Myths of Human Genetics. University of Delaware. Retrieved on 14 August 2013 from
http://udel.edu/~mcdonald/mythcleftchin.html
"Penetrance" (2014) Genetics Home Reference. Retrieved on 23 February 2014 from http://ghr.nlm.nih.gov/glossary=penetrance
"Polygenic" (2014) Genetics Home Reference. Retrieved on 11 January 2014 from http://ghr.nlm.nih.gov/glossary=polygenic
Starr, B. (2004) "Ask a Geneticist." Understanding Genetics. TheTech. Retrieved on 11 January 2014 from http://genetics.thetech.org/ask/ask47.
"Penetrance" (2014) Genetics Home Reference. Retrieved on 23 February 2014 from http://ghr.nlm.nih.gov/glossary=penetrance
"Polygenic" (2014) Genetics Home Reference. Retrieved on 11 January 2014 from http://ghr.nlm.nih.gov/glossary=polygenic
Starr, B. (2004) "Ask a Geneticist." Understanding Genetics. TheTech. Retrieved on 11 January 2014 from http://genetics.thetech.org/ask/ask47.
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