Chin Cleft

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:

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.

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

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.

Why?

Why does <insert name of body part here> look like that?

or

Why does <insert name of body part here> work like that?

These tend to come up when students learn about some feature of the human body that doesn't seem very sensical to them; favorite examples include the electron transport chain in aerobic respiration and urine production in the nephron.  When students ask these questions, my answer generally looks something like this:

Neil deGrasse Tyson said it best

... because, really, I don't know exactly why things look/work the way they do.  In these situations, I turn to evolutionary biology.

If folks think of the human body as a structure that was designed from the ground up, with goals in mind, features may indeed seem pretty nonsensical.  However, evidence suggests this was not a ground-up, goal-directed design process at all.  Evolution is more a "repurposing" - taking a feature that already exists and modifying it to perform another function.

Repurposing.

Even that is a not-entirely-accurate, wildly oversimplified way of explaining evolution, as those modifications are thought to be randomly-occurring.  In the words of Lynn Sagan in 1967, "... evolution is opportunistic and not foresighted."  What is already present may, with a few random mutations, become modified such that it performs a new function.  And that new function may increase the organism's likelihood of surviving and reproducing.  And that feature may not be the most elegant or streamlined way of performing that function, but if it works, it doesn't have to be pretty.  In the end, this process may produce some systems that resemble Rube Goldberg machines.

As I mentioned, my students often find the mechanics of the electron transport chain (ETC) to be crazy-making.  The part that confounds many is what they perceive as design flaws - "Why the heck would anyone set it up that way???"  And my answer is, "They probably wouldn't."  If we think about building a system with the goal of the ETC in mind (ultimately making a boatload of ATP), the current system looks positively nuts.  If, however, we think about the ETC as a repurposing of already-existing elements in a way that ultimately permitted this function, it may come off a bit different.

Scientific American published an article that speaks to this point of design vs. evolution in 2003.  It discusses how humans might look and function differently were we designed from the ground up.

References

Alberts, B., et. al. (2002) "The Evolution of Electron-Transport Chains." In Molecular Biology of the Cell, 4th edition. New York: Garland Science. Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK26849/ 

Sagan, L. (1967) On the origin of mitosing cells. J Theoret Biol. 14: 225-274. http://www.gps.caltech.edu/classes/ge246/endosymbiotictheory_marguli.pdf

Dimples

What causes dimples?

A not-uncommon occurrence, when learning anatomy, is to learn something new about a particular part of the body and then wonder how that applies to what you observe on the bodies around you (e.g. your classmates, your family members).  My students have been studying the neck and facial muscles for the past few weeks so it's probably not surprising that someone asked about dimples.

Mario Lopez, known forever to my generation as AC Slater from Saved by the Bell
Also, dimples.

Dimples, shown in the photo above, are a (usually) naturally-occurring indentation in the flesh on a part of the body.  Folks are often most familiar with the facial variety.  I have been fantastically unable to find information detailing the proportion of the population that has them, or I'd throw you that figure here.  

According to the few peer-reviewed papers I was able to find in the medical literature (I was stoked that there were any), there is a correlation between dimples and an anatomical variation in particular facial muscles -- specifically, in the zygomaticus major muscles.

Gray's Anatomy

On each side of the face, the zygomaticus major muscle originates at the zygomatic bone of the face and inserts, effectively, at the corner of the mouth.  When it contracts, it pulls the corner of the mouth in a lateral and superior direction (as in a smile).  Most humans have a single zygomaticus major muscle on each side of the face (as shown in the image above).  Some studies of cadaveric tissue point to an anatomical correlation between having dimples and possessing a bifid (split in 2 parts) zygomaticus major muscle on each side of the face.  This muscle has the same origin and action as the single zygomaticus major, but inserts at 2 points on each side (one above the corner of the mouth, one below the corner of the mouth).  My guess (although I have not found it directly stated) is that the dimple would occur in the space between these 2 parts of the split zygomaticus major.  Some studies suggest that folks with dimples also have connections between the hypodermis and dermis in this region (referred to as "dermal anchoring") which may contribute to the indentation.  Dimple creation surgery seems to mimic such anchoring.

In short, the research points to a connection between dimples and an anatomical variation in one set of facial muscles, possibly accompanied by a variation in the attachment of skin to the underlying tissue.

References

Gassner, H.G., Rafii, A., Young, A., Murakami, C., Moe, K.S., Larrabee, W.F. (2008) Surgical anatomy of the face: implications for modern face-lift techniques. Arch Facial Plast Surg. 10(1): 9-19. doi: 10.1001/archfacial.2007.16.

Pessa, J.E., Zadoo, V.P., Garza, P.A., Adrian, E.K., Dewitt, A.I., Garza, J.R. (1998) Double or bifid zygomaticus major muscle: Anatomy, incidence, and clinical correlation. Clin Anat. 11(5):310-3.  DOI: 10.1002/(SICI)1098-2353(1998)11:5<310::AID-CA3>3.0.CO;2-T

Lordosis

What are some medical causes for sway back (lordosis)?

Lordosis, also known as sway back, is an exaggeration of the lumbar curvature of the spine.  Folks are often most familiar with this in reference to the posture of pregnant females.

First things first - let's start with what is anatomically normal.

An anatomically normal human vertebral column has 4 curvatures that can be observed from the lateral view.

The curvatures of the thoracic and sacral regions of the spine are present at birth while those of the cervical and lumbar regions develop postnatally, during the first year of life.

A person who has an exaggerated or excessive lumbar curvature would be said to have hyperlordosis (a.k.a. sway back, saddle back).  Some resources shorthand this to just "lordosis," although that technically refers to the normally-occuring curvature in this area.

There are several reasons a person may experience lordosis.  As I stated earlier, folks who are aware of this may know it best from the posture of pregnant females.  If a person has weight added to the anterior abdomen (as in the case of pregnancy or increased visceral fat in the abdomen), that person may develop lordosis as a result, to stabilize one's center of mass (as depicted in the image below).

Whitcome, et al. (2007) Nature

Other causes noted for lordosis (particularly in children) are achondroplasia and spondylolisthesis, neither of which involves addition of weight to the anterior abdomen and both of which affect the skeleton.  Achondroplasia is a bone growth disorder which causes the most common form of dwarfism, so while folks with this condition may experience lordosis, it likely wouldn't be considered the major symptom of this condition.  Spondylolisthesis is a condition in which a vertebra in the vertebral column slips out of position – this most often happens to L5 - and would then cause a change in the lumbar curvature resulting in lordosis.  There are other conditions a person may have (like forms of muscular dystrophy) that could result in lordosis, but that person would have a lot of other symptoms as well.  

References

"Achondroplasia" (2011) PubMed Health. Retrieved 8 February 2013 from http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002544/

"Lordosis" (2012) MedlinePlus. Retrieved 16 January 2013 from

http://www.nlm.nih.gov/medlineplus/ency/article/003278.htm

"Spondylolisthesis" (2012) MedlinePlus. Retrieved 16 January 2013 from

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002240/

Whitcome, K.K., Shapiro, L.J., Lieberman, D.E. (2007) Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature. 450: 1075-1078. doi: 10.1038/nature06342.