The Genetics of Cleft Chin – What Causes Cleft Chin?

Cleft Chin of Pres. William McKinley (Image from Wikimedia Commons)What do you think Ben Affleck, John Travolta, Michael Douglas, Elijah Wood, Jessica Simpson, and Marc Anthony have in common? They simply have attractive cleft chins.  Do you have cleft chin or do you know somebody who has one? Do you want to know what causes cleft chin? Read this article to learn the genetics of cleft chins.

Overview of Cleft Chin Genetics

Cleft chin is a facial characteristic that looks like a dimple or an indentation to a person’s chin.  It is caused by the failure of the lower half of the jaw to fuse together during fetal development.  It is an inherited physical trait which is observed more prominently in men than in women. In the general population, the number of people with cleft chins is lower than those with the normal chin (it doesn’t mean that people with cleft chins are “abnormal”, they are in fact attractive). Like any of our facial characteristics (dimples, broad forehead, hook nose, etc.), genetics is what causes cleft chin.

Cleft chin is a dominant trait; it masks the expression of the gene for the normal chin. The “masked gene” is said to be the recessive gene and normal chin is the dominant trait. In genetics, a dominant trait is symbolized by a capital letter (C for cleft chin) while the recessive gene is symbolized by a lower case letter (c for normal chin).

In order for a child to have a cleft chin, he should at least inherit the gene for cleft chin from his parents. If the child inherits two cleft chin genes from his parents, he would develop cleft chin by being homozygous to the trait (CC). If he inherits only one cleft gene from his parents, he would still develop cleft chin by being heterozygous to the trait (Cc). If no cleft gene is inherited from the parents, the child will develop the normal chin.

If both parents are homozygous to cleft chin (CC x CC), there is a 100% chance that all their children would have cleft chins.  If they are both heterozygous to the trait (Cc x Cc), their children have 75% chance of developing cleft chins. If one of the parent is homozygous to the trait (CC) and the other parent is recessive (cc), the children’s chance of developing cleft chins is 100 %. If one of the parent is heterozygous to the trait (Cc) while the other parent is recessive (cc), there is 50% chance (Cc x cc) that each of their children will develop cleft chins.

Take note that the percentages above were computed using the Punnet square. Learn more about Punnet square here.

Genetic Factors that can Affect the Expression of Cleft Chin Trait

In theory, parents with cleft chins would have children with cleft chins since the trait is dominant. On the other hand, parents that do not have the cleft chins could not have children with the trait since they do not carry the genes for cleft chin. However, these instances are not always observed. It can happen that parents with cleft chins couldn’t produce children with the trait and parents with no cleft chins could produce children with the trait. These phenomena are due to variable penetrance or the presence of modifier genes.

Variable penetrance means that even a dominant trait like cleft chin cannot be expressed phenotypically. The environment can affect the expression of a gene. For example, the fetal environment has done something in the complete fusion of the lower jaws even though the fetus carries the genes for cleft chins from both of his parents.

Modifier genes are the genes that affect the phenotypic expression of other genes. It is possible that the genes that control the incomplete fusion of the chin are suppressed by a modifier gene. If this happens, even a fetus that carries his parent’s genes for cleft chin will not develop cleft chin.

Attractive?

People have different views or attitude on cleft chin. Some people consider it attractive while others consider it ugly. Do you consider cleft chin attractive? Share your answer in this poll now.


{ 1 comment… read it below or add one }

Donna@childrens author September 8, 2011 at 9:07 pm

To have or not to have a cleft chin seems to be the question. Oh dear, i am surprised that I read an article about this not so very impt. physical feature. With so many unfortunate physical differences, studying cleft chins seems a bit trite… sorry!