Cleft Chins and Bearded Ladies: The Genetics of Human Characteristics
What Causes Widow’s Peak Hair?
Widow’s peak is a V-shape in the middle of the hairline and it is a dominantly inherited trait. There are two genes for every trait; you inherit one from your mother and one from your father. Some genes are dominant, and this means that you only need to inherit one version of the gene pair to express the trait. And so if you inherit the dominant form of the widow’s peak gene as opposed to the recessive straight hairline gene, then you will have a widow’s peak hairline.
A cleft chin is another trait caused by a dominant gene, and it affects the way the jaw bones fuse together in fetal development - the lower jawbone doesn’t completely fuse. However, if a child inherits a dominant form of the gene it does not necessarily follow that a cleft chin will form. That’s the thing with the laws and rules of inheritance; like many other rules in life, they can either be broken or there are exceptions. The exception here is something called variable penetrance, which basically means that environmental cues and factors can affect whether a gene is expressed or not.
In terms of cleft chin dominance, a child may have the dominant gene, but not a cleft chin. This could be down to some intra or extracellular signals in fetal development that caused the jaw bones to fuse properly.
Tongue Rolling Genetics
The ability to roll your tongue is down to a combination of genetics and the environment. There is a dominant gene involved, but it’s not the only factor. There have been studies of identical twins (who share identical DNA) who don’t share an ability to tongue roll. One can, and the other can’t. It has been postulated that there are other factors at play that affect whether the dominant gene is expressed or not.
Excessive Hair Growth
Excessive hair growth, such as that experienced by so-called ‘bearded ladies’ has long been thought of as a genetic disorder. However, for many years the suspect genes were a mystery. But in May 2009, a team of researchers managed to identify four to eight genes that contribute to Congenital generalized hypertrichosis (CGH), the umbrella term for a range of excessive hair growth conditions.
Those genes were found by genetic analysis of three Chinese families with excessive hair growth - and the suspect genes are located on chromosome 17. The molecular mechanisms by which these genes cause excessive hair growth have still to be worked out (May 2009).