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The gene that, when present in people of Caucasian descent, results in curly hair
It has been known for a very long time that Caucasians who have curly hair have a dominant trait, while those who have straight hair have a recessive trait. This information has been gleaned from studies conducted over the course of many decades. This necessitates two distinct occurrences:1) A person will have curly hair if they carry one allele for curly hair and another allele for straight hair; 2) Having curly hair is a simple trait that is most likely determined by a single gene. 1) A person will have curly hair if they carry one allele for curly hair and another allele for straight hair. On the other hand, researchers have not yet isolated a single gene that is solely responsible for the curly (or straight) hair trait that is found exclusively in Caucasians.
A single nucleotide polymorphism (SNP) called rs11803731 was found in the TCHH gene as the result of a recent genome-wide association scan. This single nucleotide polymorphism (SNP) accounts for approximately 6% of the variation in the degree to which one's hair curls. The TCHH gene is responsible for the production of the protein known as trichohyalin. Trichohyalin has been shown to play a role in the process of cross-linking the keratin filaments that are present in hair, which is consistent with the fact that it is expressed at high levels in hair follicles, where it is also known to be present in high quantities. There is evidence to suggest that the A-allele, which is the ancestral form of this SNP, can be found in populations spanning the entire globe. At some point in the course of human evolution, a mutation took place, which led to the appearance of the T-allele (in Figure 1A, this allele is referred to as the derived allele). When the T-allele is present in the TCHH gene at position 790, an amino acid is changed from leucine to methionine. This change occurs because leucine is a precursor to methionine. It is not entirely clear what purpose this modification will serve. On the other hand, among Caucasians, those who carry the T-allele have approximately a 70 percent greater chance of having straight hair compared to those who do not carry the T-allele, who have approximately a 50 percent lesser chance of having straight hair. This leads us to believe that hair with waves or curls is the ancestral form, while hair that is straight developed much later in the history of humans.
The genes that are responsible for the thick, straight hair found in Asians
The majority of people whose ancestors originated in East Asia have thick hair that is styled in a straight fashion. This occurs at the same time as a single nucleotide polymorphism (SNP) in the EDAR gene, which is involved in the process of hair follicle formation. It is recommended that the A-allele be used because it is the ancestral form of this SNP. The recently discovered allele known as the G-allele is responsible for the development of thick, straight hair. There are certain regions of Asia (see Fig. 1B) in which the G-allele is so prevalent that practically everyone has a copy of it. Individuals who have the GG genotype at this SNP have thicker hair than those who have the AA genotype because of a change to a single amino acid in the protein. This change occurred as a result of this SNP. The presence of the SNP is responsible for bringing about this alteration. People with the AG genotype have hair that is slightly thinner than that of people with the GG genotype, but it is still relatively thick in comparison to Europeans and Africans (likely AA). People with the GG genotype have hair that is slightly thicker than that of people with the AG genotype.
Is it possible that having straight hair could give you a selection edge over those who have curly hair?
It would appear that people only relatively recently learned how to grow their hair straight. It has been linked to the newly uncovered alleles in the TCHH gene, in addition to the EDAR gene. The fact that these alleles related to straight hair are distributed in the manner shown in the figure lends credence to the theory that our human ancestors had curly hair; the development of the straight hair found in East Asians and Caucasians most likely occurred separately. Figure 1. Distribution of alleles related to straight hair.
To be more specific, what are the advantages of having straight hair? It must provide some kind of biological benefit to the hair's condition in order to be considered effective. People who have oily hair typically have straight hair, and it has been discovered that people who have oily hair and curly hair are a very rare combination. It has also been found that people who have both types of hair are extremely unusual. One school of thought maintains that the sebaceous glands produce oils that are secreted into the hair shaft, and that these oils can travel more easily down the hair shaft of straight hair.
The G-allele of the SNP rs3827760 in the EDAR gene, which is responsible for the East Asian hair type and emerged sometime within the past 65,000 years, according to recent research. This happened during the time that early humans were migrating out of Africa and into Europe, and then into Asia. It's possible that people who lived during the Ice Age benefited from having thicker hair as a form of insulation against the cold. This theory is supported by the fact that their hair was significantly longer. Additionally, the EDAR gene contributes to the proper operation of the glands found in the skin. Increased EDAR activity can lead to a number of different side effects, including enlarged eyelid glands (meibomian gland) and enlarged skin glands (sebaceous glands found in abundance on the face and scalp), as the findings of a study have shown. These improved gland functions could have led to increased skin lubrication and protection, as well as a reduction in evaporation; both of these features could have been positively selected during the cold and dry Ice Age.
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