Interaction between genetic, environmental and social forces results in the patterns of skin colour

Skin colour variation in Indians is determined not just by the environment and genetics but by sexual selection, too. A complex interaction between physical and social forces is responsible for patterns of skin colour seen in males and females in India, says a study by CCMB researchers who collaborated with an international team.

The researchers looked at how skin colour varies between 10 different socio-cultural populations varied within and between the populations in Andhra Pradesh, Tamil Nadu, Uttar Pradesh and Bihar. They also looked at variation in skin colour between males and females within and between populations. Then they studied the influence of ultraviolet radiation on skin colour and finally looked at the variations with respect to genetic data.

“Our study showed that social factors along with genetics played a strong role in shaping skin colour diversity across India,” says Dr. Kumarasamy Thangaraj from the Centre for Cellular and Molecular Biology (CSIR-CCMB), Hyderabad and a coauthor of a paper published recently in the American Journal of Human Biology.

Greater pigmentation and hence darker skin helps protect the skin from harmful UV rays near the equator while less pigmentation leading to lighter skin colour promotes season UV ray-induced vitamin D production in people living in higher latitudes. Women generally tend to have lighter skin than men highlighting the importance of cutaneous vitamin D production for enhanced vitamin D absorption during pregnancy and breast feeding.

For the study, the researchers compared the skin colour data of people living in Hyderabad and belonging to five different castes, three castes in Tamil Nadu, and from Brahmins living in Uttar Pradesh and scheduled caste living in Bihar.

Melanin index

The melanin index of people samples in Andhra Pradesh showed wide variation — 33.4 to 53. Three agricultural castes (Kapu, Naidu and Reddy) in the State had similar skin colour while Brahmins had far lighter colour and merchant caste (Vysya) had darker skin. In Tamil Nadu, Brahmins and Saurashtrians had lighter skin colour than pastoralist Yadava caste. Brahmins in Uttar Pradesh had fairer skin than scheduled caste in Bihar, and their melanin index range was nearly similar to their counterparts living in Andhra Pradesh. The melanin index range of scheduled caste in Bihar varied widely — about 46 to 79.

“Clear differences in skin colour in men and women were seen,” says Dr. Thangaraj. Males belonging to the three agricultural castes in Andhra Pradesh showed darker skin than women. Even among Brahmins in the State, women had a lighter colour than men and there is greater difference in skin colour between the sexes. Though the merchant caste (Vysya) had darker skin than the other four, they showed the least difference in skin colour between women and men. The same differences and similarities were seen in the case of Brahmins in Uttar Pradesh and scheduled caste in Bihar.

“We need to undertake a more detailed study by increasing the sample size, analysing few more genetic loci and including specific micro epidemiological factors that might be influencing skin colour for better understanding,” says Dr. Anushuman Mishra on less skin colour difference between women and men among Vysya population. Dr. Mishra is from CCMB and coauthor of the paper.

The environment apparently plays a smaller role (16%) in determining skin colour in Indians, while social factors could explain 42% variation in skin colour. “This result is consistent with the observation that in India skin colour varies markedly even among populations living in the same geographic location,” they write. And the difference in skin colour in two north Indian populations that live close to each other and share important genetic history suggests that population-level variation have a role in skin colour.

Role of gene variant

In Europeans, the SLC24A5 gene variant rs1426654-A is usually associated with lighter skin colour. But in the case of the scheduled caste population in Bihar the gene variant was found in “unusually high frequency” despite the population having dark skin. Similarly, in the case of UP Brahmins, despite the frequency of this gene variant being high, it did not have a significant effect on melanin index variation within the population.

“Our study suggests that there could be other genetic variant(s) in scheduled caste population in Bihar that have the ability to override the skin lightening effect of the gene variant rs1426654-A,” says Dr. Thangaraj. “When we look at melanin index and the genetic variant together we find in addition to genetics, the social and environment factors also play a major role in determining the skin colour of a population.”

“In our earlier study in middle Gangetic Plain of India, we have demonstrated that genetic factor decides 6.4%, while social category has 32% influence on skin colour variation. In the later study too we found 42% skin colour variation is due to social factors, although other factors also play a role,” Dr. Mishra says.

The authors conclude that numerous migrations into India and admixture of populations might have provided sufficient room for novel genetic variants that determine skin colour to emerge and spread among people in India, thus overriding natural selection.

And the population-dependent sexual selection for lighter skin and endogamy practised in India has ensured that skin colour variation has been maintained between different populations.