· Skin colour variation in Indians is determined not just by the environment and genetics but also by sexual selection. A complex interaction between physical and social forces is responsible for patterns of skin colour seen in males and females in India.
· 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.
· The researchers of Centre for Cellular and Molecular Biology ( CCMB ) 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 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.
· The melanin index of people samples in Andhra Pradesh (33.4 to 53) and scheduled castes in Bihar (46 to 79) showed wide variation.
· 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.
· 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.
· The environment apparently plays a smaller role (16%) in determining skin colour in Indians, while social factors could explain 42% variation in skin colour.
· 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.
· 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.
· That 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.
· An earlier study in middle Gangetic Plain of India, demonstrated that genetic factor decides 6.4%, while social category has 32% influence on skin colour variation. In the later study too it is found that 42% skin colour variation is due to social factors, although other factors also play a role.
· The authors conclude that numerous migrations into India and mixture 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.
Source : The Hindu