ABO and Rh(D) Blood groups
This paper describes the genetic characteristics of the Garos of Asanang village, Meghalaya. Four genetic markers are used to study this population. Although, the present study was carried out with limited genetic markers, yet it was able to reveal that the Garo population is not in genetic equilibrium in respect of the ABO blood groups. Garos are significantly deviating with most of the Mongoloid populations of Northeast India in respect of ABO blood groups. However, the Rh(D) blood group, PTC taste sensitivity and colour-blindness seem to conform with the general prevailing trend among the Mongoloid populations of North-East India which is characteristic of having low frequency occurrence.
Biological evolution occurs, from generation after generation through the operation of various evolutionary forces like natural selection, mutation, genetic drift, etc. It is obvious that any given generation of human population is a representative of the previous generation. The genetic characteristics of individuals with greater reproductive success will be predominant in the next generation. According to the Darwinian concept of evolution, the decrease in gene frequency is because of the reduced fitness and this is known as selection. On the other hand, fitness refers to the reproductive success of an individual, or a given group of individuals in terms of the number of offspring they contributed to the next generation. The most reproductively fit individuals as such are those who are better adapted to their environmental conditions. In short the outcomes of evolutionary processes are adoption of a species to its environment. As a result, evolution is believed to be a process of change in human populations and it is known as an on-going process.
In view of the above circumstances, knowledge of genetics particularly of population genetics is of considerable importance in understanding the processes of on-going human evolution and variation. Besides, population genetics contributes to a great extent in “removing misunderstanding among various population groups” as it explains the facts and nature of population variation (Das, 1981).
Objective of the study:
To describe the genetic composition of the Garos of Asanang village with the help of four classical genetic markers viz., ABO and Rh (D) blood groups, PTC taste sensitivity and colour blindness.
To compare the findings of the present study with those reported on other Mongoloid populations of Assam and Meghalaya.
Materials and Methods:
ABO Blood Groups
Blood samples on 150 individuals of which 76 males and 74 females were collected from Asanang village of West Garo Hills District, Meghalaya. For determining the ABO and Rh blood groups, the standard slide technique suggested by Lawler and Lawler (1951) and Bhatia (1977) were followed for collection of blood samples in the present study.
Phenylthiocarbamide Taste Sensitivity (PTC)
The serial dilution method, suggested by Harris and Kalmus (1949) was followed to collect data on P.T.C. taste sensitivity. A total of 150 individuals were tested of which 76 males and 74 were females.
The Ishihara chart (1959) was used to collect data on Red-Green deficiency. Like in the case of other genetic markers, a sample 150 individuals out of which 76 males and 74 females were examined for Red-green colour deficiency. The chart was kept open and plates were held at a distance, approximately two and half feet from the subjects. The subjects were asked to read number of the plates numbering 1-25 within three seconds for each plate.
In case of illiterate subjects, they were asked to trace the snake like figure or X’ of the plates 26 to 38 by means of a brush supplied to each of the subjects. The test was made utilizing the instructions attached along with the Ishihara chart.
Land and People
Data for the present study was collected from Asanang village which is under Rongram Block of West Garo Hills District, Meghalaya. It is situated at a distance of 12 km east from Tura and 192 km from the state capital, Shillong.
The Garos are sturdy and slightly dark in complexion. They have round face, high and prominent cheek bones, obliquely set eye with a prominent nose. The womenfolk are a little shorter in stature and stocky in body build. They look almost like plains tribes of Assam and an ‘outsider’ would definitely find it hard to distinguish the two when together.
The Garos are matrilineal in descent. They belong to the Tibeto-Chinese family of Tibeto-Burman Sub-family of Bodo group. They are akin to most of the aboriginal tribes of the Assam Valley such as the Kacharis, Rabhas, Meches but belong to quite a distinct stock to that from which the Khasis originated.
The findings of data analysis on four genetic markers, namely, ABO blood groups, Rh (d) blood groups, PTC taste sensitivity and Colour-blindness are given below.
Table 1. Phenotypic and genotypic allele frequencies of ABO blood groups
|ALLELIC GENOTYPE FREQUENCY
Difference between sexes: =0.46, d.f.=3, p>0.05
Goodness of fit for Hardy-Weinberg equilibrium: =48.98, d.f.=1, p<0.05
It is seen from the above table that the percentage frequencies of A, B, AB and O in males are 26.31%, 34.21%, 26.31% and 13.15% respectively. In the females, these frequencies are 18.91%, 36.48%, 27.02% and 17.56%, respectively. These differences between sexes in respect of the phenotype distribution of the ABO blood group are statistically not significant (?2=0.46, d.f. = 3, p>0.05). Since ABO locus is an autosomal character, data on both males and females were pooled together to find out the allele frequencies of the ABO blood groups. Thus, combining the data of both the sexes, the percentage frequencies of A, B, AB and O are found to be 22.67%, 35.33%, 26.67% and 15.33% respectively.
Following the methods given by Bernstein (1930) and Balakrishnan (1988), the calculated gene frequencies of p, q and r are 0.2882, 0.3836 and 0.3915 respectively. Applying the test of goodness of fit for Hardy – Weinberg equilibrium, it is found that the difference between these allele frequencies in the present population are statistically significant (?2=48.98, df=1, p<0.05). Thus, it indicates that the present population is not in genetic equilibrium in terms of ABO locus, and does not follow the Hardy-Weinberg equilibrium principle (1908) which states that, in a large randomly mating population, if there is no change in the gene frequency due to mutation, selection and migration or admixture, the frequencies of genes types will remain constant from generation to generation.
Table 2. Phenotype and allele frequencies of Rh (D) blood group
The phenotype and allele frequencies of the Rh (D) blood groups are shown in Table 2. As far as the present study is concerned no individuals with Rh- negative blood group were detected. It may be worthwhile to mention here that Rh-negative gene is either absent or present in a very low frequency among the Mongoloid populations of North-East India (Bhattacharjee, 1968; Das, 1974). The present finding among the Garos of Asanang seems to confirm such an observation.
Table 3. Distribution of PTC taste sensitivity among the Garo Males and Females
|Threshold Solution Number (TSN)||Male|
Table 3 shows the data on taste sensitivity to PTC for both males and females. It is found that the mean threshold values are 3.03±0.25 and 3.25±0.27 for males and females respectively. Thus it indicates that the mean threshold value is higher in females than in males, although the differences between the two sexes are not statistically significant (t=0.5978, d.f. =148, p>0.05). Accordingly, the present data for both the sexes are pooled together for classifying the population into tasters and non-tasters. Moreover, the gene responsible for not being able to taste PTC salt is believed to be an autosomal one.
In order to classify the individuals into tasters, we have followed the method suggested by Harris and Kalmus (1949) in which the number of individuals being able to taste PTC salt solution was plotted against the serial dilution numbers of PTC solution which was presented in the figure below. Figure1. Graph showing the distribution of PTC tasters and non-tasters among the Garos of Asanang villagez
Figure 1 shows that the distribution of PTC tasters’ sensitivity in the present population follows a bimodal distribution in which the antimode falls on 8. Thus, the cutoff point of 8 was considered for classifying the individuals into tasters and non-tasters. Thus , individuals who had the threshold value above 8 or who perceived taste in a more diluted solution were considered as tasters while those with the threshold value of 8 and below or who perceived taste in a more concentrated solutions were regarded as non- tasters. The frequency of tasters and non-tasters is given in Table 4.
Table 4 .Frequency of PTC tasters and non-tasters
x²=0.5628, d.f.=1, p>0.05
From Table 4, it is observed that the frequency of non-tasters is higher in male than in female. However, the chi-square test indicates that the difference between males and females are statistically not significant ( x2=0.5628, d.f.=1, P>0.05). The gene allele frequencies for tasters (T) and non tasters (t) are found to be 0.0134 and 0.9866 respectively.
Table 5. Frequency distribution of Colour-Blindness
The percentage distribution of colour-blindness in the Garo population is given in Table 5. As far as the present study is concerned no individual with red-green deficiency were detected, which can be said that the red-green deficient gene which is an X-linked trait though absent in the studied samples, is occurring at a very low frequency in this population. This is in concordance with the rest of the other Mongoloid population groups of the North-East India, where the colour-blindness gene has been reported at very low frequencies.
In the present paper, we have described the data on four genetic markers among the Garos of Asanang village, Meghalaya. In the study population, the frequency of ‘B’ blood group is found highest (35.33%) followed by ‘AB’ (26.67%) and ‘A’ and (22.67%) respectively. Following the methods given by Bernstein (1930) and Balakrishnan (1988), the gene frequencies of p, q and r are 0.2882, 0.3836 and 0.3915 are calculated respectively. The test of goodness of fit indicates that the allele frequencies in the present population are statistically significant ( Thus, it indicates that the Garos are not in genetic equilibrium. The Rh-negative and colour-blindness genes are absent in this population. About 97.34% of the total subjects covered under the present study were non-tasters.
We have compared our findings with those published on other neighbouring Mongoloid populations of Meghalaya and Assam with a view to understanding the genetic affinity of the present population.
Table 6. Comparison of phenotypic frequencies of ABO blood groups
|Sl No.||Populations||Sample Size(N)||Phenotype Frequency||References|
|1||Bodo||402||113||141||45||103||Das, et al.,(1980)|
|2||Mikir||245||81||65||25||74||Das,et al., (1980)|
|3||Lalung||114||30||36||10||38||Das,et al., (1980)|
|10||Lyngam||120||47||34||15||24||Ahmed,et al., (1997)|
The comparative study of phenotype frequencies of ABO blood groups among the Garo and other populations of Meghalaya and Assam are given in Table 6. The above table shows that Garo is characterized by a high frequency of blood group ‘B’ and low frequency of blood group O compared to Bodo, Mikir, Rabha and Marngar and is closer to the Khasi population as reported by Das (1978).
In order to have a better understanding of the genetic relationship of the Garo with other compared populations, the Chi-square statistic was used to test the differences between them if any with respect to distribution of the ABO blood groups.
Table 7. Chi-square value of ABO blood group between Garo population of Asanang village and the other neighboring populations of Assam and Meghalaya
|Sl No.||Populations compared||Chi-Square|
|1||Garo vs Bodo||30.32*||3||P<0.001|
|2||Garo vs Mikir||95.55*||3||P<0.001|
|3||Garo vs Lalung||20.66*||3||P<0.001|
|4||Garo vs Rabha||27.31*||3||P<0.001|
|5||Garo vs Bhoi||84.46*||3||P<0.001|
|6||Garo vs Marngar||24.26*||3||P<0.001|
|7||Garo vs Khynriam||81.83*||3||P<0.001|
|8||Garo vs Pnar||96.43*||3||P<0.001|
|9||Garo vs War||95.69*||3||P<0.001|
|10||Garo vs Lyngam||36.24*||3||P<0.001|
*Significant at 1% level of probability
Table7 shows the Chi-square (?2) value between Garo and the other neighbouring Mongoloid populations of Assam and Meghalaya. It is observed that the difference between the present population and majority of the compared populations are statistically significant with respect to the frequencies of the ABO blood groups which means that the Garo population deviate significantly from all the populations in respect of ABO blood group.
Table 8. Comparison of frequency distribution of Rh(D) blood group
|Sl No.||Populations||Sample Size(N)||Rh-Negative||Rh-Positive||References|
|1||Bodo||402||1||401||Das,et al., (1980)|
|2||Mikir||134||2||132||Das,et al., (1980)|
|3||Lalung||114||1||113||DaDas,et al., (1980)|
|4||Rabha||126||1||125||Das,et al., (1980)|
|5||Kachari||131||-||131||Das,et al., (1980)|
|7||Chutiya||64||1||63||Das,et al., (1980)|
The comparative study of frequency distribution of Rh(D) blood group among the Garo and the other neighbouring populations is given in Table 8. No individuals with Rh-negative blood group were detected in the present study
Table 9. Frequency of PTC taste sensitivity
|Sl No.||Populations||Sample Size(N)||Taster||Non-Taster||References|
|2||Lalung||114||81||33||Das,et al., (1980)|
|8||Lyngam||120||84||36||Ahmed,et al., (1997)|
The frequency distribution of tasters and non-tasters of PTC taste sensitivity among the Garos and some neighbouring Mongoloid populations is given in Table 9. Comparative study indicates that the Garos of Asanang village is characterized by very low frequency of tasters than to rest of the populations compared. An important observation noted here is that the chewing habit of areca nut, betel leaf and lime along with tobacco ingredients is prevalent in the food habit of the Garos of Asanang village, their influence seems to have interference on the perception of taste to this particular gene.
Table 10. Chi-square value of PTC tastes sensitivity
|Sl No.||Populations compared||Chi-Square|
|1||Garo vs Mikir||249.05*||1||P<0.001|
|2||Garo vs Lalung||140*||1||P<0.001|
|3||Garo vs Bhoi||200*||1||P<0.001|
|4||Garo vs Marngar||152.17*||1||P<0.001|
|5||Garo vs Khynriam||266.98*||1||P<0.001|
|6||Garo vs Pnar||180.32*||1||P<0.001|
|7||Garo vs War||267.6*||1||P<0.001|
|8||Garo vs Lyngam||137.54*||1||P<0.001|
*significant at 1% level of probability
Table 10 shows the difference between Garos and other neighbouring populations in respect of the Phenylthiocarbamide taste-blindness. The Garo population deviates significantly from all other populations taken into consideration for comparison in respect of the PTC tasting ability.
Table11. Defective Red-Green colour vision
|Sl No.||Populations||Sample Size(N)||Normal||Red-Green|
|5||Bodo-Kachari||201||186||15||Mukherjee and Guha (1990)|
The frequency distribution of colour blindness among the Garos and other neighbouring mongoloid populations are shown in Table 10. Comparative study indicates that the Garo is characterized by zero frequency of Red-Green deficiency as compared to the other populations. Very low frequency of this trait is observed among the Hajong, Khynriam, Bodo-Kachari, and is completely absence among Pnar, Bhoi and Mikir.
Although, the present study was carried out with limited genetic markers, yet it was able to found out that Garo population is not in genetic equilibrium in respect of the ABO blood groups. Since, the Hardy-Weinberg Law, states that genetic equilibrium occurs when the populations is large and mating occurs at random, along with the absence of other evolutionary forces such as natural selection, mutation, genetic drift, migration and others. Therefore, with the present set of data is difficult to give a proper explanation whether the evolutionary forces such as mutation and natural selection are not playing significant roles in regulating the gene frequencies in the present populations.
When the Garos were compared with the other mongoloid populations in respect of the four genetic markers, it showed that they deviate significantly from the compared populations in respect of ABO blood group. This indicates that there is likely a low admixture rate or intermarriage with other populations which might have brought about genetic differences between them.
The findings on the Rh(D) blood group and colour-blindness seem to conform with the general prevailing trend among the Mongoloid population of North-East India which is characteristic of having low frequency occurrence of the Rh-ve blood group and also less frequency of the Red-Green deficient gene for colour-blindness. Thus, the present finding on genetic markers among the Garo population of Asanang needs to be further studied with the larger sample size along with the inclusion of the additional genetic markers which could provide more insight into the overall genetic composition of the population.
- Bhatia, H.M, 1977.Procedures in blood banking and Immunohaematology. Bombay: Blood Group Reference Centre.
- Chumikam, B.T. 2002. A genetic study of the Marngar population of Ri-Bhoi District, Meghalaya.Unpublished M.Sc. Dissertation, Dept. Anthropology, NEHU, Shillong.
- Das, B.M. 1981.Microevolution (Gen.Ed) L.P. Vidhyarti.RanchiAnthro.Series 4.
- Harris, H and Kalmus, K. 1949. The Measurement of Taste Sensitivity to PTC. Ann Eugene, 15:24-31.
- Ishihara, S. 1959. Tests for colour Blindness.Japan:KanncharaShuppan co. Ltd. Tokyo.
- Lawler, S.D. and L.J.Lawler, 1951. Human Blood Groups and Inheritance. London: William Heinemann Ltd