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R H Ward

Publications and source records attributed to R H Ward.

At least 127 records · Page 7Linked to original sources
PubMed1973

Admixture studies and the detection of selection.

Three methods can be used to search for evidence of natural selection from admixture studies. These include classification of the admixture estimates into two groups; calculation of the rank correlation between estimates from more than one population; and the testing of admixture estimates for homogeneity. The use of these methods is discussed with special reference to black-white admixture in the United States. Using revised estimates of African gene frequencies, derived from a consideration of the geographical origin of the slaves, we calculated admixture estimates and their variances for five U.S. black populations; Claxton (Georgia); Sapelo Island (Georgia); James Island (South Carolina); Charleston (South Carolina); Oakland (California). Two out of the five populations yielded heterogeneous admixture estimates but all other tests were non-significant. The data provide little evidence for the action of selection. The few, inconsistent significant results are more indicative of the action of random drift or biased gene frequency estimates than natural selection, and in general these effects cannot be differentiated. It seems doubtful that admixture studies can ever provide unequivocal evidence for the action of natural selection in human populations. In the search for natural selection, perhaps admixture studies should only be used as a preliminary screening device.

Black People↗
PubMed1972

The genetic structure of a tribal population, the Yanomama Indians. VI. Analysis by F-statistics (including a comparison with the Makiritare and Xavante).

The infra-structure of three relatively undisturbed tribes of American Indians (Yanomama, Makiritare, Xavante) has been investigated by means of the F-statistics of Wright, using 8, 9 and 6 codominant systems respectively. The data for the first two mentioned tribes are much more extensive (37 and 7 villages) than for the third (3 villages), and much of the argument is based on the first two. An additive model partitioning F(IS) into an average effect (F(A)) and deviations due to deme size, systems effects, village effects, and random error has been employed. The Cannings-Edwards formulation suggests that the small size of the demes alone would result in an F(IS) of -0.008 for the Yanomama and -0.007 for the Makiritare. There is no evidence for significant village or systems effects. Despite considerable scatter, F(A) values are not significantly heterogeneous and tend to be negative (-0.012 to -0.023). On the basis of a computer simulation model, it appears that there is an excess of consanguineous marriage over random expectation, i.e. the negative F(A) values are probably not due to avoidance of close inbreeding in a subdivided population in which demes are small. Aspects of population structure which could contribute to negative F(A) values are identified. These include unequal gene frequencies in the sexes and occasional marked differential fertility. It is at this point unnecessary to introduce overdominance as a cause of the negative F(A) values, since a computer simulation program which does not incorporate selection satisfactorily reproduces the observed F(IS) values. If population breeding structure alone can result in negative F(IS) values, then this may constitute a mechanism for retarding random fixation.-Mean F(ST) values are 0.063 for the Yanomama and 0.036 for the Makiritare. While truly comparable data are lacking, it seems likely these will be found to be relatively high values for human populations. F(IT) values have been calculated by both direct and indirect approaches. The direct approach yields a value of 0.045 for the Yanomama and -0.009 for the Makiritare; the respective indirect values are 0.085 and 0.017. The primary identifiable reason for this difference between tribes is the greater genetic heterogeneity among Yanomama villages. The assumptions underlying the indirect approach to the calculation of F(IT) do not appear to be met in these populations.

Analysis of Variance↗
PubMed1970

Village and tribal genetic distances among American Indians, and the possible implications for human evolution.

Pair-wise genetic distances based on six genetic systems (Rh, MNSs, Kidd, Duffy, Diego, and haptoglobins) are presented for seven villages of Makiritare Indians, seven villages of Yanomama Indians, and 12 Indian tribes of Central and South America. It is shown that the mean genetic distance between Indian villages is 85-90 per cent of the distance between tribes. Since in the past, the initial event in the formation of a new tribe was probably the breaking away of one or several related villages from an established tribe, it is clear that this initial event could have profound consequences for subsequent tribal gene frequencies. By the criterion of changes in gene frequency in polymorphic genetic systems, the maximal rate of evolution in the American Indian has been approximately 100 times more rapid than the mean rate suggested by calculations based on amino acid substitutions in certain polypeptides of a variety of organisms. The above-mentioned findings could account in part for this 100-fold difference. Some other factors which might diminish the apparent difference between the results of these two types of calculations are mentioned.

Acculturation↗
PubMedDate not supplied

Is maternal alpha-fetoprotein screening still of value in a low-risk area for neural tube defects?

Estimation of maternal serum alpha-fetoprotein (AFP) was used as a screening method for the detection of neural tube defects (NTDs) in 6344 women over three years. Of 88 (1.4 per cent) who had one or more serum AFP levels equal to, or greater than, 2.5 multiples of the median (MoM) for the relevant gestational age, 43 (0.68 per cent) underwent amniocentesis. There were eight NTDs. Four of these were screened by serum AFP, and all cases of spina bifida had serum AFP levels greater than 3.0 MoM, including one small open defect which was not seen on ultrasound. The other four cases of NTD, which were not screened, were identified by ultrasound. Of 64 singleton pregnancies 32 (50 per cent) had serum AFP levels between 2.5 and 3.0 MoM, and low birthweight (less than or equal to 2500 g) occurred in 29 per cent. Because of improvements in ultrasound techniques and the apparent falling incidence of NTD, the role of serum AFP as the primary screening procedure should be regularly reviewed. Effective screening is dependent on mothers booking early.

Amniocentesis↗