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It is often assumed that Afrikaners stem from a small number of Dutch immigrants. As a result they should be genetically homogeneous, show founder effects and be rather inbred. By disentangling my own South African pedigree, that is on average 12 generations deep, I try to quantify the genetic heritage of an Afrikaner. As much as 6% of my genes have been contributed by slaves from Africa, Madagascar and India, and a woman from China. This figure compares well to other genetic and genealogical estimates. Seventy three percent of my lineages coalesce into common founders, and I am related in excess of 10 times to 20 founder ancestors (30 times to Willem Schalk van der Merwe). Significant founder effects are thus possible. The overrepresentation of certain founder ancestors is in part explained by the fact that they had more children. This is remarkable given that they lived more than 300 years (or 12 generations) ago. DECONSTRUCT, a new program for pedigree analysis, identified 125 common ancestors in my pedigree. However, these common ancestors are so distant from myself, paths of between 16 and 25 steps in length, that my inbreeding coefficient is not unusually high (f ≈ 0.0019).
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‘After three centuries of evolution the population structure of the Afrikaners is still far from stable, and there does not appear to be much prospect of its ever attaining uniformity. … The numerous and often mutually contradictory genetic statements frequently made about them can consequently all be simultaneously true. The Afrikaner is a product of miscegenation, the last ‘pure European’, pathologically inbred and a manifestation of hybrid vigour, all at the same time.’ (Nurse et al. 1985)
Afrikaners are often considered a rather homogeneous, probably rather inbred, white population of Dutch ancestry. Yet, as the above quotation illustrates, there are uncertainties about the genetic composition of Afrikaners. Due to Afrikaners' high linkage disequilibrium, they are seen as a fruitful hunting ground for genes associated with disease (Hall et al. 2002). It is thus important that we have a clear appreciation of the Afrikaners' genetic heritage. In what follows I address the questions of racial admixture, nationalities, founder effects and inbreeding in the Afrikaner. I do so in a novel way: rather than taking a sample of modern Afrikaners and genotyping them, I start with one living Afrikaner and trace most of his South African ancestors. In this way I cast a net into his past and hope to get an impression of what the genetic heritage of a typical Afrikaner may be.
In lectures I often jokingly suggest that the Afrikaner population is a good example of inbreeding. The fact that Afrikaans children address any adults, related or not, as ‘oom’ and ‘tannie’, the Afrikaans for uncle and aunt, lends some credence to this suspicion. However, it was with mixed feelings that I, an Afrikaner, realized that the Afrikaner population has become a text book example of founder effects (Ridley, 2004). A founder effect refers to the phenomenon where the gene frequency of a new population is very different from that of its parent population. This happens when one or more of the immigrants have an unusual genotype and when the total number of immigrants are few. Although Ridley's suggestion that almost all Afrikaners stem from one ship-load of Dutch immigrants, who landed at the Cape of Good Hope in 1652, is most certainly incorrect he may well be correct about Afrikaners being a good example of founder effects. This simplistic one-ship-load suggestion is a common sentiment in many recent papers on Afrikaners. The unusually high incidence of a number of familial diseases among Afrikaners suggests that there has been a significant founder effect (Botha & Beighton, 1983a, b; Nurse et al. 1985). However, de Villiers & Pama (1966c) list just over 4000 founders who emigrated to South Africa between 1652 and 1806. Even with such a large number of emigrants significant founder effects are possible, when emigration occurred over a long time, so that earlier arrivals contributed disproportionately more to the population. For instance, a number of founder effects in Saguenay-Lac Saint Jean (Quebec, Canada; Heyer & Tremblay, 1995; 1997) can be explained by cultural transmission of fitness (Austerlitz & Heyer, 1998; Heyer et al. 2005). Data collected by Heese (1971) show how this is possible in the case of Afrikaners (Table 1). For the 150 years recorded in Table 1, it is clear that there was a steady stream of emigration rather than a single event. Also notice how, over the years, more and more emigrants got married to locally born individuals. This is a typical situation where early emigrants can contribute disproportionately to the gene pool, leading to founder effects.
Table 1. Number of emigrant couples married at the Cape, and a breakdown of the origin of the children these immigrants had, for five 30-year time windows.
|Number of couples||Both Parents emigrants (%)||NL||DE||FR||non-European||Small European contributors||Unknown||UK|
Several diseases with an unusually high frequency in Afrikaners have been suggested to be the result of such founder effects: porphyria variegate (Dean, 1963), Beukes familial hip dysplasia (Cilliers & Beighton, 1990), familial hypercholestrolemia type I (Jenkins et al. 1980), Huntington's chorea (Hayden et al. 1980), Fanconi anaemia (Rosendorff et al. 1987), pseudoxanthoma elasticum (Torrington & Viljoen 1991), progressive familial heart block type I (Torrington et al. 1986), lipoid proteinosis (Heyl, 1970) and sclerosteosis (Beighton et al. 1977). Several studies have aimed to identify the actual founder ancestors who brought these disease-causing alleles to South Africa. Curiously, the founder Willem Schalk van der Merwe and his wife Elsje Cloete have been credited with introducing at least four diseases to the Afrikaner: Huntington's chorea (Hayden et al. 1980), pseudoxanthoma elasticum (Torrington & Viljoen, 1991), lipoid proteinosis (Heyl, 1970) and schizophrenia (Karayiorgou et al. 2004).
A founder effect normally goes hand in hand with elevated levels of inbreeding. Nurse et al. (1985) suggest that there may well be medically important levels of inbreeding in the Afrikaner population, stemming not only from the small population but also from a tendency to marry local people. The degree of inbreeding is normally calculated as the inbreeding coefficient (Wright, 1922), f, which is the chance that two alleles taken from a locus of an individual are identical by descent. In humans from Western societies close kin unions (uncle-niece and first cousin) are uncommon (Bittles, 2001), and the inbreeding coefficient in Western populations tends to be very low, typically in the order of 0.001 (Bodmer & Cavalli-Sforza, 1976). In isolated populations of small size and originating from few founders, the inbreeding coefficient can be in excess of 0.01 (Mange, 1964; Bodmer & Cavalli-Sforza, 1976). In contrast, consanguineous unions are very common in west, central and south Asia, and north Africa (Bittles, 2001).
By comparing observed and expected heterozygosity, Lucassen (2005) found that the inbreeding coefficient of white South Africans is slightly negative (−0.001). Since this estimate includes not just Afrikaners, it may not be entirely reflective of the Afrikaner population. However, if there is a significant substructure in the white population, then a Wahlund effect may in fact push this empirical estimate of Lucassen higher than it really is. This suggests that the current mating pattern is panmixia. To my knowledge there has not been extensive calculation of inbreeding coefficients for Afrikaners based on pedigrees. However, founder effects can result in a positive pedigree inbreeding coefficient even if the population is panmictic (Jacquard, 1975).
Given that genealogists could show that as much as 7% of Afrikaner genetic heritage is not of European descent (Heese, 1971), I find it curious that a system such as apartheid worked in South Africa. Seven percent is not a trivial amount, and is equivalent to having slightly more than a great-great-grandparent who was non-European. Since most of this non-European genetic heritage came into the Afrikaner population via female slaves, one would expect that as much as 14% of Afrikaner mitochondrial DNA is not even European. This female bias influx stems from the fact that emigrants were predominantly male, resulting in a male biased sex ratio of adults (Gouws, 1981).
Similarly, genetic studies also give support for this mixed racial ancestry. Working with a number of blood group gene frequencies, Botha & Pritchard (1972) estimated that beween 6–7% admixture between western European and slaves from Africa and the East, and/or Khoikhoi, would be required to explain the allele frequencies. Nurse et al. (1985) listed a number of alleles typical to the Khoisan and Bantu-speaking peoples that are found in low frequencies in Afrikaners (ABO system: Abantu; glucose-6-phosphate dehydrogenase: GdA− and GdA; Rhesus: R°; Haemoglobin C).
Another argument concerning the heritage of the Afrikaner has concerned the relative contributions of Dutch, German and French immigrants. Although earlier authors suggested a mostly Dutch origin, more recent work has suggested almost equal contributions from these three groups (Heese, 1971). However, as Pama (de Villiers & Pama, 1966a) points out, the ever moving borderlines in the 1600s, and the regional distribution of customs and people in the 1600s make the current distinction somewhat artificial.
Heese's (1971) method needs to be explained. He recorded all the wedding dates, number of fertile children and origin of each immigrant. He divided the period from 1657 to 1837 into six 30 year periods. Since people who came to South Africa earlier contributed more to the nation, he multiplied each “blood unit” (fertile child) from the respective periods by 32, 16, 8, 4, 2 and 1. This approach clearly makes some mistakes, but given the numbers of people involved would probably give an answer fairly close to reality. This calculation, however, will work for Afrikaners as a whole, but for any individual it may vary largely from his estimate. This is because a more recent ancestor will contribute a larger proportion of a focal individual's DNA, whereas that recent ancestor contributes less to the population.
Recently, a number of studies on the human mating system and life history have made effective use of old church records (Helle et al. 2002, 2004; Voland & Beise, 2002; Cavalli-Sforza et al. 2004; Pettay et al. 2005). Afrikaners are in the unique position that their genealogies from the 1600s up to the early 1800s are very well constructed and recorded in reference books (de Villiers & Pama, 1966a, b, c; Heese & Lombard, 1986, 1989, 1992a, b; GISA, 1999, 2001, 2002a, b, 2003, 2004a, b, 2005, 2006). This makes the Afrikaner population ideal for studies of this nature.
I took advantage of these resources and recorded my own ancestral charts up until my ancestors immigrated to South Africa, and I tried to give some clarity to the questions raised above: founder effect, inbreeding coefficient, nationality composition and racial composition. These statistics will strictly only apply to my siblings and myself but it is of value to see how these calculations differ from those of Heese (1971). Deviations will reflect the influences of recent emigrants, but may also point to cultural inheritance of fitness (Heyer et al. 2005).
Very early on in this investigation, it became clear that certain founder ancestors seem to have contributed a disproportionate amount of DNA to me. Such an effect could simply be the result of chance drift, but as the population grew so fast (despite three smallpox epidemics it averaged 2.8% per year for the period 1735–1800 (Gouws, 1981)) this is unlikely. Two further alternative explanations are possible: it could result from fitness differences in these founders, as was found for the Saguenay population (Austerlitz & Heyer, 1998; Heyer et al. 2005), or it could stem from local mating groups that were established by specific founder ancestors, which was again illustrated for the Saguenay-Lac Saint Jean region (Lavoi et al. 2005). The data I accumulated allow me to test the fitness hypothesis. To do so sensibly, we need a brief digression to human life history theory.
One of the central tenets of life history theory is that there is a tradeoff between the quantity and quality of offspring (Lack, 1947; Smith & Fretwell, 1974). In humans, this tradeoff has been demonstrated inconsistently, with some studies supporting its existence (Strassmann & Gillespie, 2002; Hagen et al. 2006; Penn & Smith, 2007) and others not (Pennington & Harpending, 1988; Borgerhoff Mulder, 2000). Theoretically, for a specific amount of resources a mother will have an optimal number of offspring. If all mothers have similar resources we can expect an inverse U relationship between the fertility of mothers and their fitness, with mothers at the extremes producing too few or too many offspring. On the other hand, if mothers vary in the amount of resources that can be channelled to offspring, and each mother produces the optimal number of offspring, we expect a positive linear relationship between fertility and fitness. This suggests that the outcome depends on resource variation in different communities, and that without controlling for resources interpretation may be ambiguous.
In these studies fitness is measured as the number of children reaching the age of 5, or 10 years or the number of grandchildren. Here we have a unique opportunity to test this tradeoff, by comparing the fertility of founders with their genetic contribution to a specific Afrikaner living ±12 generations later.
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Most approaches to human population genetics start by sampling a population. Here I have taken the opposite approach, and used one individual living in the present to sample more than a thousand entangled lineages running back into the past. In this way I have tried to elucidate information both about the founder ancestors of the Afrikaners, as well as present day Afrikaners.
Even though my pedigree is 17% incomplete, the accumulation curves suggest that as few as 7 more founder ancestors remain to be discovered. This is due to the substantial ancestor loss to which I will return below. It does mean that what I have calculated here with regards to my composition is unlikely to change substantially as additional lineages are completed. The fact that my average lineage, including myself, is almost 12 generations long and that the average gene in me is now spending almost its 11th generation on this continent show that, although there are some founder ancestors that immigrated here more recently, most contributions stem from the early immigrants. One may thus expect a fairly close agreement between my composition and the average Afrikaner as calculated by Heese (1971).
Considering the nationalities of contributors I contain 12% more French genetic heritage than Heese's calculations (Table 3). This increase is balanced by an 8% loss from German, a 3% loss from other European, and a 1% loss from non-European genetic heritage. The fact that most non-European genetic heritage came into the Afrikaner population via German immigrants (Heese, 2005) probably explains the drop in non-European genetic heritage. However, my high relatedness to the more recent immigrant, Rosalyn van Macao, who contributed 25% of my non-European genetic heritage, ‘reconciled’ this mismatch between my data and that of Heese's (1971).
It is not clear if my higher estimate of French contribution is because of a systematic mistake in Heese's (1970) estimate, or if it is because of a quirkiness in my own ancestry. It seemed to be the case that when a lineage hit the French Huguenots it stayed in this group. It will be interesting to compare the degree of inbreeding of the early generations of Huguenots to the other early immigrants. In the light of the calculations of Heyer et al. (2005) there is an interesting possibility that the cultural inheritance of fitness may have led to a systematic bias in Afrikaners, since Huguenots tended to be more educated and trained than German emigrants who tended to be soldiers. We are currently investigating this hypothesis.
It is unclear how the 2% contribution by women known only as van die Kaap should be allocated. At least one of them, Maria Bastiaans van die Kaap, carried an M mtDNA haplotype, which suggests an Asian and possibly Indian ancestry (Maca-Meyer et al. 2001). Although more slaves came to the Cape from Madagascar than from Guinea, Guinea contributed almost 5 times more to my gene pool than Madagascar. This is through the fertile contributions of a woman known as Lijsbeth Sanders van die Kaap. From her social associations it is believed that she was from Guinea (Hattingh, 1980). Of the slaves, however, a large number of women from India contributed to my genes.
My estimate of 6% non-European genetic contribution is in close agreement with genealogical (Heese, 1971) and genetic (Botha & Pritchard, 1972) estimates. I am not aware that this estimate has been validated for any other Afrikaner individual, but it will be interesting if this can be confirmed for more Afrikaners. Presently, most white and black South Africans are equally incredulous at the prospect that Afrikaners have such a rich genetic heritage. Hopefully, with time all South Africans will celebrate the fact that Afrikaners are, and continue to be, a proudly south African concoction.
The unusually large linkage disequilibrium in Afrikaners (Hall et al. 2002) is thus explained by this heterogeneous starting population, as well as the relatively few generations (12) since the origins of the Afrikaner.
Of 1101 lineage tips only 299 are unique founder ancestors. According to Pearl's (1917) inbreeding estimator, that measures the fraction of lineages that have coalesced, 73% of my lineages coalesced within 12 generations. Pattison (2004) estimated that the same degree of coalescence would only have occurred between 1450 and 1600 for Britain, India, Japan, Europe and China. This suggests that the Afrikaner population is indeed small and suffers from a founder effect. However, Pattison (2004) assumed that the entire population of each of these areas mated at random, a very unlikely scenario, and this will certainly inflate these coalescence times.
With only 299 founder ancestors, and with some ancestors being so dominant in their contributions, it is very likely that founder effects could have played an important role. Two points need to be mentioned. First, if any of the people in Table 2 carried a disease allele it will now occur at a high frequency in Afrikaners. Second, Table 2 is a list of the usual suspects for introducing diseases, but it is not clear if this is because they were in fact fitter (see below) or whether they actually carried the diseases. Willem Schalk van der Merwe will probably be a common ancestor for any two Afrikaners and will thus always be a possible candidate. Further support for this cautionary message comes from the fact that my father and mother share 125 common ancestors. In other words, if they both carried the same familial disease, 125 people could have been the possible donor. One should thus be cautious to identify the actual founder individuals before complete pedigrees have been obtained.
In support of the fitness/carrier debate I can list the following examples where I am related to proposed carriers, often several times: first, Willem Schalk van der Merwe and/or his wife allegedly brought Huntington's chorea (Hayden et al. 1980), pseudoxanthoma elasticum (Torrington & Viljoen, 1991), lipoid proteinosis (Heyl, 1970) and schizophrenia (Karayiorgou et al. 2004) to South Africa, but I am related to him at least 30 times. Interestingly, Hayden (1980) argued that all the Huntington's chorea lines ran through Sophia van der Merwe, a daughter of Willem Schalk. Although I am related to Willem Schalk 30 times, this is through 4 of his 10 other children who left descendents in South Africa but not through Sophia. This suggest that this link may well be less likely than the regular van der Merwe link, and that Hayden et al. (1980) were correct to argue that this could have been due to a new mutation in Sophia.
Second, Ignasius Ferreira may have brought progressive familial heart block to South Africa (Torrington et al. 1986), but I am related to Ferreira twice. However, Ferreira's wife was a Terblanche who on her mother's side was a le Febre. To these surnames I am related 17 times. Again, it becomes pretty likely that anyone may be linked to these founder ancestors. Similarly to Huntington's chorea, all the lineages run to Ferreira via one of his sons, Thomas Ignatius, suggesting that this may have been a new mutation too (van der Merwe et al. 1994).
Third, Gerrit Jansz van Deventer or his wife Arriaenje Jacobs Adriaanse apparently carried porphyria variegata (Dean, 1963). To van Deventer and his wife I am related 7 times with a further one link to Arriaenje's sister, Willemyntjie Ariens de Wit.
Fourth, there are four possible families, van der Merwe, Burger, Visser and Smit, who have been suggested as the possible origin of psedoxanthoma elasticum (Torrington & Viljoen, 1991). I am, respectively, related to these families 30, 12, 17 and two times.
Given the amount of ancestor loss it comes as a bit of a surprise that my inbreeding coefficient is only 0.0019. It is about twice as high as estimates for other Western countries, but these estimates are normally based on ancestries that are only three generations deep. All my inbreeding comes from much deeper common ancestors (Figure 2d). Given the small magnitude of the inbreeding coefficients calculated for Western societies, a mistake of 0.0019 is not trivial. Even though the founding of the Cape population seems very unlike stable communities in Europe, many communities in the 1600s were very stagnant, and similar founder effects could be important. This low level of inbreeding given the high ancestor loss is probably explained by the rapid expansion of the population (Gouws, 1981; Halliburton, 2004).
If generations did not overlap, one would expect path lengths to show a peak at every second path length because each generation adds two parents. The fact that it does not (Figure 2d) is the result of generation overlap. This can also be seen from the fact that so many common ancestors were on paths of varying length, up to 5 steps in difference. Such generation overlaps can be expected for populations like these, where girls got married at a very young age and sometimes had more than ten children.
The fact that founder ancestors who had more children are more related to me, and related to me more times, vindicates our use of number of offspring as a measure of fitness, as originally proposed by Darwin (1859). One may have expected that the fitness values of the subsequent 12+ generations would have destroyed this signal. However, the cultural transmission of fitness (Heyer et al. 2005) would make this observation more likely. The inverse U relationship expected under a quantity-quality tradeoff was not found. This suggests that the quantity-quality tradeoff may not have existed in this population. The fact that the Afrikaner population grew almost 5 times faster per year than the average Scandinavian population at the same time (Gouws, 1981) implies that competition for resources may not have been limiting. However, we cannot exclude the possibility that larger families simply had more resources.
The DNA typing confirmed and clarified places of origin of certain founders. W is a relatively rare haplotype that is most common in Western Eurasia, reaching frequencies of 17% in the Sindhi population of South Eastern Pakistan (Quintana-Murci et al. 2004). However, W does occur in southern Europe (Torroni et al. 1996; Maca-Meyer et al. 2001) and is not at odds with Claudine Eloy from Bordighera being the carrier. The M haplotype of my father's mtDNA was donated by a slave born at the Cape. We can now speculate that her mother must have come from Asia, possibly India (Maca-Meyer et al. 2001), rather than from Africa. My Y chromosome is a common European genotype and could have come from Germany (Wells et al. 2001).
The approach followed in this study offers unique advantages, in that it links history to its genetic consequences. Although my ancestry sampled more than 1000 lineages, i.e. was potentially all inclusive, the high degree of ancestor loss may suggest that these conclusions are not generally true for Afrikaners. Of the potential 63 couples married before 1687 in the Cape I am related to only 33. This skew is in part explained by the fitness differences between the founders (Figures 3a & b), but these models were overdispersed suggesting that other factors are also important. I suspect that the extra noise could be explained by the fact that marriage partners were normally obtained from the local population, so that place of origin can tie a number of families together. To answer these questions similar analyses needs to be conducted on more Afrikaners and geography needs to be incorporated.