Research Notes

Inbreeding Isn't All Bad A geographically isolated population subjected to a bottleneck event and then the inevitable inbreeding seems a recipe for genetic disaster, yet the Chillingham cattle of northern England, highly inbred for three centuries, are remarkably healthy and fertile. In 1947, the herd consisted of five males and eight females, and by October 30, 2000, their numbers had grown to just 49 animals. Their secret to success: probably luck, explains Peter Visscher, a reader in animal genetics at the University of Edinburgh. "We suggest that this herd has been lucky in being able to purge the bad alleles, whereas on average we would expect a build-up of bad alleles in the population," he says. The Chillingham cattle live under unnatural conditions, although they are a feral herd. "The Chillingham estate wishes to keep the herd isolated, so the cattle are prevented from going outside, and other cattle are prevented from coming in, using fences and natural barriers. The herd is genetically very different from other cattle breeds in the [United Kingdom] and elsewhere. As far as we know, this is the first example of a feral population that is more than 99 percent inbred," says Visscher. He and colleagues at the University of Edinburgh, nearby Roslin Institute, and De Montfort University in Grantham, U.K., scrutinized the animals' genetic uniformity by screening 25 microsatellite markers known to vary greatly among cattle, taking DNA from hair roots (P.M. Visscher et al, "A viable herd of genetically uniform cattle," Nature, 409:303, Jan. 18, 2001). Microsatellites are repetitive DNA sequences. All but one marker was identical in two copies (homozygous) in all 13 animals examined. The exceptional, heterozygous, marker varied in the same way in nine of the 13 cattle, so this genotype is apparently not hazardous. But whether it has been retained because it confers a benefit isn't known. "Because of the small sample size in our study, and because we have not measured any 'fitness' traits on individual animals, such as survival and fertility, we cannot say whether the persistent heterozygosity at one site is due to selection or due to chance. All we can say is that our observations are consistent with a population of small size that has been isolated for 300 years," concludes Visscher. --Ricki Lewis Breast Milk Medical authorities regard breast milk as the premier source of nutrition for infants, but some mothers may shy away from it because of a belief that environmental contaminants in the milk pose unacceptable hazards. That perception comes from numerous studies, which present an incomplete picture of levels of contaminants in breast milk in the United States (J.S. LaKind et al., "Infant exposure to chemicals in breast milk in the United States: What we need to learn from a breast milk monitoring program," Environmental Health Perspectives, 109:75-88, January 2001). Systematic breast milk sampling programs in other countries, says LaKind, an environment risk management consultant, show that breast milk levels of such contaminants as dioxins, PCBs and furans are in fact decreasing. A breast milk monitoring program, adapted for socioeconomic, racial, and environmental factors in the United States, would help determine if that is in fact true in this country, as LaKind suspects it may be. The program would also help provide basic information on depuration--how environmental contaminants are eliminated from the mother via breast-feeding. Having that information would provide a better picture of the level of environmental contaminants to which breast-fed infants are exposed. The levels of environmental chemicals appear to be decreasing, says LaKind, as breast-feeding continues. "And if we see new chemicals coming in breast milk, we can move quickly to try and figure out where the exposures are coming from and whether we need to do something about it," she says. --Harvey Black

Research Notes

Inbreeding Isn't All Bad

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