Genetic¹ resistance to a parasitic² nematode（线虫） that infects sheep has been discovered by a team of scientists with the U.S. Department of Agriculture (USDA) and the International Livestock³ Research Institute (ILRI). The researchers are the first to detect quantitative⁴ trait loci (QTL), genetic locations on chromosomes⁵, for resistance to gastrointestinal（胃肠的） nematode parasites⁷ in a double-backcross population derived⁸ from African native sheep. The parasites, common in tropical regions, cause significant economic and production losses in Africa each year. Sheep infected with parasites suffer from diarrhea（腹泻） , anemia⁹（贫血） , weight loss and sometimes death.

Geneticist Tad Sonstegard at the Agricultural Research Service (ARS) Henry A. Wallace Beltsville Agricultural Research Center in Beltsville, Md., and researchers at ILRI in Kenya hope to identify genes¹⁰ that increase tolerance¹¹ to parasites and improve production of grazing animals. ARS is USDA's chief intramural（内部的） scientific research agency, and this research supports USDA's priority of promoting international food security.

In one study, researchers mapped the regions of the genome that control resistance to gastrointestinal nematode parasites in a sheep population bred by ILRI. Hybrid¹³ rams¹⁴ were produced by mating a Red Maasai, which is tolerant to gastrointestinal parasites, to a Dorper, a breed that is more susceptible¹⁵ to the parasite⁶. Several of the hybrid ram¹² offspring were then bred to either Red Maasai or Dorper ewes to complete the backcross.

Scientists genotyped 20 percent of the backcross progeny¹⁶（子孙，后裔） to map QTL that affect parasite-resistance traits. Blood packed-cell volume and fecal egg count-indicators¹⁷ of parasites-were collected for three months from more than 1,060 lambs that grazed on parasite-infected pastures. Scientists selected lambs for genotyping based on parasite indicators. They detected significant QTL for average fecal egg count and packed-cell volume on chromosomes 3, 6, 14, and 22.

Future studies will focus on genotyping the same animals using the OvineSNP50, according to Sonstegard. The OvineSNP50 is a powerful tool that can examine more than 50,000 locations in the genome.

Findings from this research were published online in Animal Genetics in May 2011.