more than three million children have been born as a result of assisted reproductive technologies（辅助生育技术） since the birth of the first "test tube baby" in 1978. while the majority of these children are healthy and normal, as a group they are at greater risk of certain kinds of birth defects（先天缺陷） and being low birth weight, which is associated with 1, hypertension and type 2 2（糖尿病） later in life. carmen sapienza, a geneticist at temple university school of medicine, studies the impact of the environment on 4 by comparing one type of 5（染色体） 6, called "7 methylation（甲基化） " between children conceived in the traditional fashion with children born as a result of assisted reproduction.
"we found that 5-10 percent of these chromosome 8 were different in children born through assisted reproduction, and this altered the expression of nearby genes," he said. "but we have not yet 9 whether this is caused by assisted reproductive technologies or other factors such as the couple's 10."
because some of the 11 genes are involved in the development of fat tissue and the 12（新陈代谢） of 13（葡萄糖） , sapienza believes it will be important to monitor these children long-term to determine whether they have higher rates of obesity or diabetes.
he notes that even though there were measurable differences in dna methylation and 3 expression between the two groups, only a small fraction（部分） of the assisted reproduction children were found to be outside the "normal" range.
收听单词发音 
