All the evidence so far, scientists say, indicates that the breathtakingly rapid reprogramming in cloning can introduce random errors into the clone's DNA, subtly altering individual genes with consequences that can halt embryo or fetal development, killing the clone. Or the gene alterations may be fatal soon after birth or lead to major medical problems later in life. Some scientists say they shudder to think what might happen if human beings are cloned with today's techniques. While arguments over the ethics of human cloning have dominated the debate, these scientists say the real issue is the likelihood that clones would have genetic abnormalities that could be fatal or subtle but devastating. Until that problem is solved, they say, human cloning should be out of the question.
Scientists say they see what appear to be genetic problems almost every time they try to clone.
For example, some mouse clones grow fat, sometimes enormously obese, even though they are given exactly the same amount of food as otherwise identical mice that are not the products of cloning. The fat mice seem fine until an age that would be the equivalent of 30 for a person, when their weight starts to soar, said Dr. Ryuzo Yanagimachi, a University of Hawaii researcher who first cloned these animals and has studied cloning's consequences in them. Cloned mice also tend to have developmental abnormalities, taking longer to reach milestones like eye opening and ear twitching, Dr. Yanagimachi has found. Dolly herself, while apparently healthy, grew fat and had to be separated from the other sheep and put on a diet. But her experience is difficult to interpret since it is hard to draw conclusions about a propensity to obesity from one animal.