单选题 .  ①Founder mutations are a class of disease-causing genetic mutations, each derived from its own ancestral ''founder" in whom the mutation originated. ②While most disease-causing mutations are found in humans at a rate of one in a few thousand to one in a few million people, founder mutation can occur at much higher rates. ③This apparent anomaly is partially explained by the fact that most founder mutations are recessive: only a person with copies of the affected gene from both parents becomes ill. ④Most people with only one copy of the gene—"carriers"—survive and pass the gene to offspring. ⑤Furthermore, the single copy of a founder mutation often confers a survival advantage on carriers. ⑥For example, the hereditary hemochromatosis mutation protects carriers from iron-deficiency anemia because the mutated gene allows increased efficiency of iron absorption.24.  The passage indicates which of the following about founder mutations? ______