脊柱裂和无脑畸形常有性别比率上的差异,这是否提示X染色体上的基因起作用呢?若干年来这些情况一直看作是“阈缺陷”的例证,即在引起缺陷的多种因素中,遗传作用是由数种基因调节,而产生疾病所需的基因数则随环境影响而异。最近Burn Does this suggest a role for the genes on the X chromosome? These cases have been seen for years as an example of “threshold defects,” that is, among the many factors that cause defects , The genetic effect is regulated by several genes, and the number of genes required to produce the disease varies with the environment. Recently Burn