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| Sex-limited genes ： ウィキペディア英語版 |
Sex-limited genes
Sex-limited genes are genes that are present in both sexes of sexually reproducing species but are expressed in only one sex and remain 'turned off' in the other. In other words, sex-limited genes cause the two sexes to show different traits or phenotypes, despite having the same genotype. This term is restricted to autosomal traits, and should not be confused with sex-linked characteristics, which have to do with genetic differences on the sex chromosomes (see sex-determination system). Sex-limited genes are also distinguished from (sex-influenced ) genes, where the same gene will show differential expression in each sex. Sex-influenced genes commonly show a dominant/recessive relationship, where the same gene will have a dominant effect in one sex and a recessive effect in the other (for example, male pattern baldness).
Sex-limited genes are responsible for sexual dimorphism, which is a phenotypic (directly observable) difference between males and females of the same species. These differences can be reflected in size, color, behavior (ex: levels of aggression), and morphology. An example of sex-limited genes are genes which instruct the male elephant seals to grow big and fight, at the same time instructing female seals to grow small and avoid fights. These genes are also responsible for some female beetles' inability to grow exaggerated mandibles, research that is discussed in detail later in this article.
The overall point of sex-limited genes is to resolve intralocus sexual conflict. In other words, these genes try to resolve the "push-pull" between males and females over trait values for optimal phenotype. Without these genes, organisms would be forced to settle on an ''average ''trait value, incurring costs on both sexes. With these genes, it is possible to 'turn off' the genes in one sex, allowing both sexes to attain (or at least, approach very closely) their optimal phenotypes.
== A brief history ==
Unsurprisingly, the idea of sex-limited genes was initially developed by Charles Darwin himself in 1871 in his book ''The Descent of Man and Selection in Relation to Sex.'' He does not distinguish between sex-limited, sex-linked, and sex-influenced genes, but refers to any gene that expresses differently between sexes as sex-limited. While this concept was still in its infancy, Darwin catalyzed the further development of sex-related selection. Thomas Hunt Morgan, fully aware of this confusing terminology, published an article in ''The American Naturalist ''in 1914 titled "Sex-Linked and Sex-Limited Inheritance." This article directly acknowledges that Darwin applied the term sex-limited whenever a characteristic seemed specific to one sex. Morgan proposes the definitions for sex-linked genes and sex-limited genes that we still use today (and that were defined in the introduction above). This paper helped to distinguish between these two similar concepts and clarify much confusion in the scientific community at the time. Morgan's paper was followed by several others involving sex-limited genes and their expression as traits. One of the more notable examples is John H. Gerould's "Inheritance of White Wing Color, a Sex-Limited (Sex-Controlled) Variation in Yellow Pierid Butterflies," published in ''Genetics ''in 1923 (and edited slightly in 1924). Gerould observed that in this species of butterfly, females naturally occur as yellow or white, while males only occur with yellow coloration. He extensively explores this apparently sex-limited trait from a genetic perspective in this ground-breaking 50 page paper. To conclude the notable advancements in the early stages of the development of sex-limited genes, a brief discussion of R. A. Fisher is necessary. Commonly hailed as one of the best evolutionary biologists of his time, Fisher was also a talented geneticist. His book ''The Genetical Theory of Natural Selection'', published in 1930, over 20 years before the double-helix shape of DNA was discovered, was the first attempt to explain Darwin's theories within the foundation of genetics. Chapter 6 of this book is titled "Sexual Reproduction and Sexual Selection" and includes a genetic interpretation of Darwin's initial idea of sex-limited genes. After these groundbreaking works, papers continue to be published further exploring the causes, mechanisms, evolutionary advantages, and more of sex-limited genes.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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