i*****y
发帖数: 3449 | 1 男与男、女与女，为甚么会有同性恋，科学上迄今仍是谜。但有美国科学家最新以「表
观遗传学」（epigenetics）解释同性恋原因。表观遗传学主要研究环境如何改变基因
表现（gene expression），据指出，当胚胎在母体发育初期，能影响基因表现的「外
延标记」（epi-marks），会向与性别相关的基因发出指令，保护胎儿免受环境影响，
像避免胎儿免受过多睾丸素影响，这足以影响胎儿生殖器官发展、性取向和性别认同。
一般而言，外延标记不会遗传下一代，否则会以父传女、母传子的方式遗传下来，
令女儿较男性化、儿子较女性化，特别强势的外延标记就会导致同性恋倾向。但这理论
仍须研究确定。
http://www.sciencedaily.com/releases/2012/12/121211083212.htm
Epigenetics May Be a Critical Factor Contributing to Homosexuality, Study
Suggests
Dec. 11, 2012 — Epigenetics -- how gene expression is regulated by
temporary switches, called epi-marks -- appears to be a critical and
overlooked factor contributing to the long-standing puzzle of why
homosexuality occurs.
According to the study, published online today in The Quarterly Review
of Biology, sex-specific epi-marks, which normally do not pass between
generations and are thus "erased," can lead to homosexuality when they
escape erasure and are transmitted from father to daughter or mother to son.
From an evolutionary standpoint, homosexuality is a trait that would not be
expected to develop and persist in the face of Darwinian natural selection.
Homosexuality is nevertheless common for men and women in most cultures.
Previous studies have shown that homosexuality runs in families, leading
most researchers to presume a genetic underpinning of sexual preference.
However, no major gene for homosexuality has been found despite numerous
studies searching for a genetic connection.
In the current study, researchers from the Working Group on Intragenomic
Conflict at the National Institute for Mathematical and Biological Synthesis
(NIMBioS) integrated evolutionary theory with recent advances in the
molecular regulation of gene expression and androgen-dependent sexual
development to produce a biological and mathematical model that delineates
the role of epigenetics in homosexuality.
Epi-marks constitute an extra layer of information attached to our genes'
backbones that regulates their expression. While genes hold the instructions
, epi-marks direct how those instructions are carried out -- when, where and
how much a gene is expressed during development. Epi-marks are usually
produced anew each generation, but recent evidence demonstrates that they
sometimes carry over between generations and thus can contribute to
similarity among relatives, resembling the effect of shared genes.
Sex-specific epi-marks produced in early fetal development protect each sex
from the substantial natural variation in testosterone that occurs during
later fetal development. Sex-specific epi-marks stop girl fetuses from being
masculinized when they experience atypically high testosterone, and vice
versa for boy fetuses. Different epi-marks protect different sex-specific
traits from being masculinized or feminized -- some affect the genitals,
others sexual identity, and yet others affect sexual partner preference.
However, when these epi-marks are transmitted across generations from
fathers to daughters or mothers to sons, they may cause reversed effects,
such as the feminization of some traits in sons, such as sexual preference,
and similarly a partial masculinization of daughters.
The study solves the evolutionary riddle of homosexuality, finding that "
sexually antagonistic" epi-marks, which normally protect parents from
natural variation in sex hormone levels during fetal development, sometimes
carryover across generations and cause homosexuality in opposite-sex
offspring. The mathematical modeling demonstrates that genes coding for
these epi-marks can easily spread in the population because they always
increase the fitness of the parent but only rarely escape erasure and reduce
fitness in offspring.
"Transmission of sexually antagonistic epi-marks between generations is the
most plausible evolutionary mechanism of the phenomenon of human
homosexuality," said the study's co-author Sergey Gavrilets, NIMBioS'
associate director for scientific activities and a professor at the
University of Tennessee-Knoxville.
The paper's other authors are William Rice, a professor at the University of
California, Santa Barbara, and Urban Friberg, a professor at Uppsala
University in Sweden.
http://www.sciencedaily.com/releases/2012/12/121211083212.htm |
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