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The Organizational-Activational Hypothesis states that steroid hormones permanently organize the nervous system during early development, which is reflected in adult male or female typical behaviors. [1] In adulthood, the same steroid hormones activate, modulate, and inhibit these behaviors.
In humans, barring intersex conditions causing aneuploidy and other unusual states, it is the male that is heterogametic, with XY sex chromosomes.. Haldane's rule is an observation about the early stage of speciation, formulated in 1922 by the British evolutionary biologist J. B. S. Haldane, that states that if — in a species hybrid — only one sex is inviable or sterile, that sex is more ...
The optimality hypothesis states too much variability in the MHC can result in a failure of T-cells to distinguish themselves non-selves, and thereby increase the risk of autoimmune disease. This would confer greater fitness to individuals without a large degree MHC diversity. [6] [13] Autoimmune diseases are associated with MHC loci. In humans ...
Kidney and nerve tissue cells can form memories much like brain cells, one new study has found. ... to its former state of obesity.” To prove this hypothesis, the researchers, in the first ...
In human cells with more than one X chromosome, two long non-coding RNAs are produced: Tsix is produced by one X chromosome, and Xist is produced by all of the other X chromosomes. Tsix is a long non-coding RNA that prevents repression of an X chromosome, while Xist is a long non-coding RNA that acts to repress and condense an entire X chromosome.
Best-Man (Williams, 1966; Emlen, 1973; Treisman, 1976): The Best-Man hypothesis proposes that, on average, sexually produced offspring may be of somewhat lower fitness than the asexually produced offspring, but the much greater diversity of the sexually produced offspring implies that they will include a few individuals of extraordinary high ...
1. All living organisms are composed of one or more cells 2. The cell is the most basic unit of life. Schleiden's theory of free cell formation through crystallization was refuted in the 1850s by Robert Remak, Rudolf Virchow, and Albert Kolliker. [5] In 1855, Rudolf Virchow added the third tenet to cell theory.
The Lyon hypothesis states that in cells with multiple X chromosomes, all but one are inactivated early in embryonic development in mammals. [ 3 ] [ 4 ] The X chromosomes that become inactivated are chosen randomly, except in marsupials and in some extra-embryonic tissues of some placental mammals, in which the X chromosome from the sperm is ...