X Chromosome

choi

Carl Zimmer, Seeing X in a New Light; Scientists bring color to efforts to unravel a nagging chromosome mystery. New York Times, Jan 21, 2014
www.nytimes.com/2014/01/21/scien ... -in-a-new-light.htm

Except in the window of print: Dazzling mosaics show how X chromosomes are shut down in certain cells.

Note:
(a) X chromosome “got its name in 1891 from a baffled biologist named Hermann Henking [who] examined cells under a simple microscope. All the chromosomes in the cells came in pairs. All except one."
(i) Hermann Henking
en.wikipedia.org/wiki/Hermann_Henking
(1858-1942; discovered the X chromosome in 1890 or 1891. The work was the result of a study in Leipzig of the testicles of the Firebug Pyrrhocoris during which Henking noticed that one chromosome did not take part in meiosis)(A) Radomir SOCHA, Pyrrhocoris apterus (Heteroptera) - an Experimental Model Species: A Review. European Journal of Entomology, 90: 241-286 (1993)
(“The diploid number of chromosomes in P apterus [Pyrrhocoris apterus] is 2n = 24 in females and 211 = 23 in males (Henking, 1891; Ueshima, 1979); the bug thus belongs to the XO-type of sex determination”)
* The copy in the Web was pirated and likely ordered to removed. But Google search with the title with return a cache.
* It is possible “211” was a copy error--should be “2n” for diploid--from pdf file to html file when Google cached it. But the meaning is clear. This firebug determines sex this way: female is XX and male is XO, meaning one X but no other sex chromosome (such as Y). Due to XO in male, there is no counterpart [homologous chromosome; 同源染色體 in Taiwan], no pairing (synapsis; 聯會 in Taiwan)--in meiosis 減數分裂.  Biologists have learned that sex determination is complicated, varying from species to species.
* X and Y (the sex chromosomes in humans) are not homologous chromosomes. Also in humans--in fact, in most mammals--X and Y do synapsis in meiosis, only through pseudoautosomal region (PAR; X and Y each has one, and one
only).* firebug
en.wikipedia.org/wiki/Firebug
(it is distributed throughout the Palaearctic from the Atlantic coast of Europe to northwest China)
(B) He was German.

(b) "If an egg inherits an X chromosome from both parents, it becomes female. If it gets an X from its mother and a Y from its father, it becomes male."

The journalist is talking about most--but not all--mammals and some insects (including Drosophila).

(c) The scientific communication--dealing with which, but not why,  X chromosome (paternal or maternal) is inactivated in what cells of a female--that is the center of the NYTimes report:

Wu H et al, Cellular Resolution Maps of X Chromosome Inactivation: Implications for Neural Development, Function, and Disease. Neuron, 81: 103-119 (Jan 8, 2014; abstract and photos)
www.sciencedirect.com/science/article/pii/S0896627313010039
("we have defined the topographies of XCI mosaicism for multiple CNS cell types and of retinal vascular dysfunction in a model of Norrie disease")
(i) What is an eutherian? Generally speaking, it is a placental mammal. There were nonplacental eutherians, but these were extinct. See
placental mammal. Encyclopaedia Britannica, undated
www.britannica.com/EBchecked/topic/462577/placental-mammal(“Some scientists classify placental mammals in the clade Placentalia, which includes all living placental mammals and their most recent common ancestor. This grouping attempts to avoid the potential for confusion associated with fossil
evidence of extinct nonplacental eutherians, which were the predecessors of modern-day placentals”)(ii) Norrie disease. Genetics Home Reference, National Library of Medicine (NLM), US National Institute of Health (NIH), undated
ghr.nlm.nih.gov/condition/norrie-disease
(an inherited eye disorder that leads to blindness in male infants at birth; "Mutations in the NDP gene cause Norrie disease. The NDP [whose full name is "Norrie disease (pseudoglioma)"] gene provides instructions for making a protein called norrin")

Wikipedia says the disease was “named after [a] famous Danish ophthalmologist, Gordon NORRIE (1855–1941),” who had nothing to do with the discovery of, or
contribution to, the disease. An honorific title, so to speak.

(d) "A Japanese biologist, Susumu Ohno, first recognized X-chromosome inactivation in the late 1950s. In every female cell that he and his colleagues studied, they found that one of the two X chromosomes had shriveled into a dormant clump. Scientists would later find that almost no proteins were being produced from the clump, indicating that it had been shut down."

That clump is
Barr body
en.wikipedia.org/wiki/Barr_body
(named after discoverer Murray Barr [A Canadian who made the discovery in 1948])

(e) Scientists have "identified a number of the molecules that do the silencing. The leader of this molecular team is known as Xist."

The Xist (acronym for "X-inactive specific transcript") is a large noncoding (ie, not producing protein) RNA that is exclusively expressed in the inactivated X chromosome.
(f) The last figure in the NYTimes report displays left adn right retinas, showing a bit of bias (not random, that is) in X chromosome inactivation. The retina is an incomplete ball (open in front), which are cut into quarters in each eye. What is intriguing is the X chromosome inaction pattern in an adult female mouse, which suggests preference in the left half and right half.