An Excess of X
(June 12, 2008) Human females possess two X chromosomes, whereas males have one X and one small Y chromosome. This is in addition to 22 pairs of autosomal chromosomes unrelated to sex. Males prove that just one X is sufficient, so how do females cope with an excess of X chromosomes? They do so by inactivating one of their X chromosomes, compacting it into a clump named a Barr body. In the past, female Olympic athletes have been required to reveal their Barr bodies for gender verification. The Barr exam was abandoned because it could be confused by XXY males and X females.
Silencing of the extra X in females occurs randomly during early embryo genesis. Some cells will repress the maternally derived X while other cells repress the paternal X, and these choices are then transmitted into succeeding generations of cells. The resulting embryo is a mosaic of X-linked genes, often expressing different gene alleles in different regions (see diagram). We are ready to understand why colorful calico cats are almost always female.
Sex determination in all mammals parallels that in humans. In the common female calico cat, genes for ginger and dark fur occur within different X chromosomes. Thus, the calico’s colors flaunt the maternal-paternal mosaicism which is usually invisible in humans. The white areas in a calico are produced by an overiding, melanin-suppressing gene located on an autosomal chromosome. One in 3,000 calico cats is male. These rare tomcats are either XXY males or chimeras, fusions of 2 different embryos.
A question for you: Would you like human hair color to be sex-linked, as it is in calico cats?
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Today
Winter Classic Table Tennis Tournament
sports / 11:30 a.m. Arcata Community Center, 321 Community Parkway. Compete in 12 and under, beginners, intermediate, advanced or seniors groupings. Prizes for winners. $10/$5 kids 12 and under. 601-5447.
Audubon Society Field Trip
outdoors / 9 a.m. Humboldt Bay National Wildlife Refuge, 1020 Ranch Road, Loleta. Meet at Refuge Visitor Center off Hookton Road. Leisurely, two- to three-hour trip intended for people wanting to learn birds of Humboldt Bay area. 822-3613.
Open Celtic Music Session
music / 3 p.m. Cafe Veritas/Mosgo's, 180 Westwood Center, Arcata. Informal monthly gathering of musicians playing Irish and other Celtic music. Hosted by Seabury Gould. seaburygould.com. 845-8167.
Nonviolence Action Camp
etc. / 10 a.m. Chinmaya Mission near Piercy. Weekend-long direct action orientation features workshops, role playing, seminars, ceremonies and field trips. Bring food, bedding, warm clothes, signs, banners, bikes, drums, acoustic instruments. Pre-register. saverichardsongrove.org. 932-5898.
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