Water Is Weird
Owing to some curious characteristics of its molecular bonds, water displays a number of peculiar properties—and these become more pronounced at extreme temperatures. Scientists have known for a while that ultrapure water can remain liquid well below 32 F (ice crystals need a speck of something solid in order to form). Now researchers at SLAC are using the powerful LCLS X-ray laser to probe the structure of H2O at temperatures below minus 42 F to determine whether there's a critical point where water reaches maximal weirdness.
Did You Know?
- Hot water forms ice at a higher temperature than cold water—so it freezes sooner during a cooling process, though not necessarily faster.
- There are at least five different liquid phases of water and 15 different solid phases of ice.
- Water is densest at precisely 4 degrees Celsius (39 F). Either raising or lowering the temperature will cause it to expand.
Mexico's Mix
The first large-scale study of genetic diversity in Mexico has revealed striking regional variations that can be traced to pre-Columbian indigenous ancestry. These differences have the potential to affect health and point to the need for finer-grain distinctions among subpopulations in clinical and research settings.
Andres Moreno-Estrada, a research associate in Carlos Bustamante's lab, and Christopher Gignoux, '01, MS '06, a postdoctoral scholar, were co-lead authors of the paper, which appeared in Science in June. Working with a team of collaborators from Stanford, UCSF and the Mexican National Institute of Genomic Medicine, they analyzed the genomes of 511 individuals representing 20 of the country's 65 indigenous groups, as well as 500 individuals of mixed descent ("mestizos") living in 10 Mexican states and Los Angeles.
The results showed pronounced genetic differences among the present-day Native American populations that increased with geographic distance: The Seri in the northwest are as distinct from the Lacandon in the southeast as Europeans are from East Asians. What's more, cosmopolitan mestizo populations in Mexico show a similar spatial distribution of indigenous gene variants.
"We had expected that 500 years of population movements, immigration and mixing would have swamped the signal of pre-Columbian population structure," says Bustamante, a professor of genetics who was co-senior author on the study. (Conversely, the people of Mexican descent living in Los Angeles, having come from all over, showed no coherent pattern.)
Finally, the researchers were interested in whether variations in indigenous ancestry might impact medically relevant traits known to vary among ethnic groups. They found that in some regions, average values for a test of lung function were slightly higher than the "Mexican" reference standard used to diagnose asthma and other pulmonary diseases, while in other regions it was slightly lower.
"Variations in genetic composition clearly give a different physiological response," concludes Gignoux.
'We're moving beyond blanket definitions like Mexican or Latino . . . Those broad terms imply common ground among populations, but we're finding that it's much more like a mosaic.' —Andres Moreno-Estrada
