2,716 research outputs found
Genetic Dominance & Cellular Processes
In learning genetics, many students misunderstand and misinterpret what “dominance” means. Understanding is easier if students realize that dominance is not a mechanism, but rather a consequence of underlying cellular processes. For example, metabolic pathways are often little affected by changes in enzyme concentration. This means that enzyme-producing alleles usually show complete dominance. For genes producing nonenzymatic proteins such as collagen or hemoglobin, the amount of product matters, and dominance relationships are more complicated. Furthermore, with hemoglobin, dominance can change depending on what aspect of the phenotype is being studied and on the environmental conditions. X-linked genes are a special case, whether enzymatic or not. Because of X-chromosome inactivation, only one X-linked allele can be active in a cell, which means that the concept of dominance cannot be applied at the cellular level. Instead, a type of dominance is demonstrated at the individual level; but even so, dominant traits may fail to be expressed, and recessive traits can be expressed. Teaching not only what is happening but why it\u27s happening will give students a deeper understanding, not only of dominance relationships, but of the underlying cellular processes as well
Delayed Recombination
Under the standard model for recombination of the primeval plasma, and the
cold dark matter model for structure formation, recent measurements of the
first peak in the angular power spectrum of the cosmic microwave background
temperature indicate the spatial geometry of the universe is nearly flat. If
sources of Lya resonance radiation, such as stars or active galactic nuclei,
were present at z ~ 1000 they would delay recombination, shifting the first
peak to larger angular scales, and producing a positive bias in this measure of
space curvature. It can be distinguished from space curvature by its
suppression of the secondary peaks in the spectrum.Comment: submitted to ApJ
Human Evolution
A common characteristic of humans is a desire to know their roots. This is evident not only in the tracing of family ancestries, but in the excitement about and attention given to the study of the roots of our entire species - the study of human evolution. We have, over the past few million years, evolved from an ape-like ancestor to modern Homo sapiens. Like evolution in general, the fact of human evolution is as firmly established as anything in science. There are, however, differences among scientists concerning details of this process. In this paper I will give an historical overview and summary of our knowledge of human evolution, and will focus on current disagreements
On the Method to Infer an Atmosphere on a Tidally-Locked Super Earth Exoplanet and Upper limits to GJ 876d
We develop a method to infer or rule out the presence of an atmosphere on a
tidally-locked hot super Earth. The question of atmosphere retention is a
fundamental one, especially for planets orbiting M stars due to the star's
long-duration active phase and corresponding potential for stellar-induced
planetary atmospheric escape and erosion. Tidally-locked planets with no
atmosphere are expected to show a Lambertian-like thermal phase curve, causing
the combined light of the planet-star system to vary with planet orbital phase.
We report Spitzer 8 micron IRAC observations of GJ 876 taken over 32
continuous hours and reaching a relative photometric precision of 3.9e-04 per
point for 25.6 s time sampling. This translates to a 3 sigma limit of 5.13e-05
on a planet thermal phase curve amplitude. Despite the almost photon-noise
limited data, we are unable to conclusively infer the presence of an atmosphere
or rule one out on the non-transiting short-period super Earth GJ 876d. The
limiting factor in our observations was the miniscule, monotonic photometric
variation of the slightly active host M star, because the partial sine wave due
to the planet has a component in common with the stellar linear trend. The
proposed method is nevertheless very promising for transiting hot super Earths
with the James Webb Space Telescope and is critical for establishing
observational constraints for atmospheric escape.Comment: Published in Ap
Theoretical Transmission Spectra During Extrasolar Giant Planet Transits
The recent transit observation of HD 209458 b - an extrasolar planet orbiting
a sun-like star - confirmed that it is a gas giant and determined that its
orbital inclination is 85 degrees. This inclination makes possible
investigations of the planet atmosphere. In this paper we discuss the planet
transmission spectra during a transit. The basic tenet of the method is that
the planet atmosphere absorption features will be superimposed on the stellar
flux as the stellar flux passes through the planet atmosphere above the limb.
The ratio of the planet's transparent atmosphere area to the star area is
small, approximately 10^{-3} to 10^{-4}; for this method to work very strong
planet spectral features are necessary. We use our models of close-in
extrasolar giant planets to estimate promising absorption signatures: the
alkali metal lines, in particular the Na I and K I resonance doublets, and the
He I - triplet line at 1083.0 nm. If successful, observations
will constrain the line-of-sight temperature, pressure, and density. The most
important point is that observations will constrain the cloud depth, which in
turn will distinguish between different atmosphere models. We also discuss the
potential of this method for EGPs at different orbital distances and orbiting
non-solar-type stars.Comment: revised to agree with accepted paper, ApJ, in press. 12 page
On the Insignificance of Photochemical Hydrocarbon Aerosols in the Atmospheres of Close-in Extrasolar Giant Planets
The close-in extrasolar giant planets (CEGPs) reside in irradiated
environments much more intense than that of the giant planets in our solar
system. The high UV irradiance strongly influences their photochemistry and the
general current view believed that this high UV flux will greatly enhance
photochemical production of hydrocarbon aerosols. In this letter, we
investigate hydrocarbon aerosol formation in the atmospheres of CEGPs. We find
that the abundances of hydrocarbons in the atmospheres of CEGPs are
significantly less than that of Jupiter except for models in which the CH
abundance is unreasonably high (as high as CO) for the hot (effective
temperatures K) atmospheres. Moreover, the hydrocarbons will be
condensed out to form aerosols only when the temperature-pressure profiles of
the species intersect with the saturation profiles--a case almost certainly not
realized in the hot CEGPs atmospheres. Hence our models show that photochemical
hydrocarbon aerosols are insignificant in the atmospheres of CEGPs. In
contrast, Jupiter and Saturn have a much higher abundance of hydrocarbon
aerosols in their atmospheres which are responsible for strong absorption
shortward of 600 nm. Thus the insignificance of photochemical hydrocarbon
aerosols in the atmospheres of CEGPs rules out one class of models with low
albedos and featureless spectra shortward of 600 nm.Comment: ApJL accepte
Ecology of Iowa Drosophila II. Lowland Forest and Sand Prairie
The seasonal abundances of Drosophila species collected from a lowland forest community and from a sand prairie community in northeastern Iowa are compared. With some important exceptions, the patterns seen in the lowland forest community were similar to what had been observed in a previous collection (Jennings et al. 1985). Fewer species and many fewer individuals were collected from the sand prairie community, although in general the patterns seen were similar to those of the lowland forest community. Strong evidence of microhabitat differentiation was seen in both communities. In contrast to our previous study, none of the seasonal abundance patterns were significantly correlated with temperature
Theoretical uncertainty in baryon oscillations
We discuss the systematic uncertainties in the recovery of dark energy
properties from the use of baryon acoustic oscillations as a standard ruler. We
demonstrate that while unknown relativistic components in the universe prior to
recombination would alter the sound speed, the inferences for dark energy from
low-redshift surveys are unchanged so long as the microwave background
anisotropies can measure the redshift of matter-radiation equality, which they
can do to sufficient accuracy. The mismeasurement of the radiation and matter
densities themselves (as opposed to their ratio) would manifest as an incorrect
prediction for the Hubble constant at low redshift. In addition, these
anomalies do produce subtle but detectable features in the microwave
anisotropies.Comment: 4 pages, REVTeX, 1 figure. Submitted to PR
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