High School Biology-Chemistry SMILE Meeting
29 March 2005
Notes Prepared by Porter Johnson

Marva Anyanwu [Wendell Green Elementary School]             Genetics
As a followup to her miniteach of last time concerning genetics, Marva brought an exercise illustrating the Punnett square, which is a classic device used to work out elementary genetics problems (see http://anthro.palomar.edu/mendel/mendel_2.htm). In the example that Marva provided, we were looking at a hypothetical case involving inheritance of tail color in "critters". The "T" gene determines tail color, the t allele (recessive) determining orange tail and the T allele (dominant) determining blue tail. We first use the Punnett square (and blue and orange crayons as a mnemonic for the alleles) to predict the fraction of blue and orange tail offspring in a mating between two blue tailed critters, each with a "genotype" of Tt. In this typical example of a monohybrid cross, we expect genotypes in the offspring of TT to Tt to tt in a ratio of 1 to 2 to 1; due to the dominant and recessive relationships, the expected "phenotypes" in the offspring will be blue-to-orange in a ratio of 3 -to-1. Two other examples allowed us to extend our familiarity with this technique/set of concepts.

We continued with a lively discussion of other topics in genetics such as XX, XY sex determination, X-chromosome inactivation, etc.

Enlightening, Marva.

Terri Donatello [ST Edwards, science]                  Physical Properties
Terri followed up on her last miniteach, specifically in her set-up with hot and room temp water (in insulated containers) connected by a metal bar. Actually, from a microscopic statistical viewpoint, heat flows in both directions (from room temp to hot as well as the expected hot to room temp), but the flow from hot to room temp is much greater than the reverse so that the net flow is from hot to room temp. Cool stuff, Terri.

Ken Schug [IIT Chemistry]      Identifying Gases by Color?
Ken showed us two sealed glass tubes that were brown in color; we could tell by careful observation that the glass itself was not brown, but that there was a brown gas inside. As near as we could tell the two tubes were identical. Ken asked what gasses are colored. Some suggestions were bromine (brownish; Marva); and chlorine and iodine (yellow and reddish; Walter). He then asked what we could do experimentally to investigate this phenomenon further. Ken put the base of one tube in hot water (from the coffee pot) and the other in ice. The cooled one got lighter colored at the end immersed in ice, and a bit of liquid began to form; the heated tube got darker.

The explanations of these phenomena are as follows. NO2 was the gas; it is brown, but does not have a complete set of pairs of valence electrons. As the NO2 cools down, NO2 dimers form to complete the pairs of valence electrons, and this is colorless (and it condenses to a liquid). The process is reversed under high temperatures as the NO2 dimers have more vibrational energy and tend to break apart.

This led to a very interesting discussion of how absolute zero was determined (from extrapolation of volume versus temperature curves of various gasses to what would correspond to zero volume). Colorful Physical Chemistry. Thanks, Ken!

Notes prepared by Benjamin Stark.