BIOL 114
Biology Dept
Kenyon College
Genes, Environment, and Chance
Fall SectionSpring Section 1Spring Section 2
The character of all living organisms result from the interaction of environment and genes.  For example, the risk of colon cancer is increased by dietary factors such as high fat and low fiber.  But a certain genetic allele (version of a gene sequence) in certain individuals confers a high risk of colon cancer, even with a low-risk diet.
 
A SAMPLING OF ENVIRONMENTAL GENES
Polymorphism Class Environmental exposure Associated disease
CYP1A1 Activation Smoking Lung cancer
NAT2 Detoxification Smoking Bladder, breast cancer
GSTT1 (null) Detoxification Chlorinated solvents Cancer, toxicity
Paraoxonase Detoxification Nerve agents, pesticides Nervous system damage
HLA-H Nutritional factors Iron in diet Hemochromatosis
TGF-alpha Growth factor Maternal smoking Cleft lip & palate
Locus on chrom. 17 in mice Immune/inflammatory response Ozone Lung inflammation
HLA-DP bet1 marker Immune response Beryllium Chronic beryllium disease (lung disorder)
ALAD Biosynthesis Lead Lead poisoning
Jocelyn Kaiser, Science Magazine,  278:569 - 570,  24 Oct 1997
Genes contain the information of a cell that is inherited by future cells.  Each cell needs to copy its chromosome exactly and pass on an identical copy to each daughter cell.   Of course rare "mistakes" occur -- cells actually have evolved to favor rare mistakes.Why?  (See Week 8, Molecular Evolution)

Experiments designed to demonstrate the genetic or environmental component of a trait generally aim to keep all components constant except the one tested.  As a result, someone who studies Down's Syndrome might conclude that the basis of intelligence is genetics; whereas someone studying lead poisoning would conclude that the basis of intelligence is environmental. Actually, all traits -- appearance, development, behavior -- depend on BOTH genes and environment.

Chance also determines the development of an organism. For example individuals with the chromosome abnormality of Down's Syndrome may develop by chance to a wide range of levels -- some develop so badly they die before birth; others have severe heart defects and other physical problems; mental delays vary from severe to minor; some show few physical problems, and can attend college. None of this variation can be predicted from the genes, only from chance effects in development.

Levels of organization in living systems

The content of the Biology core courses can be viewed as a continuum:
      BIOL 113 ------------------------------------------------ BIOL 112

      Molecule - Cell - Organism - Population - Ecosystem - Biosphere
       

    BIOL 114 -- Transfer of information between levels, by gene expression and development, and by genetic change.
Genomes
A genome is the total of all genetic sequence in an organism.  See The Human Genome Project.

The genome of Escherichia coli contains 4.6 million base pairs, encoding 4,400 genes.

The human genome contains 3 billion base pairs in the nucleus, but only 60,000 genes (estimated), taking up 3% of the sequence.  The rest includes regulator regions and large stretches of repetitive sequence of unknown function.

The entire genome has been sequenced for several microbes, and for several animals and plants:

Human (Homo sapiens)
Mouse
Fruit Fly (Drosophila)
Nematode (Caenorhabditis elegans.
Arabidopsis -- model plant
Take a closer look at three genomes:

T. A. Brown, Genomes, BIOS


  • Genomes of microbes usually contain mainly genes encoding proteins.
  • Multicellular eukaryotes (animals and plants) have their genes interspersed between large stretches of repetitive sequence (Satellite DNA).
  • Much of repetitive DNA (50% of total human genome) comes from viruses that copied their DNA into a human chromosome many generations ago.  The viral DNA mutated, so it could no longer make viruses; it just kept getting copied as human chromosomes replicated.
  • Genes of humans and other eukaryotes are interrupted by introns that do not encode RNA or protein products.  Some introns contain regulatory sequences
  • As genomes evolve, some genes accidentally make extra copies, which degenerate through mutations, becoming pseudogenes.
Genomes from different organisms have a lot in common.   Thus, we can use model systems to make hypotheses about the biology of humans.  We can learn a surprising amount of human biology from genomes of yeast, C. elegans, Drosophila, and the mouse.  The latest model genome proposed is the chimpanzee.
  • The chimpanzee's genome could give us cancer-resistant genes.
  • To create "model systems" for human diseases, we could put human disease genes into chimps.

    • What might the chimps have to say about it?