Behavioral OBJECTIVES
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1. to understand how genes determine cell chemistry and thus determine cell specialization
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2. to begin to understand how cell specialization and individual characteristics in humans vary
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3. to begin to understand the laws of heredity as Mendel proposed them
As studied in the previous lesson, specific loci on the DNA an responsible for the coding in m-RNA which puts together peptides for enzyme synthesis in the ribosomes of the cells.
An outstanding development in biochemical genetics was the concept that each gene is responsible for producing a specific enzyme. The theory was proposed by G. W. Beadle and E. L. Tatum. They experimented with the mold Neurospora They found that when Neurospora, Men exposed to radiation, did not synthesize a vitamin which it needed. Destruction of the gene by radiation does not only effect the syntheses of amino acids but of other polypeptides.
In 1976, Har Bobind Hhorana of the Massachusetts Institute of Technology succeeded in synthesizing an artificial gene. . . . The structural part of the gene has 126 nucleotides.
Traditionally biology has studied genomes and accepted variations within genetic material to account for the differences between organisms within a species and between individuals of different species.
Remarkably, the genetic code is the same for all living things, Only the mitochondrial codes are slightly different from the rest. Both protists and humans use GCU as the code word for alanine. This is another example of the unity of life and the relationship of living things.
We are trained by socialization to distinguish individuals belonging to different racial groups.
During the last forty years immunology and protein chemistry studies have identified a large number of genes that code for proteins and specific enzymes. Seventy-five percent of the different kinds of proteins are identified in all human tested.
Human genetic traits are inherited in a mendelian manner. There are over 3,000 human traits and some of them are shown in the table below. There are many others such as sex determination and ABO blood types.
(figure available in print form)
Over 2,000 human disorders are known to be inherited. Genetic counseling and typing are recommended for the recognition of many of these traits. Some are:
Sickle Cell Anem
ia where red blood cells are crescent shaped. They last 10-25 days whereas normal cells last 120 days.
Change of one codon recessive
Huntington’s chorea incurable insanity after age 35 dominant Tay Sacks destruction of nervous system in children recessive failure of enzyme to eliminate fat from nerve cells
PKU failure to convert enzymes and mental retardation occurs recessive Cystic fibrosis recessive where mucous forms in the lungs and pancreas Prior to genetic engineering which has helped to emphasize the importance of DNA sequencing, Gregor Mendel studied hereditary factors. He knew nothing about DNA, transcription and translation by RNA. His material did know that each parent contributed half the inherited material. Thus many ‘hidden’ factors produce traits that appear in successive generations.
Like humans, pea plants have certain characteristics such as flower color, blossom placement, plant size and pod color, and shape to name a few. He made thousands of crosses and recorded results from generation to generation. He knew nothing about DNA and genes determining polypeptide synthesis but his findings were:
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1. that traits are determined by paired factors
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2. that in paired factors one of the pair seems to be expressed more often than its homologous pair and is thereby said to be dominant
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3. in gamete formation (germ cell) the paired factors for one trait assort independently from the paired factors representing another trait
The occurrence of traits from generation to generation is predictable by representing the trait appearance in all gametes and then finding the possibilities from crossing all gametes.
GLOSSARY
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Factors
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Traits
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Law of Dominance
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Recessive
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Law of Independent Assortment
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Gamete
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Germ Cell
CONCEPT QUESTIONS
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1. What is meant by a pattern of inheritance?
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2. What is one of the accomplishments of genetic engineering?
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3. How are amino acids related to proteins?
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4. What did the findings of Gregor Mendel disclose?
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5. What were some of the dominant and recessive traits discovered by Mendel?
