|Bizarre Facts in Biology|
A Typical Human: Brown Hair, Color Vision, Freckles, and Six Fingers!
by Gail Gasparich
Greg Harris, a pitcher for the Montreal Expos, upon his induction into the Baseball Hall of Fame in Cooperstown, New York, donated one of his two specially designed gloves for exhibition. Harris’s glove is unique because it was made to accommodate his six fingers. Why do people look the way that they do? Where does the information come from to have six fingers rather than five? These human traits are passed down from parents to their children as genes on their DNA.
By studying the common pea plant, Gregor Mendel (a Czech monk) was the first to investigate how offspring inherit specific traits from their parents. When Mendel crossed two true-breeding plants with alternative traits—say, a purple-flowered plant with a white-flowered plant, the offspring showed only one of the two traits. Mendel observed that whenever he looked at a trait with two possible alternatives and used true-breeding plants, one trait was fully expressed (dominant) and the other had no noticeable effect (recessive). However, when Mendel crossed the offspring of these parents with each other, the recessive trait would reappear in one quarter of the new generation. The recessive trait had not been eliminated; it simply was not obvious in the presence of the dominant trait. We now know that dominant and recessive traits are products of the different alleles of a gene.
More than 100 human genetic disorders are known to be inherited as Mendelian traits (either as a dominant or recessive allele). Many dominant traits are not necessarily the "better" or "stronger" or most common in a population (for example, polydactyly —extra fingers and/or toes); it is simply the allele that produces a noticeable effect, a phenotype. The allele responsible for polydactyly occurs in one of the many genes responsible for directing the development of the embryo. When an allele causes such a gene product to be overexpressed, or expressed at an inappropriate time during development, then we see the effect—six fingers.
What makes an allele common or rare in a population? Again, it doesn’t depend on whether the allele is dominant or recessive. An allele can become rare when the phenotype produced by the presence of the allele is selected against in the environment. In the extreme case, alleles become rare when the homozygous state (two of the same alleles) produces a lethal condition before those individuals can produce children. In the case of polydactyly, two dominant alleles probably prevents proper embryonic development, resulting in a miscarriage. One dominant allele provides extra fingers or toes, and, maybe, a Hall of Fame pitcher.
How Are Human Genetic Disorders Investigated?, page 226
How Are Human Disorders Caused by Single Genes Inherited?, page 227
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