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The difference between genes and alleles

A gene is an informational unit that describes how to construct a particular type of molecule.  (Let’s leave aside for the moment how the information is “written” and who/what does the construction.)  By themselves, genes don’t do anything other than provide information on how to make something that does do something.  In other words, a gene is simply a set of instructions that describes how to build a specific type of molecule that will perform some concrete activity or job in a cell.  In general, you can think of the molecule described by a gene as a kind of machine designed for a specific job, usually some biochemical reaction.


For any given gene, plants and animals usually carry two copies.  A key thing to know is that for any specific gene, there can be different versions (also called alleles).  Oftentimes the two copies of a gene in an organism are different versions of the same gene, or sometimes the same version.  Here’s a crude analogy (the first of many in this series) to illustrate…


I recently baked a cake.  The recipe called for one cup of oil.  Not being an experienced baker, I didn’t know which oil to use.  Vegetable oil?  Peanut oil?  Whale oil?  Pennzoil?  All of these are oils, but they would no doubt produce cakes that taste quite different.  Now imagine the situation if instead of one cup of oil, the recipe called for two cups – and you could mix different types of oil in your cake.  Here are the possibilities:


vegetable oil                 vegetable oil

vegetable oil                 peanut oil

vegetable oil                 whale oil

vegetable oil                 Pennzoil

            peanut oil                     peanut oil

peanut oil                     whale oil

peanut oil                     Pennzoil

whale oil                     whale oil

whale oil                     Pennzoil

Pennzoil                       Pennzoil


You’d get some pretty weird cakes with some of those combinations! 


Hopefully this baking example illustrates something about genetics: each different kind of oil is an “allele” of oil.


For each gene, humans and most orchids carry two copies (there are exceptions).  One copy comes from the mother, and the other from the father.  Now, each copy can be one of potentially many alleles that exist in the general population.


Let’s assume that we’re talking about just one gene, call it gene G.  Assume also that twelve different alleles of this gene exist out in the population.  Although everyone (orchid or human) in the population has two copies of the gene, a particular individual may carry any two of the twelve possible alleles.  (In fact, an individual may carry two copies of the same allele, an exceedingly common occurrence.) 




In the above illustration, each parent carries two different alleles, and all four of these parental alleles are different from each other.  Hence, during a mating, these parents could only contribute to their offspring one of the four combinations of alleles shown.


OK, now that we understand more clearly the difference between genes and alleles, let’s go back to how alleles of genes (i.e., different versions of genes) affect traits.


A trait is basically what you see: it is the physical expression of the genes themselves.  Brown hair, blue eyes, yellow flowers, long petals – all of these traits derive from some combination of alleles.  The term for this physical expression is phenotype.  The corresponding term, genotype, describes the genes that produce the phenotype.


In his plant breeding experiments, Mendel studied pea plant phenotypes that presented themselves clearly.  For example, the pea seeds were either smooth or wrinkled, the flowers white or violet, the pods full or constricted.  His selection of clearly discernible, unblended traits proved critical to Mendel’s conclusions of how traits passed from one generation to another.

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