Dr. Ray L. Winstead
Professor of Biology, Indiana University of Pennsylvania

Definitions in Genetics

1. gene: The basic unit of heredity that produces the identifying 
    characteristics of an individual.  It is a unit of DNA that occupies a 
    locus - or location - on a chromosome and codes for the production of a 
    single chain of amino acids (a polypeptide chain), i.e., usually one 
    protein, e.g., a specific enzyme.  New research is stimulating some 
    proposals of revisions to this definition.

2. allele: A particular form of a gene at a specific locus on a chromosome.  
    There could be two or more forms, e.g., a gene for basic eye color 
    could be either basic brown or basic blue.

3. homologous chromosomes: A pair of chromosomes that are similar in size 
    and shape and also have genes that code for the same traits and 
    characteristics.  One homologous chromosome is inherited from the 
    father, the other from the mother.

4. (a) homozygous: Refers to the condition when the corresponding alleles 
         on homologous chromosomes for a specified trait are the same alleles, 
         i.e., the same form of a gene, e.g., both alleles are genes for having 
         blue eyes or both are genes for having brown eyes.

   (b) heterozygous: The corresponding genes on homologous chromosomes are 
        different alleles, e.g., one is for blue eyes, the other for brown 
        eyes.

5. (a) dominant: This refers to an allele or characteristic that is 
         actually expressed, regardless of what other allele is on the other 
         homologous chromosome, e.g., if one allele is for brown eyes and the 
         other allele is for blue eyes, the person will actually have brown 
         eyes.

    (b) recessive: This refers to an allele or characteristic that is not 
         expressed when a dominant allele is present, or is expressed only when 
         the individual is homozygous for that trait, e.g., having blue eyes.

6. (a) genotype: The sum total of all the alleles present in an individual 
         - both expressed and not expressed - for the specified trait(s), e.g., 
         includes  both the brown and blue eye genes when both are present.

    (b) phenotype: The expressed characteristics of an individual, e.g., 
         even if the blue eye allele is present, refers only to the expressed 
         brown eyes.

7. (a) monohybrid: An individual that is heterozygous for one particular 
         pair of specified alleles.  A monohybrid cross is a mating of 
         monohybrid parents.

    (b) dihybrid: An individual that is heterozygous for two particular 
         pairs of specified alleles.  A dihybrid cross is a mating of dihybrid 
         parents.


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Dr. Ray L. Winstead
Direct e-mail Link: RWinstea@iup.edu