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