Understanding the genetics that govern the Paradigm trait first requires a solid grasp of the simplest case in which
a character is determined by one gene for which there are only two alleles, one completely dominant to the other.
Confused already? Not to worry! We will start by defining some basic terms:
Genetics - the scientific study of heredity and hereditary variation.
DNA - or deoxyribonucleic acid, is the basic molecular substance of genes.
Chromosomes - structures within cells, each of which includes one very long DNA molecule usually
consisting of hundreds or thousands of genes arranged along its length.
Gene - the basic unit of inheritence, a gene is a segment of DNA that transmits information
defining a trait from parent to offspring.
Alleles - alternative versions of genes that account for variations in inherited characteristics; for
example, the gene for flower color in pea plants exists in two versions, or alleles, one for purple flowers and the other
for white.
Locus - a gene's specific location along the length of a chromosome.
Loci - the plural of locus.
Normal - the most common phenotype in natural populations; synonymous with wild type.
Wild type - the most common phenotype in natural populations; synonymous with normal.
Heterozygote - an organism that has two different alleles for a character is said to be heterozygous
for the gene controlling that character, and is called a heterozygote with respect to that character.
Heterozygous - having two different alleles for a character.
Homozygote - an organism having a pair of identical alleles for a character is said to be homozygous
for the gene controlling that character, and is called a homozygote with respect to that character.
Homozygous - having a pair of identical alleles for a character.
Character - a heritable feature, such as flower color, that varies among individuals.
Trait - each variant for a character, such as purple or white color for flowers, is called a trait.
The terms character and trait are sometimes used synonymously.
Genotype - an organism's genetic makeup in terms of its pairs of alleles.
Phenotype - an organism's traits that result from
its genotype. When considering color and pattern morphs (visual traits) in boas, the phenotype can be thought of
simply as the boa's appearance.
Complete Dominance - one allele of a pair is completely dominant with respect to the other; in
this situation, the phenotypes of the heterozygote (Aa) and the dominant homozygote (AA) are indistinguishable. In other
words, the phenotype is completely determined by the dominant allele, and the recessive allele has no discernable
effect on the organism.
Recessive - a recessive allele has no discernable effect on an organism's phenotype when paired with
a dominant allele in the heterozygote (Aa).
These definitions should be sufficient to get us through this introductory step, but we will first present a brief discussion
to provide further clarification of the terminology and its usage.
Other than in reproductive cells, chromosomes exist in pairs with one chromosome of the pair received from each
parent. Except for the sex chromosomes (XY in humans), the chromosomes in a pair are homologous, which means that
each contains the same sequence of genes controlling the same inherited characters; each locus on one chromosome has a mate
on the second chromosome of the pair. Thus, each chromosome pair is a series of gene pairs that determine specific
characters in the organism. The gene pair will be comprised of alleles (versions of that gene),
one inherited from each parent, that may be the same or different. Thus we could also say that
each chromosome pair is a series of allele pairs that determine specific characters in the organism.
For example, the gene that controls flower color in pea plants exists in two forms, a dominant allele that produces purple
flowers (P) and a recessive allele that produces white flowers (p). There are three possible pairings of these
two alleles at the flower color locus on the chromosome pair. The dominant homozygote (PP) and the heterozygote (Pp)
both produce purple flowers that are indistinguishable from one another. The recessive homozygote (pp) produces white
flowers.