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. 

 

 

Until the development of the Paradigm boa (more on this later), the gene controlling Sharp-strain albinism was thought to occur at one locus with two possible alleles, one for normal and the other for Sharp-albino.  The normal allele is completely dominant to the Sharp-albino allele, and so the heterozygote is of the normal phenotype.  Using classical Mendelian notation, a capital "A" could represent the dominant normal allele, and a lower-case "a" would then represent the recessive Sharp-albino allele.  This is more simply summarized below.

 

Two of the four (or 1/2) of the possible allele combinations result in Sharp-albinos (aa), and the other two of the four (1/2) possible combinations result in het Sharp-albinos (Aa).  Thus we would expect (on average) that half of the offspring produced by these parents would be Sharp-albino, with the other half being het Sharp-albino.  It could also be said that for each offspring the odds are 50/50 as to whether it will be born a Sharp-albino or a het Sharp-albino; each baby has a 50% probability of being born a Sharp-albino and a 50% probability of being born a het Sharp-albino.  A summary of the expected offspring genotype and phenotype frequencies is presented below.

 

This analysis is more classically presented in the form of a Punnett square, which provides the same results but uses a simpler, more compact notation than the arrow diagram above.  The alleles of one parent are placed in the top grey boxes, and the alleles of the other parent are placed in the grey boxes at the left.  The Punnett square is then completed by carrying the alleles across the rows and down the columns, filling in each of the white boxes in the square.