Lecture 8
Unit Factors, Genes, and Homologous Chromosomes

I. Science as a Process

Unit Factors:  Mendel (1856) found that genetic characters are controlled by unit factors with unknown nature that exist in pairs in individual organisms.

Chromosomal Theory of Inheritance:

Sutton and Boveri (1902) discovered that behavior of chromosomes during meiosis follows Mendelian principles of segregation and independent assortment.  They pointed out that the separation of chromosomes during meiosis could serve as the cytological basis of these two postulates.

Each species possesses a specific number of chromosomes in each somatic cell (all body cells, except sex cells) nucleus except in gametes (sex cells).  For diploid organism, this number is called the diploid number (2n) and is characteristic of that species.  During meiosis, this number is precisely halved in each gamete (n), and when two gametes combine during fertilization, the diploid number is reestablished.

The diploid number of chromosomes is composed of homologous pairs identifiable by their morphological appearance and behavior.  The gametes contain one member of each pair.

The Currently Accepted Genetic Concept:

Chromosome is composed of a large number of linearly ordered, discrete information-containing units (DNA, RNA), called genes.

Unit Factors constitute a pair of genes located on one pair of homologous chromosomes.

Locus (pl. loci):  The location on a given chromosome where any particular gene occurs is called its locus.

Alleles:  The different forms taken by a given gene is called alleles, which contain slightly different genetic information that determines the same character (The color of the pea plant:  Purple allele and white allele).

Multiple alleles:  Most genes have more than two allelic forms.  The typical example is Human A B O blood type.  One gene (II) can have alleles, such as AI, BI, and i.  The combination of these three alleles determines four different blood types.