Genes & Chromosomes

Chromosome Theory of Heredity  The factors (genes) that affect the expression of traits are found on chromosomes.  The chromosome theory of heredity states: That genes are located on the chromosome and each gene occupies a specific place on that chromosome. Each chromosome contains just one allele for each of its genes.

Crossing-Over  Crossing-Over: The exchange of a section of DNA between chromosomes.  Recombinants: Individual organism with a new combination of genes due to crossing-over.

Mutations  Mutation: Change in genetic material.  Chromosomal Mutation: Change in number or structure of chromosomes. Nondisjunction: Failure of chromosomes to separate. Polypoidy: Extra set of chromosomes.  Gene Mutation: Change in the nucleotides of DNA. Point mutations: Single nucleotide. Frameshift mutation: The addition or deletion of a base, altering DNA from the point of mutation on.

Sex Linkage  Discovered by Nettie Stevens.  Sex Chromosome: Chromosome that is different in males (XY) and females (XX), determine the sex of an organism.  Autosomes: Chromosomes that are the same in males and females.  Sex-Linked: Trait that is determined by a gene located on a sex chromosome.

Regulation of Gene Expression  Incomplete Dominance: Inheritance in which an active allele does not entirely compensate for an inactive allele, results in a mixed phenotype (blended trait). Ex. Red Flower + White Flower = Pink Flower  Co-dominance: Condition in which both alleles of a gene are expressed. Ex. A person with one blue eye and one green eye.  Polygenic Inheritance: A trait that is controlled by a multiple alleles of a genes (3 or more).