Lecture 6 April 19, 2012 “The Chromosome Theory of Inheritance Continued” ****Continuation of Presentation 3**** Mitosis: one cell divides into two cells and have exact genetic information MeiosisInterphaseExplaining the Principle of SegregationThe Origins of Genetic Variation - Offspring of sexual reproduction are genetically different from their parents and from one anoher o Independent Assoirtment o Random Fertilization o Recombination by Crossing Over - Result: New allelic combinations The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygoteWhen Meiosis Goes Awry in Humans - What happens when errors occur in meiosis? o Chromosome number in adults is either less than or larger than 46 o Aneuploidy: a numerical change in part of the genome o Most chromosome number abnormalities results in lethalityTrisomy - Trisomy: an individual carrier one extra chromosome (2n + 1 ) o Most trisomies in humans are lethal - Example: trisomy 21 or down syndrome (47, + 21_ o Viable trisomy in humans Sex linked