BI226 2009 Section 6: Alterations in chromosome number and structure
• Alterations in chromosome number and structure influe nce phenotypic expression
of traits and speciation.
• Aneuploidy is an aberration in which ch romosomes are missing or added so that
chromosome sets are unbalanced.
• Polyploidy, in which there are three or mo re sets of chromosomes in a cell, is
common in plants and is important in plan t evolution, but is rare in animals.
• Alterations in chromosome structure include deletions, duplications, inversions,
translocations, fissions, and fusions. These alterations have important genetic
• Most chromosome alterations in humans do not permit survival of the fetus.
Those chromosome alterations that pe rmit survival usually have medical
syndromes associated with them.