Variation in Chromosome Number Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 22, 2011


Variations in chromosome number may occur that do not involve whole sets of chromosomes, but only parts of a set. The term aneuploidy is given to variations of this nature, and the suffix "-somic" generally refers to a particular organism and its chromosome number (which may be an abnormal situation).

  1. Monosomic. Diploid organisms that are missing one chromosome of a single pair are monosomics with the genomic formula 2n – 1. The single chromosome without a pairing partner may go to either pole during meiosis, but more frequently will lag at anaphase and fails to be included in either nucleus. Monosomics can thus form two kinds of gametes, (n) and (n – 1). In plants, the n – 1 gametes seldom function. In animals, loss of one whole chromosome often results in genetic unbalance, which is manifested by high mortality or reduced fertility.
  2. Trisomic. Diploids that have one extra chromosomeare represented by the chromosomal formula 2n + 1.One of the pairs of chromosomes has an extra member, so that a trivalent structure may be formed during meiotic prophase. If two chromosomes of the trivalent go to one pole and the third goes to the opposite pole, then gameteswill be (n + 1) and (n), respectively. Trisomy can produce different phenotypes, depending upon which chromosome of the complement is present in triplicate. In humans, the presence of one small extra chromosome (autosome 21) results in Down syndrome, which is a condition characterized by mental retardation and a variety of physical features and malformations.
  3. Tetrasomic. When one chromosome of an otherwise diploid organism is present in quadruplicate, this is expressed as 2n + 2. A quadrivalent may form for this particular chromosome during meiosis that then has the same problem as that discussed for autotetraploids.
  4. Double Trisomic. If two different chromosomes are each represented in triplicate, the double trisomic can be symbolized as 2n + 1 + 1.
  5. Nullosomic. An organism that has lost a chromosome pair is a nullosomic. The result is usually lethal to diploids (2n – 2). Some polyploids, however, can lose two homologues of a set and still survive. For example, several nullosomics of hexaploid wheat (6n – 2Þ exhibit reduced vigor and fertility but can survive to maturity because of the genetic redundancy in polyploids.

Practice problems for these concepts can be found at:

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