Molecular Genetics

Aneuploidy assists in assigning linkage groups to morphological chromosomes.

Facts

Aneuploidy refers to the chromosomal state of a cell with abnormal numbers of specific chromosomes or chromosome sets. Multiploidy refers to possession of greater than normal numbers of chromosomes. There are also specific terms for certain numbers of abnormal numbers of chromosomes, such as triploid, tetraploid, hexaploid, etc.

		The r-x1 mutation in maize, when present in the female parent results in monosomic (one chromosome present in just a single copy instead of the normal two) progeny at high frequency. Pollen from a homozygous mutant is used to fertilize an ear on an r-x1 plant (homozygous w.t. for the test marker). F1 progeny (left) are screened for those with the mutant phenotype. Mutant phenotype plants are examined for which chromosome is monosomic.
Phenotype	Genotype
w.t.	A/a
w.t.	A (monosomic)
mut.	a (monosomic)
For many species, trisomic lines can be produced and propagated. These lines have three copies of one chromosome and two copies of all the rest. Crossing a homozygous mutant with a line trisomic for a chromosome other than the one containing the gene results in a standard 3:1 segregation in the F2 (right, upper). Crossing a homozygous mutant with a line trisomic for the chromosome on which the mutant gene is located results in a deviation from the standard 3:1 segregation in the F2 (right, lower).

Interpretations

• Consistent identification of monosomy for one chromosome in progeny with the mutant phenotype is good evidence for the assignment of that gene to that chromosome.
• All genes linked to the gene studied are also assigned to this morphological chromosome.
• Identification of aberrant segregation with a trisomic line is good evidence for the assignment of the test gene to the chromosome that is trisomic.
• All genes linked to the gene studied are also assigned to this morphological chromosome.

Further information

• Similar systems for assigning linkage groups to chromosomes are available for other organisms.
• Monosomy and trisomy result from recombination after pericentric inversions.
• Human genetic diseases may result from trisomy or monosomy. Trisomy and monosomy may play a role in prion diseases.

This is page 1211 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2000-1, 2003