The three basic types of chromosome mutations are (1) chromosome rearrangements, which are changes in the structure of chromosomes; (2) aneuploidy, which is an increase or decrease in chromosome number; and (3) polyploidy, which is the presence of extra chromosome sets.
Chromosome rearrangements include duplications, deletions, inversions, and translocations.
In individuals heterozygous for a duplication, the duplicated region will form a loop when homologous chromosomes pair in meiosis. Duplications often have pronounced effects on the phenotype owing to unbalanced gene dosage. Segmental duplications are common in the human genome.
In individuals heterozygous for a deletion, one of the chromosomes will loop out during pairing in meiosis. Deletions may cause recessive alleles to be expressed.
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Pericentric inversions include the centromere; paracentric inversions do not. In individuals heterozygous for an inversion, the homologous chromosomes form inversion loops in meiosis, and reduced recombination takes place within the inverted region.
In translocation heterozygotes, the chromosomes form crosslike structures in meiosis.
Fragile sites are constrictions or gaps that appear at particular regions on the chromosomes of cells grown in culture and are prone to breakage under certain conditions.
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Nullisomy is the loss of two homologous chromosomes; monosomy is the loss of a single chromosome; trisomy is the addition of a single chromosome; and tetrasomy is the addition of two homologous chromosomes.
Aneuploidy usually causes drastic phenotypic effects because it leads to unbalanced gene dosage.
Primary Down syndrome is caused by the presence of three full copies of chromosome 21. Familial Down syndrome is caused by the presence of two normal copies of chromosome 21 and a third copy that is attached to another chromosome through a translocation.
All the chromosomes in an autopolyploid derive from one species; chromosomes in an allopolyploid come from two or more species.