Chapter08_SSM - 65781_CH08_152_174.qxd 8/1/08 12:58 PM Page...

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152 Chapter 8: Human Karyotypes and Chromosome Behavior Chapter Summary The normal human chromosome complement consists of 22 pairs of autosomes, assigned numbers 1 through 22 from longest to shortest, and one pair of sex chromosomes (XX in females and XY in males). Fetuses that contain an abnormal number of autosomes usually either fail to complete normal embryonic development or die shortly after birth, though people with Down syndrome (trisomy 21) sometimes survive for several decades. Persons with excess sex chromosomes survive, because the Y chromosome contains relatively few genes other than the master sex-controller SRY , and because only one X chromosome is genetically active in the cells of females (dosage compensation through the single-active-X principle). The mosaic orange and black pattern of the female “calico” cat results from X inactivation, because these alternative coat color alleles are X linked in cats. Most structural abnormalities in chromosomes are duplications, deletions, inversions, or translocations. In a duplication, there are two copies of a chromosomal segment. In a deletion, a chromosomal segment is missing. An organism can often tolerate an imbalance of gene dosage resulting from small duplications or deletions, but large duplications or deletions are nearly always harmful. Chromosome rearrangements may affect gene expression through position effects. Although most genes can be expressed whatever their location in the genome, the level of expression may vary substantially according to position. A major effect of gene location on expression is observed in position-effect variegation (PEV), in which a wildtype allele repositioned in or near heterochromatin is unable to be expressed in a fraction of the cell lineages. A chromosome that contains an inversion has a group of adjacent genes in reverse of the normal order. Expression of the genes is usually unaltered, so inversions rarely affect viability. However, crossing-over between an inverted chromosome and its noninverted homolog in meiosis yields abnormal chromatids. Crossing-over within a heterozygous paracentric inversion yields an acentric chromosome and a dicentric chromosome, both of which also have a duplication and a deficiency. Crossing-over within a heterozygous pericentric inversion yields monocentric chromosomes, but both have a duplication and a deficiency.
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153 Two nonhomologous chromosomes that have undergone an exchange of parts constitute a reciprocal translocation. Organisms that contain a reciprocal translocation, as well as the normal homologous chromosomes of the translocation, produce fewer offspring (this is called semisterility) because of abnormal segregation of the chromosomes in meiosis. The semisterility is caused by the aneuploid gametes produced in adjacent-1 and adjacent-2 segregation. Alternate segregation yields equal numbers of normal and translocation-bearing gametes. In genetic crosses, the semisterility of a heterozygous translocation behaves
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Chapter08_SSM - 65781_CH08_152_174.qxd 8/1/08 12:58 PM Page...

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