Overview Centromere holds sister chromatids together attachment of mitotic spindle fibers allows chromosomes to be pulled to opposite poles during anaphase variations in position metacentric centromere in the middle submetacentric centromere offset slightly towards one end acrocentric near complete displacement of centromere to one end Nomenclature long = q short = p remember: p = petite translocation = t deletion = del Types of chromosomal alterations Nondisjunction homologous chromatids do not separate properly during meiosis stage of nondisjunction affects gamete production outcome nondisjunction in meiosis I results in 2 gametes with x 2 and 2 gametes x 0 nondisjunction in meiosis II results in 2 normal gametes, 1 gamete x 2 and 1 gamete x 0 zygote receiving 3 copies = trisomy zygote receiving 1 copy = monosomy risk greatly ↑ with ↑ in maternal age more common in oogenesis than spermatogenesis Translocation exchange genetic info between nonhomologous chromosomes by breakage and repair balanced where exchanged fragment is still functional on another chromosome unbalanced where exchanged fragment cannot function properly common in cancers types Robertsonian balanced always involve two acrocentric chromosomes 13, 14, 15, 21, 22 results in loss of short arm and fusion of two long arms of different chromosomes no clinical presentation because short arms of acrocentrics contain no vital info is a translocation carrier problems with gametogenesis and therefore reproduction miscarriage, aneuploidy depends on how chromosomes segregate during homologous pair separation reciprocal exchange of DNA between two non-homologous chromosomes as long as no DNA is lost the phenotype is normal for that generation is a translocation carrier Inversion type of rearrangement where part of chromosome is inverted in orientation types pericentric inverted chromosomal segment includes centromere remember: pericentric involves centromere paracentric inverted chromosomal segment does not include centromere Ring chromosomes causes product of two breakage sites on the chromosome and the segment lost circularizes ends of chromosomes join circularizing entire chromosome usually lost during gametogenesis → monosomy Isochromosome replication of one arm of a chromosome with loss of the other p-q → p-p' lethal for autosomes can be observed on sex chromosomes Deletions loss of chromosome segment types terminal end of chromosome interstitial within the chromosome