Genetics 3301

Chapter 15: Chromosome Alterations

Changes in chromosome number:

Ä Aberrant euploidy alters the number of chromosome sets (T15-1); Monoploids are found in certain insects and develop by parthenogenesis.

Ä Polyploidy is mainly seen in plants and results in larger size (15-4); Autopolyploids arise from within a species; Triploids are autopolyploids that produce inviable aneuploid progeny (15-5); Autotetraploids can be generated or spontaneous and can generate gametes to produce viable progeny (15-6, 15-7); allopolyploids are hybrids between two species; Crossing cabbage and radish to produce an amphidiploid that is viable and fertile (15-8); Allopolyploids within the Brassica genus and for wheat (15-9, 15-10); Propagating monoploids by cold treating pollen grains to create embryoids (15-11); Autotriploids like bananas and watermelons generate fruits without seeds; Autotetrapolids generate larger fruits than diploids (15-12).

Ä Polyploid animals are found in some amphibians and reptiles, salmon, and triploidy has been exploited in oysters to avoid the spawning season.

Ä Aneuploid animals can be trisomic (2n + 1), monosomic (2n -1) and nullosomic (2n - 2); Aneuploid sex chromasome nomenclature; Nondisjunction during meiosis is the cause of most aneuploidy (15-13); Nondisjunction is correlated with lack of meiotic crossing-over, suggesting that crossing-over is important for normal disjunction.

Ä Human monosomics for the X-chromosome (Turner Syndrome) occurs in 1:5000 births; Turners Syndrome phenotypes (15-14).

Ä Human trisomics for the X chromosome (Klinefelters Syndrome) occur in 1:1000 births; Phenotypes of Klinefelters Syndrome; XYY males and XXX females are apparently phenotypically normal and fertile; Trisomy 21 (Down Syndrome) occurs spontaneously in 0.15% of births; Down Syndrome phenotypes (15-17); Down Syndrome increases with the age of the mother (15-18); Other autosomal trisomics rarely live; Gene dosage balance is important because gene products often must be present within a narrow range; The X chromosome is dosage compensated by doubling the expression of X chromosome genes in male Drosophila or inactivating one X chromosome in female humans.

Changes in Chromosome Structure:

Ä Inversions result from breaks within a chromosome (15-20); The centromere is outside the inverted part of the chromosome in a paracentric inversion and span the centromere in a pericentric inversion; Crossing-over within a paracentric inversion loop results in a dicentric bridge and an acentric fragment and two normal products (15-22); Crossing-over within a pericentric inversion generates two normal and two abnormal products (15-23).

Ä Reciprocal translocations exchange parts of two chromosomes; Semisterility of products from adjacent 1 and alternate segregation (15-24)

Ä Deletions can be used to map genes via pseudodominance (15-29); Adverse v affects of deletions in human chromosomes ≠ cri du chat syndrome and Williams syndrome (15-30, 15-31).

Key terms: Know all of these except balanced rearrangement, balancer, deletion loop, genetic load, homeologous, insertional duplication, position effect variegation, pseudolinkage, tandem duplication.

Problems: 1 - 3, 5, 9, 11 -16, 20-23, 24, 26, 27.