1. CHAPTER 16 Variations of Chromosome Structure and Number Text authored by Dr. Peter J. Russell Slides authored by Dr. James R. Jabbur

2. Types of Chromosomal Mutations Variations in chromosome structure or number arise spontaneously or are induced, and can be detected by:  Genetic analysis (observing changes in linkage)  Microscopic examination (karyotype assay) Chromosomal aberrations contribute to miscarriage, stillbirth and genetic disorders  About 50% of spontaneous abortions result from major mutations  Visible mutations occur in about 0.6% of all live births  About 11% of men with fertility problems and 6% of the mentally institutionalized have mutations

3. Variations in Chromosome Structure Mutations involving changes in chromosome structure occur in four common types  Deletions, duplications, inversions and translocations Structural mutations begin with a break in the DNA, producing ‘sticky’ ends that may adhere to other ends Variations in structure are studied using polytene chromosomes, which are readily found in the salivary glands of Drosophila  Polytene chromosomes are composed of bundles of chromatids produced by DNA synthesis without mitosis or meiosis  Homologs are tightly paired, joined at the centromeres by a proteinaceous chromocenter.  Polytene chromosomes are easily detectable, possessing detailed banding patterns (they are approximately 1,000 times the normal size with each band averaging 30kb of DNA)

5. Deletion In a deletion, part of the chromosome is missing Deletions start with chromosomal breaks, which are induced by:  Heat or radiation (especially ionizing)  Viruses  Chemicals  Transposable elements  Errors in recombination Deletions can not revert, because the DNA is missing

6. The effect of a deletion depends on what was deleted  A deletion in one allele of a homozygous wild-type organism may give a normal phenotype, while the same deletion in the wild-type allele of a heterozygote would produce a mutant phenotype  Deletion of the centromere results in an acentric chromosome that is lost, usually with serious or lethal consequences. (No known living human has an entire autosome deleted from the genome)  Large deletions can be detected by unpaired loops, which are seen in karyotype analysis Deletion mapping indicates the location of a gene on the chromosome; deletion of the dominant allele in a heterozygote results in the recessive phenotype  The expression of a recessive trait caused by the absence of a dominant allele is termed pseudodominance

7.  Demerec and Hoover studied a fly strain heterozygous for the X-linked recessive mutations y, ac and sc  Genetic analysis shows the 3 loci linked at the left end  Deletion experiments correlated the loss of the dominant alleles with a loss of dominant phenotype and the appearance of pseudodominance

8. Human disorders caused by large chromosomal deletions are generally seen in heterozygotes, since homozygotes usually die Examples of human disorders caused by large chromosomal deletions include:  Cri-du-chat (“cry of the cat”) syndrome, resulting from deletion of part of the short arm of chromosome 5. The deletion results in severe mental retardation and physical abnormalities  Prader-Willi syndrome, occurring in heterozygotes with part of the long arm of one chromosome 15 homolog deleted. The deletion results in feeding difficulties, poor male sexual development, behavioral problems and mental retardation

9. Cri-du chat syndrome

10. Duplication Duplications result from the doubling of chromosomal segments and occur in a range of sizes and locations  Tandem duplications are adjacent to each other  Reverse tandem duplications result in genes arranged in the opposite order of the original  Tandem duplication at the end of a chromosome is a terminal tandem duplication Multigene families (i.e. globin) result from duplications

11.  An example of duplication is the Drosophila eye shape allele, Bar, that reduces the number of eye facets, giving the eye a slit-like rather than oval appearance (the Bar allele resembles an incompletely dominant mutation)  The Bar allele results from the duplication of a small segment (16A) of the X chromosome

12.  Inversion results when a chromosome segment excises and reintegrates oriented 180° from the original orientation  There are two types of inversion:  Pericentric inversions include the centromere  Paracentric inversions do not include the centromere Inversions

13. Inversions generally do not result in lost DNA, but altered phenotypes can arise if the breakpoints are in genes or in regulatory regions*** When close enough, linked genes are often inverted together. The meiotic consequence depends on whether the inversion occurs in a homozygote or a heterozygote  A homozygote will have normal meiosis (always)  The effect in a heterozygote depends on whether crossing-over occurs If there is no crossing-over, no meiotic problems occur. If crossing-over occurs in the inversion, an unequal crossover may produce serious genetic consequences (next slide)

14.  Different recombinant chromosomes are produced by a crossover in a heterozygote, depending on the centromere’s involvement  Paracentric inversions, where no centromere is involved, result in visible inversion loops between homologous chromosomes  Crossover in the inversion region results in unbalanced sets of genes produced for gametes Animation: Inversion Heterozygote Animation: Inversion Heterozygote Crossover

15.  Pericentric inversions that undergo a single crossover result in:  2 viable gametes, one with genes in the normal order, the other with the inversion -and-  2 inviable gametes, each with some genes deleted and others duplicated

16. Translocations A change in location of a chromosome segment is a translocation. No DNA is lost or gained Simple translocations are of two types: Intrachromosomal, with a change of position within the same chromosome Interchromosomal, with transfer of the segment to a nonhomologous chromosome Reciprocal transfer involves a 2 segment exchange

17.  Gamete formation is affected by translocation, resulting in altered gene linkage and unbalanced duplication and/or deletion  Strains homozygous for a reciprocal translocation form normal gametes  Strains heterozygous for a reciprocal translocation must pair a normal set of chromosomes with a translocated set, producing a cross-like structure with partial homology to others in the group  Anaphase segregation is alternate or adjacent (2 forms) Animation: Meiosis in a Animation: Meiosis in a Translocation Heterozygote Translocation Heterozygote

18. Chromosomal Mutations and Human Tumors Most human malignant tumors have chromosomal mutations, commonly in the form of a translocation Examples include:  Chronic Myelogenous Leukemia, involving a reciprocal translocation of chromosomes 9 and 22 Physiologicaly, Myeloblasts (stem cells of white blood cells) replicate uncontrollably. 90% of CML patients have the Philadelphia chromosome reciprocal translocation. The hybrid gene arrangement causes expression of a leukemia-producing gene product, tyrosine kinase. The drug Gleevec blocks the tyrosine kinase, reducing white blood cell proliferation.  Burkitt Lymphoma involves a reciprocal translocation of chromosomes 8 and 14 Burkitt’s is induced by a virus, causing B cells to secrete antibodies as they proliferate The reciprocal translocation positions myc next to an active immunoglobulin gene, causing overexpression

19. Position effect Sometimes inversions or translocations change the phenotypic expression of genes by the position effect; by moving a gene from euchromatin to heterochromatin, or vice-versa (transcription generally occurs in euchromatin but not in heterochromatin) This is an example of an epigenetic effect since the DNA sequence of the gene is not affected An example is the white-eye (w) locus in Drosophila:  An inversion moves the wt + gene (w  ) from a euchromatin region of the X chromosome to a position in heterochromatin  In a hemizygous wt + male (w  ) or a heterozygous female (w  /w), where the wt + gene (w  ) is involved in the inversion, the eyes will have white spots resulting from the cells where the wt + allele (w  ) was moved and inactivated

20. Fragile Sites and Fragile X-syndrome Chromosomes in cultured human cells develop narrowings or unstained areas called fragile sites A well-known example is fragile X syndrome, in which a region at position Xq27.3 is prone to break Fragile X inheritance follows Mendelian patterns, but only 80% of males with a fragile X are mentally retarded The 20% with fragile X but a normal phenotype are transmitting males  A normal transmitting male can pass the chromosome to his daughter(s), where further DNA alteration takes place  Sons of those daughters frequently show mental retardation  About 33% of carrier (heterozygous) females show mild mental retardation  Sons of carrier females have a 50% chance of inheriting the fragile X  Daughters of carrier females have a 50% chance of being carriers

21. Molecular analysis shows a repeated 3-bp sequence, CGG, in the FMR-1 (fragile X mental retardation-1) gene, at the fragile X site  Normal individuals have 6 to 54 repeats, with an average of 29  Normal transmitting carrier males, their daughters, and some other carrier females have 55 to 200 repeats, but do not show symptoms  Individuals with fragile X syndrome have 200 to 1300 repeats, indicating that tandem amplification of this sequence is tolerated until a threshold number of copies is reached The FMR-1 product (FMRP) is an RNA-binding protein, affecting expression of certain mRNA’s in the brain at neuronal synapses Individuals with the full mutation undergo Cytosine (CGG) methylation, silencing the FMR-1 gene, resulting in mental retardation

22. Variations in chromosome number An organism or cell is euploid when it has one complete set of chromosomes or exact multiples of complete sets Eukaryotes that are normally haploid or diploid are euploid, as are organisms with variable numbers of chromosome sets (polyploids) Aneuploidy results from variations in the number of individual chromosomes, so that the chromosome number is not an exact multiple of the haploid set of chromosomes

23. Changes in One or a Few Chromosomes Aneuploidy occurs due to nondisjunction during meiosis I or II Autosomal aneuploidy is not well tolerated in animals and in mammals is detected mainly after spontaneous abortion. Aneuploidy is much better tolerated in plants***…

24. There are four main types of aneuploidy  Nullisomy involves the loss of one homologous chromosome pair (2N-2)  Monosomy involves the loss of a single chromosome (2N-1)  Trisomy involves one extra chromosome, so the cell has three copies of one and two copies of the others (2N+1)  Tetrasomy involves an extra chromosome pair, so the cell has four copies of one and two copies of all the others (2N+2) More than one chromosome or chromosome pair may be lost or added  A double monosomic aneuploidy has two separate chromosomes present in only one copy each (2N-1-1)  A double tetrasomic aneuploidy has two chromosomes present in four copies each (2N+2+2)

25. Human examples of Aneuploidy Why are aneuploidies more common for sex chromosomes?  Autosomal trisomies account for about half of fetal deaths; only a few result in live birth  Most (trisomy-8, -13 and -18) result in early death, with only trisomy-21 (Down syndrome) surviving to adulthood

26. Trisomy-21  Occurs in an estimated 3,510/10 6 conceptions and 1,430/10 6 births  Down syndrome individuals are characterized by a reduced IQ, epicanthal eye folds, short and broad hands, and below-average height  Risks for having afflicted children include increased maternal and paternal age, and maternal smoking with the use of oral contraceptives (oral contraceptives alone do not increase risk)

27. Animation: Robertsonian Animation: Robertsonian Translocaton & Down Syndrome Translocaton & Down Syndrome  Robertosian translocation is a centric fusion of nonhomologous, acrocentric chromosomes, resulting in production of 3 copies of the long arm of chromosome 21  Mating of a heterozygous carrier and a normal individual has a high risk of Down syndrome offspring

28. Trisomy-13 (Patau syndrome)  Occurs in 2/10 4 live births; most die within the first three months  Characteristics include a cleft lip and palate, small eyes, polydactyly (extra fingers and toes), mental and developmental retardation, cardiac and other abnormalities

29. Trisomy-18 (Edwards syndrome) occurs in 2.5/10 4 live births; 90% die within 6 months About 80% of Edwards syndrome infants are female Characteristics include a small size with multiple congenital malformations throughout the body, clenched fists, an elongated skull, low-set ears, mental and developmental retardation

30. Changes in Complete Sets of Chromosomes Monoploidy & polyploidy involve complete sets of chromosomes, thus both are cases of euploidy Monoploidy & polyploidy can result when either round of meiotic division lacks cytokinesis or when meiotic nondisjunction occurs for all chromosomes Even-number polyploids are more likely to be at least partially fertile, unlike odd- number polyploids, which are usually sterile or have increased zygote death

31. Polyploidy is cherished in agriculture, where polyploids include most crops, common flowers and all commercial grains (i.e. bread wheat, Triticum aestivum, an allohexaploid of three plant species) – hybrids are more competetive!  Autopolyploidy results when all sets of chromosomes are from the same species, i.e. seedless watermelon or grapes (who likes eating seeds?)  Allopolyploidy results when the chromosomes are from two different organisms, typically from the fusion of haploid gametes followed by chromosome doubling. An example is a cross between the cabbage and radish of note…