CHAPTER 6:CHROMSOME MUTATIONS:VARIATION IN CHROMSOME NUMBER AND ARRANGEMENTIntroduction- Most members of diploid species normally contain precisely two haploid setsof chromosomes- Chromosome Mutations or Chromosome Aberrations are variations from this normal pattern, which include:o A change in the total number of chromosomeso The deletion or duplication of genes or segments of a chromosome,o Rearrangements of the genetic material either within or among chromosomes- Some of these changes can result in some form of phenotypic variation and may be lethal in some cases- The Chromosome is the unit of genetic transmission and therefore chromosome aberrations are passed to offspring in a predictable manner6.1 Variation in Chromosome Number: Terminology and Origin- Aneuploidy – a condition in which anorganism gains or loses one or morechromosomes and has other than an exactmultiple of the haploid set o The loss of a single chromosomefrom an otherwise diploid genomeis called monosomy(2n-1)o The gain of one chromosomeresults in trisomy(2n+1)- Euploidy – condition where completehaploid sets are present o Polyploidy occurs when morethan two sets ofchromosomes arepresento Organisms with threesets are triploidy(3n)o Organisms with foursets are tetraploidy (4n) and so on.- Klinefelter Syndrome – gainof an X chromosome (47,XXY)- Turner Syndrome – loss of an X chromosome (45,X) Example of monosomy- Nondisjunction – random error during the production of gametes that results in loss or gain of a sex chromosome (Turner and Klinefelter are examples). Paired homologs fail to disjoin during meiosis I or II leading to a variety of conditions in humans and other organisms6.2 Monosomy and Trisomy Result in a Variety of Phenotypic Effects- Monosomy – the loss of one chromosome (2n-1)o Monosomy of Autosomes is not usually tolerated in humans or other animals.- Haploinsufficiency – a phenomenon in which a single copy of a recessive gene, due to monosomy is sufficient to provide life-sustaining function for the organism- Trisomy – the gain of one chromosome (2n+1)o More sustainable than monosomyo For the plant Jimson weed, Datura, whose diploid number is 24, 12 trisomy conditions are possible, each providing a different phenotype.- Down Syndrome: Trisomy 21 o Trisomy of chromosome 21 (47, 21+) is a result of non-disjunction of the chromosome during meiosis, thus it is not to be inherited.o Average life span of about 50 yearso It has 12-14 characteristics and affected individuals express 6-8 on averageo Down Syndrome Critical Region (DSCR) – the critical portion of chromosome 21 that contains genes that are dosage sensitive and are responsible for the many phenotypes associated with Trisomy 21o Individuals with down syndrome have a decreased risk of developing a number of cancers due to the presence of an extra copy of the DSCR1 gene which encodes a protein that suppresses vascular endothelial growth factor (VEGF)o The ovum is the source of the additional chromosome in about 95% of47, 21+ trisomy cases- Monosomy and trisomy can occur for both sex chromosomes- Monosomy involving any autosome is fatal and results in miscarriage early inpregnancy- Trisomy involving autosomes often result in the death of the fetus. Trisomy 21 (Down syndrome) is the only one in which survival into adult hood is possible- The trisomy of 13th and 18th chromosomes is Patau Syndrome (47, 13+) and Edwards syndrome (47,18+) respectively. These are the only other two trisomy’s besides down syndrome that survive to term but they manifestsevere malformations and early lethality.- Amniocentesis and Chronic Villus Sampling (CVS) are both prenatal diagnostic techniques, where fetal cells are obtained from the amniotic fluid or the chorion of the placenta, respectively- Noninvasive Prenatal Genetic Diagnosis (NIPGD) is a newer technique in which fetal cells and DNA are derived directly from the maternal circulation. Posing no risk to the fetus.o Fetal cells are cultured and the karyotype can be determined by cytogenetic analysis- Normal embryonic development requires a precise diploid complement of chromosomes to maintain the delicate equilibrium in the expression of genetic information6.3 Polyploidy, In Which More Than Two Haploid Sets of Chromosomes Are Present, Is Prevalent in Plants- Polyploidy – instances in which more than two multiples of the haploid chromosome set are found.o Triploid has 3n chromosomes o Tetraploid has 4n, Pentaploid has 5n chromosomes and so forth- Odd numbers of chromosome sets are not maintained from generation to generation because a polyploidy organism with an uneven number of homologs often does not produce genetically balanced gametes.- Therefore, triploid, pentaploid and so on are not usually found in plant species that depend solely on sexual reproduction for propagation- Polyploid originates in two ways: o The addition of one ormore extra sets ofchromosomes, identicalto the normal haploidcomplement of the samespecies resulting in Autopolyploidyo The combination ofchromosome sets fromdifferent speciesoccurring as a consequence of hybridization resulting in Allopolyploidy - In Autopolyploids each additional set of chromosomes is identical to the parent species but they are much larger- Autotriploids can arise in several ways:o Failure of chromosomes to segregate during meiotic divisions produces a diploid gamete which can be fertilized by a haploid gameteto produce a zygote with three sets of chromosomeso Two sperm may fertilize a ovum simultaneously to produce a triploid zygoteo Diploid X Tetraploid produces a triploid zygote as well- Autotetraploids (4n) are more likely to be found in nature then autotriploids because they have an even number of chromosomes and produce balanced gametes- Tetraploids arise when chromosomes have replicated and the parent cell fails to divide and instead entering interphase: the chromosome number will have duplicated. - Colchicine interferes with spindle formation andthus prevents replicated chromosomes fromseparating during anaphase. Once it is removedthe cell can reenter interphase to produce apolyploidy organism (4n in the case of thediagram). - G1 cyclins are repressed when ploidy increases.o They facilitate the cells movement through G1 of the cell cycle which is delayed when expression of theses genes is repressed- An allotetraploid/amphidiploid results from hybridization of two closely related species.o Polyploid contains four haploid genomes
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