CHAPTER 6 CHROMSOME MUTATIONS VARIATION IN CHROMSOME NUMBER AND ARRANGEMENT Introduction chromosomes Most members of diploid species normally contain precisely two haploid sets of Chromosome Mutations or Chromosome Aberrations are variations from this normal pattern which include o A change in the total number of chromosomes o The deletion or duplication of genes or segments of a chromosome o Rearrangements of the genetic material either within or among Some of these changes can result in some form of phenotypic variation and may chromosomes be lethal in some cases The Chromosome is the unit of genetic transmission and therefore chromosome aberrations are passed to offspring in a predictable manner 6 1 Variation in Chromosome Number Terminology and Origin Aneuploidy a condition in which an loses one or more chromosomes and has multiple of the haploid set o The loss of a single chromosome diploid genome is called monosomy 2n 1 o The gain of one chromosome results in trisomy 2n 1 Euploidy condition where complete haploid sets are present organism gains or other than an exact from an otherwise o Polyploidy occurs when more than two sets of chromosomes are present o Organisms with three o Organisms with four sets are triploidy 3n sets are tetraploidy 4n and so on Klinefelter Syndrome gain 47 XXY of an X chromosome 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 organisms 6 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 monosomy o 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 years o It has 12 14 characteristics and affected individuals express 6 8 on average o 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 21 o 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 of 47 21 trisomy cases Monosomy and trisomy can occur for both sex chromosomes Monosomy involving any autosome is fatal and results in miscarriage early in pregnancy 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 manifest severe 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 information 6 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 or chromosomes identical to complement of the same Autopolyploidy o The combination of more extra sets of the normal haploid species resulting in chromosome sets from different species occurring 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 gamete to produce a zygote with three sets of chromosomes o Two sperm may fertilize a ovum simultaneously to produce a triploid zygote o 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 and replicated chromosomes from separating during Once it is removed the cell can reenter interphase to produce a polyploidy organism 4n in the case of the diagram G1 cyclins are repressed when ploidy increases thus prevents anaphase 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 derived from separate species o Sterile hybrids can undergo a natural or induced chromosomal doubling
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