Bio 240 Chapter 8 notes!
Bio 240 Chapter 8 notes! Bio 240
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This 5 page Class Notes was uploaded by Izabella Nill Gomez on Saturday October 10, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 39 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 10/10/15
Biology 240 Chapter 8 notes Chromosome mutationsaberrations modi cations in the total number of chromosome sets deletions duplication of genes or segment 5 of a chromosome and rearrangements of genetic material within or among chromosomes These are passed on to offspring in a predictable manner Aneuploidy an organism gains or loses one or more chromosomes but not a complete set Loss of one chromosome from a diploid genome is monosomy Gain of a chromosome is triploidy Euploidy complete haploid sets of chromosomes are present Polyploidy refers to the addition of more than 2 sets With three sets are triploid four are tetraploid Loss of one chromosome results in 2n1 or monosomy Ex 45 X Turner syndrome monosomy of autosomes is generally not tolerated in mammals Monosomy unmasks recessive lethals that are otherwise tolerated in heterozygotes carrying wild type alleles Haploinsuf ciency may also occur Aneuploidy is better tolerated in plants although they are less viable Trisomy 2n1 addition of one chromosome produces somewhat more viable individuals in both animal and plant species than does the loss of a chromosome In plants trisomy is viable but the phenotype may be alteredresults in slower growth Down syndrome Trisomy 21 found in approximately 1800 live births Causes speci c characteristics such as the simian fold on the hands epicanthic fold of the eyes and at face and round head Exhibits mental retardation and poor muscle tone It is suggested that a critical region of the chromosome 21 contains dosage sensitive genes that are responsible for altered phenotypes the region portion is Down Syndrome critical region DSCR Findings related to the region developed that these individuals are at a lower risk for cancerous tumors Correlated to an extra copy of the DSCR1 gene which encodes protein for suppressing of vascular endothelial growth factor VEGF In turn it blocks angiogenesis Ovum is the source of 95 of trisomy cases Passing 45 the woman is more likely to develop errors in meiosis related to the ovum which leads to Down39s Amniocentesis or CVS allows women to culture eggs to test if they are at risk for a Down39s child In a new approach noninvasive prenatal genetic diagnosis NIPGD is used and DNA and fetal cells are tested Down syndrome is a random accident of nondisjunction of chromosome 21 but can be inherited in some cases familial Down Syndrome involves the translocation of chromosome 21 Patau and Edwards syndrome 47 13 47 18 lethal diseases in which individuals live only for a few years 20 of all conceptions terminate in spontaneous abortion 30 of aborted fetuses have a chromosomal abnormality These observations support the hypothesis that normal embryonic development requires a precise diploid complement of chromosomes to maintain the delicate balance in the expression of genetic information Polyploidy originates in two ways 1 Addition of one or more extra sets of chromosomes identical to normal haploid complement of the same species which results in autoploidy or 2 The combination of chromosome sets from different species occurring as a consequence of hybridization resulting in alloploidy A normal organism is represented simply as AA A is the haploid set of chromosomes Autoploids arises either by failure of the chromosomes to segregate during meiotic divisions and is fertilized by haploid gametes or two sperm fertilize an ovum Autotetraploids more easily found in nature due to the even number of chromosomes ln crops colchicine is added to plants to inhibit spindle formation and create replicated and triploid fruit generally sterile due to the imbalanced number of chromosomes In general autopolyploids are larger due to larger cell size Ex triploid strawberries bananas watermelons Amphidiploid hybridization of two species results in imbalanced numbers and inviable zygotes normally but some species can create balanced chromosomes to produce a zygote Ex cotton Amphidiploids often express traits of both parents Endopolyploidy condition in which only certain cells in an otherwise diploid organism are polyploid In such cells the set of chromosomes replicates repeatedly without nuclear division Ex human liver cells Although their role is not clear proliferation of chromosome copies often occurs in cells where high levels of certain gene products are required If an aberration is found in one homolog of a chromosome but not the other an individual is said to be heterozygous for the aberration Unusual pairing of the arrangement of chromosomes often leads to gametes that are duplicated or de cient for some chromosomal regions Offspring of quotcarriersquot are more likely to demonstrate phenotype changes Deletion when a chromosome breaks in one or more places and a portion of it is lost Can occur near one end terminal deletion or within the interior of the chromosome intercalary deletion The portion that contains the centromere region is usually maintained when the cell divides whereas the segment without it is eventually lost in the progeny cells following meiosismitosis For synapsis to occur between a chromosome with a large intercalary deletion and a normal homolog the unpaired region must buckle out into a compensation loop Cri du chat syndrome results from deletion of small terminal portion of chromosome 5 Considered to be a segmental deletion Associated with the loss of a small variable part of the short arm of chromosome 546 5p lnfants exhibit malformations and often mentally retarded Abnormal development of the larynx leads to the characteristic cry 12550000 births Results from sporadic loss of the small piece of chromosomal material in gametes Missing piece encodes for telomerase reverse transcriptase essential for maintaining of telomeres during DNA replication Duplication any part of the genetic material is present more than once in the genome As in deletions pairing loop can be used to compensate Duplications can arise because of unequal crossing over between chromosomes during meiosis or replication error before meiosis rDNA codes for rRNA and multiple genes code for itgene redundancy Sometimes even having multiple genes to code for rRNA is not enough to supply for construction of ribosomes needed for oocytes of the ovum and essential in early development of embryo Gene ampli cation increases the amount of rRNA that can be encoded Genes that code rRNA are located in the nucleolar organizer region NOR Associated with nucleolus To amplify rDNA is selectively replicated and form new NORs Duplications can cause phenotypic variation that may appear to be caused by a simple gene mutation Ex bareye phenotype in Drosophila Process of gene duplication seems to be the major source of new genes based on supposition that products of many genes are indispensable to survival of members of any species in evolution Therefore unique genes are not free to accumulate mutations to alter their primary function and produce new genes However if duplication occurs of the unique genes mutations will be tolerated because they are essential for survival The Duplication can receive many mutational changes However over time it may change enough to diverge and produce a new role in a cell New function may be an adaptive advantage Gene families groups of contiguous genes whose products perform the same or similar functions Share a common origin and arose through gene duplication Ex different human hemoglobin polypeptide chains Dupications of portions of genes occur on a regular basis Copy number variants CNVs variations in copies of any duplicated gene that represent differences in the number of large DNA sequences found both in coding and noncoding regions of the genome Play crucial role in many of individual traits Have both positive and negative associations with diseases Inversion type of chromosomal aberration in which a segment of a chromosome is turned around 180 degrees within a chromosome Rearranges linear gene sequence Requires two break points and reinsertion of the inverted segment May or may not include the centromere if the centromere is not part of the rearranged chromosome it is a paracentric inversion It the centromere is part of it it is a pericentric inversion If only one member of a homologous pair of chromosomes has an inverted segment normal linear synapsis not possible Inversion heterozygotes are organisms with one inverted chromosome and a noninverted homolog Two chromosomes in meiosis can only pair with an inversion loop If crossing over does not occur within inverted segments of the inversion loop the homologs will segregate which results in two normal and two inverted chromatids distributed into gametes However if crossing over occurs within inversion loop abnormal chromatids are produced When crossover occurs within a paracentric inversion one recombinant dicentric chromatid with 2 centromeres and one recombinant acentric chromatid without a centromere are produced Both have duplications and deletions of chromosome segments During anaphase acentric may be lost or randomly distributed Dicentric is pulled in both directionsproduces dicentric bridges and the dicentric chromatid usually breaks at some point Usually zygote produced is abnormal if it even develops Similar chromosomal imbalance occurs in pericentric inversion Recombinant chromatids have duplications and deletions lnviable embryos usually produced In inversion heterozygotes the inversion has the effect of suppressing the recovery of crossover products when chromosome exchange occurs Viability therefore greatly diminished Because recovery of crossover is suppressed groups of speci c alleles in adjacent loci within inversions may be preserved from generation to generation If the alleles confer a survival advantage the inversion is bene ciary Translocation movement of a chromosomal segment to a new location in the genome Reciprocal translocation involves exchange of segments between two non homologous chromosomes Occurs if the arms come close to each other so that an exchange is facilitated Genetic consequences similar to inversions Genetic information not lost or gained only rearrangement Homologs heterozygous for reciprocal translocation undergo unorthodox synapsis in crosslike con guration Some gametes produced are genetically imbalanced Alternate segregation at the rst meiotic division can occur Second pattern is adjacent segregation where homologous centromeres migrate to the same pole When genetically imbalanced gametes participate in fertilization in animals resultant offspring generally do not survive Result in parent having semisterility where only few offspring are produced if fortunate Robertsonian translocation Involves breaks at the extreme ends of the short arms of two nonhomoogous acrocentric chromosomes Small segments are lost but the larger segments fuse at their centromeric region Creates large submetacentric chromosome Accounts for one instance of familial Down syndrome where one parent has a 1421 DG translocation One parent is phenotypically normal through shehe has only 45 chromosomes and is a balanced translocation carrier All offspring that parent has will have Down syndrome due to the double copy of chromosome 21 Fragile Xsyndrome is most common form of inherited mental retardation This syndrome affects 14000 males and 18000 Gene responsible is FMR1 in which a sequence of 3 nucleotides is repeated many times expanding the size of the gene
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