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PCB3063: Chapter 8

by: Brittany Woody

PCB3063: Chapter 8 PCB3603

Brittany Woody

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About this Document

Powerpoint, book, and lecture notes for chapter 8
Dr. W. Brad Barbazuk
Class Notes
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This 4 page Class Notes was uploaded by Brittany Woody on Wednesday September 28, 2016. The Class Notes belongs to PCB3603 at University of Florida taught by Dr. W. Brad Barbazuk in Fall 2016. Since its upload, it has received 5 views. For similar materials see Genetics in Genetics at University of Florida.


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Date Created: 09/28/16
Chapter 8 Chromosome Mutations: Variations in Chromosome Number and Arrangement - Although most diploid species normally contain precisely two haploid chromosome sets, there are many known variations: • a change in the total number of chromosomes • the deletion or duplication of genes or segments of a chromosome • rearrangements of the genetic material either within or among chromosomes - Variations in chromosome number are known as aneuploidy when an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set (2n plus or minus 1); includes monosomy, disomy, trisomy, tetrasomy - In euploidy, complete haploid sets of chromosomes are present (multiples of n); includes diploidy, polyploidy, triploidy, tetraploidy, etc - Chromosomal variation can arise from nondisjunction, in which chromosomes or chromatids fail to disjoin and move to opposite poles during meiosis I or II - The loss of one chromosome to produce a 2n – 1 complement is called monosomy - Although monosomy for the X chromosome occurs in humans, monosomy for any of the autosomes is usually not tolerated in humans and other animals - Another possible cause of lethality of aneuploidy is that a single copy of a recessive gene may be insufficient to provide adequate function for sustaining the organism, a phenomenon called haploinsufficiency - Trisomy(2n + 1 chromosomes) for the sex chromosomes has a less dramatic phenotype than trisomies for autosomes, which are often lethal - In trisomy, three copies of one chromosome are present, so pairing configurations are usually irregular - At any particular region along the chromosome length, only two of the three homologs may synapse, though different regions of the trio may be paired - Down syndrome (trisomy 21); Patau syndrome (trisomy 13); Edwards syndrome (trisomy 18) 1 - Trisomies are often found in spontaneously aborted fetuses, but monosomies are not; this suggests that monosomic gametes may be very functionally impaired - The naming of polyploids is based on the number of sets of chromosomes found: a triploid has 3n chromosomes; a tetraploid has 4n chromosomes; a pentaploid, 5n chromosomes - Polyploidy can originate by: • the addition of one or more sets of chromosomes identical to the haploid complement of the same species (autopolyploidy) • or the combination of chromosome sets from different species as a consequence of interspecific matings (allopolyploidy) - In autopolyploidy, tetraploids more common than triploids; may arise if chromosomes replicate, but parent cell fails to divide before entering - An allotetraploid arises from hybridization of two closely related species - If the sterile hybrid undergoes a natural chromosomal doubling, a fertile amphidiploid is produced - Amphidiploid plants can be produced by somatic call hybridization - Endopolyploidy: condition in which only certain fells in an otherwise diploid organism are polyploid 2 - Rearrangements of chromosome segments include: deletions, duplications, inversions. nonreciprocal translocations, and reciprocal translocations - Deletion: When a chromosome breaks in one or more places and a portion of it is lost, the missing piece is referred to as a deletion (or a deficiency); the deletion can occur near one end (terminal deletion) or from the interior of the chromosome (intercalary deletion) • For synapsis to occur between a chromosome with a large intercalary deletion and a normal complete homolog, the unpaired region of the normal homolog must loop out of the linear structure into a deletion or compensation loop • segmental deletion: partial monosomy small portion of a chromosome is deleted; causes cri du chat syndrome on chromosome 5 - Duplications: repeated segment of the genetic material; duplications arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis • Gene duplication may play a role in evolution • This hypothesis is supported by the discovery of genes that have a substantial amount of their DNA sequence in common, but whose gene products are distinct • Other support includes the presence of gene families, regional groups of genes whose products perform the same general function • Copy number variations (CNVs): quantitative differences in the number of large DNA sequences - Inversion involves a rearrangement of the linear gene sequence rather than the loss of genetic information • an inversion requires two breaks in the chromosome and subsequent reinsertion of the inverted segment • an inversion may arise from chromosomal looping • organisms with one inverted chromosome and one on inverted chromosome are called inversion heterozygotes • dicentric chromatid has two centromeres; eccentric chromatid is lacking a centromere 3 • paracentric inversion allows arm ration to remain unchanged; pericentric inversion changes the arm ratio • Synapsis of inverted chromosomes requires an inversion loop - Translocation is a movement of a chromosomal segment to a new location in the genome • A reciprocal translocation involves the exchange of segments between two nonhomologous chromosomes; has an unusual synapsis configuration during meiosis; shows two possible segregation patterns: - one that leads to a normal and a balanced gamete (called alternate segregation) - one that leads to gametes containing duplications and deficiencies (called adjacent segregation) 4


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