Chapter 7 Outline A 7 1 Chromosome Mutations Include Rearrangements Anueploids and Polyploids 1 Chromosome mutations variations in the number and structure of chromosomes They 2 Metacentric The centromere is located approximately in the middle so the chromosome 3 Submetacentric The centromere is displaced toward one end creating a long arm and a 4 Acrocentric the centromere is near one end producing a long arm and a knob or satellite play an important role in evolution has two arms of equal length short arm short arm p long arm q at the other end 5 Telocentric The centromere is at or very near the end of the chromosome Acrocentric Telocentric Metacentric Submetacentric B 7 2 Chromosome Rearrangement Alter Chromosome Structure 1 Chromosome rearrangement mutations that change the structure of individual chromosomes 2 Chromsome duplication a mutation in which part of the chromosome has been double a AB CDEFG b tandem duplication the duplicated region is immediately adjacent to the original segment AB CDEFEFG i i i c Displaced duplication the duplicated segment is located some distance from the original segment either on the same chromosome or on a different one d Reverse duplication an inverted duplication AB CDEFGEF AB CDEFFEG e An individual homozygous for a duplication carries the duplication mutated sequence on both homologous chromosomes f An individual heterozygous for duplication has one unmutated chromosome and one chromosome with duplication The problems arise in chromosome pairing at prophase I of meiosis because the two chromosomes are not homologous in length g The only change is the presences of additional copies of normal sequences likely due to an imbalance in the amounts of gene products h The amount of a particular protein synthesized by a cell is often directly related to the number of copies of its corresponding gene i If the amount of one protein increases while the amounts of others remain constant problems can result 3 Chromosome Deletion the loss of a chromosome segment that causes the chromosome to be noticeably shorter a AB CDEFG to AB CDG b Individuals heterozygous for deletions the normal chromosome must loop during the pairing of homologs in prophase I of meiosis to allow the homologous regions of the two chromosomes to align and undergo synapsis c If the deletion includes the centromere the chromosome will not segregate in meiosis or mitosis and will usually be lost d Individuals homozygous for deletions are lethal because all copies of any essential genes located in the deleted region are missing e Individual heterozygous for a deletion may have multiple defects for three reasons i ii iii iv v The heterozygous condition may produce imbalances in the amounts of gene products similar to the imbalances produced by extra gene copies Recessive mutations on the homologous chromosome lacking the deletion may be expressed when the wild type allele has been deleted Pseudodominance the expression of a recessive mutation that is an indication that one of the homologous chromosomes has a deletion Some genes must be present in two copies for normal function Haploinsufficient gene a single copy of a gene is not sufficient to produce a wild type phenotype 4 Chromosome Inversions a chromosome segment is inverted turned 180 a AB CDEFG to AB CFEDG b Paracentric inversions inversions that do not include the centromere such as the one above i ADC BEFG has been altered c Pericentric inversion inversions that include the centromere such as d Inversions have neither lost nor gained any genetic material just the gene order e Position effect positions are altered by an inversion they may be expressed at inappropriate times or in inappropriate tissues f Homozygous individuals for inversions have no special problems arise in meiosis and the two homologous chromosomes can pair and separate normally g Heterozygous individuals for inversions have a differing gene order and the two homologous sequences can align and pair only if the two chromosomes form an inversion loop They also exhibit reduced recombination When crossing over does take place the result is abnormal gametes that result in nonviable offspring h Dicentric chromatid has two centromeres i Acentric chromatid lacks a centromere In anaphase I of meiosis the centromeres are pulled toward opposite poles and the two homologous chromosomes separate The previous action stetches the dicentric chromatid across the center if the nucleus forming a dicentric bridge Eventually the bridge breaks and the acentric fragment is lost Recombination is reduced within a pericentric inversion The recombinant chromosomes have too many copies of some genes and no copies of others so gametes that receive the recombinant chromosomes cannot produce viable progeny 5 Translocation entails the movement of genetic material between nonhomologous chromosomes or within the same chromosome a Nonreciprocal translocation genetic material moves from one chromosome to another without any reciprocal exchange AB CDEFG to MN OPQRS AB CDG to MN OPEFQRS b Reciprocal translocation a two way exchange between of segments between chromosomes AB CDEFG to MN OPQRS AB CDQRG to MN OPEFS c Translocations can create new linkage relations that affect gene expression i i ii iii iv i i d The chromosomal breaks that bring about translocations may take place within a gene and disrupt its function e Robertsonian translocation long arms of two acrocentric chromosomes become joined to a common centromere through a translocation generating a metacentric chromosome with two long arms and another chromosome with two very short arms The smaller chromosome often fails to segregate leading to an overall reduction in chromosome number f The effects of translocation on chromosome segregation in meiosis depend on the nature of the translocation g Heterozygous for a reciprocal translocation with the above translocation would posses one normal copy of each chromosome and one translocated copy i Each of the chromosomes contains segments that are homologous sequences pair in prophase I of meiosis crosslike configurations consisting of all four chromosomes h Whether viable or nonviable gametes produced depends on how the chromosomes in these crosslike configurations separate i Only about half of the gametes from an individual heterozygous for a reciprocal translocation are expected to be functional and so these individuals frequently exhibit reduced fertility 6 Fragile sites chromosomes of
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