Chromosomal Mutations Genetic Variation at the Chromosomal level o in Ch 14 we looked at how mutations occur at the nucleotide level but large scale rearrangements of genetic material also occurs can have profound consequences o Can include changes in number of chromosomes deletion of segments of chromosomes rearrangements of chromosomes Terminology for altered chromosomal number o plants are much more tolerant of ploidy than animals o Aneuploidy 2n x chromosomes Monosomy 2n 1 Disomy 2n Trisomy 2n 1 Tetrasomy Pentasomy etc 2m 2 2n 3 etc o Euploidy multiples of n Diploidy 2n Polyploidy 3n 4n 5n Triploidy 3n Tetraploidy pentaploidy etc 4n 5n etc Autopolyploidy mult of the same genome Allopolyploidy amphidiploidy mult of closely related genomes o Non disjunction causes changes in chromosome number outcome depends on when nondisjunction occurs in meiosis I or meiosis II monosomy can result in haploinsufficiency o Monosomy 2n 1 o Downs syndrome loss of a single or part of a chromosome can have large effects ex cri du chat syndrome has a segmental deletion a piece of the short arm of one member of the chromosome 5 homolog caused by an extra copy of chromosome 21 occurs in about 1 800 births series of about 12 characteristics associated and individuals live to about 50 prone to respiratory and heart malformations one region of the chromosome seems to be critical for this condition incidence increases with maternal ages although extra X chromosome may be donated by father in 95 of cases the extra chromosome comes from the egg possibly because the eggs have a long life formed before birth o Trisomies few are viable in humans other than trisomy 21 only trisomy 15 Patau syndrome and 18 Edwards syndrome survive to term with severe malformations and early lethality Estimated that about 20 of all conceptions about natural about 30 of these have chromosomal abnormalities o Polyploidy 2n prevalent in plants is where there is more than 2n chromosomes few animals are polyploidy but not uncommon in plants polyploids tend to be even numbered Autopolyploids have mult of the same chromosomes Allopolyploids have mult of chromosomes from two closely related species o Colchicine spindle inhibitor that can induce autopolyploidy in cells inhabits spindle formation causing anaphase to fail and the chromosome number to double if the cell re enters interphase autopolyploid cells tend to be larger than diploid cells this is important in many plants such as potatoes and bananas How can allopolyploids form o the origin and propagation of an amphidiploid a allotetraploid where he parent species are known ex American cotton plant a3 o species 1 contains genome A consisting of 3 distinct chromosomes a1 a2 and o Species 2 contains genome B consisting of two distinct chromosomes b1 and b2 o following fertilization between members of the two species and chromosome doubling a fertile amphidiploid containing two complete diploid genomes AABB is formed deletion of part of a chromosome o involves chromosome breaks and repair a terminal deletion an intercalary deletion pairing occurs between a normal chromosome and one with and intercalary deletion by looping out The undeleted portion to form a deletion or compensation loop Duplication is repeated part of a chromosome o mispairing of a tetrad during synapsis results in a deficient and duplication chromosome then non crossover chromosomes remain unchanged o in some cells some parts of chromosomes are selectively duplicated many times ex egg cells the nuclear region can be amplified hundreds of times may be visible as micronucleoli The Bar mutation in fruit flies o results in a duplication of part of chromosome 16A o normal eyes B B have 800 facets but B B have only 350 and B B just 70 o BD B flies have even fewer and are termed double bar o Duplication can be seen in stained chromosomes Role of gene duplication in evolution o Ohno 1970 proposed that duplication is essential to the origin of new genes o Idea that existing genes are indispensable but by making an extra copy there is one to tinker with while original keep on doing its job o The new copy is free to accumulate mutations some of which may prove during evolution advantageous o this idea is now supported by the identification of genes with closely related sequence but different functions ex hemoglobin gene family and many others o major jumps in evolution appear to have involved duplication of entire genomes Inversions o change linear order of genes along a chromosome o during an inversion no information is lost o the inverted region may be short or long and may or may not include the centromere o if it does it is called a pericentric inversion and if not it s a paracentric inversion o Paracentric inversion the effects of a single crossover SCO within an inversion loop in a paracentric inversion heterozygote where two altered chromosomes are produced one acentric and one dicentric Both chromosomes also contain duplicated and deficient regions o Pericentric inversion the effects of a crossover in a pericentric inversion heterozygote where two altered chromosomes are produced both with duplicated and deficient regions Translocation o alter the location of a chromosomal segment in the gene o reciprocal exchange of genetic material occurs between nonhomologous chromosomes four breaks are required two on each chromosome the resulting translocated chromosomes do not readily line up with the other homolog during meiosis Synapsis of a translocation heterozygote o an unusual cross shaped configuration occurs during synapsis o different segregation patterns can ensue resulting in balanced and unbalanced games being formed Familial passed down in families o down syndrome is due to a translocation o a large segment of chromosome 21 is translocated onto chromosome 14 o the translocation carrier is phenotypically normal because there is essentially nearly two copies of all genetic information on chromosome 14 and 21 o only the offspring with the two chromosome 21 and the translocation show symptoms similar to trisomy 21 Fragile sites o In humans are susceptible to chromosome breakage o the underlying cause is unknown but may be due to less tight coiling of chromatin in a certain region of a chromosome o Fragile X Martin Bell syndrome causes mental retardation in about 30 of males and females that carry this X chromosome dominant effect o Also a link to cancer mutation within this region FHIT may be due to the large number of