chapter 9: chromosome variation - university of idaho · 2012-10-29 · 3 the human y chromosome...
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Chapter 9: Chromosome Variation
Lectures 13 and 14
32 + 31and don’t pair correctly
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Metacentric
Submetacentric
Acrocentric
Telocentric
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The human Y chromosome
Chromosome Rearrangements
duplications
deletions
Inversions
translocations
Looping out of duplicated chromosome allows homologous regions to pair.
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Duplications can be produced by unequal crossing over.
Normal gene dosage
Duplications can lead to problems, sometimes due to abnormal gene dosage.Not completely understood.
Deletions
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Mechanisms for formation of deletions and inversions
1. Legitimate recombination: between similar DNA sequences= THE most common type of recombination
A. Intrachromosomal recombination between repeats oriented in the same direction deletion.
B. Intrachromosomal recombination between inverted repeats inversion.
2. Illegitimate recombination: between DNA sequences with only a few nucleotides of sequence similarity
3. Other mechanisms (including movement of transposons)
Phenotypic Effects of Inversions
--no loss or gain of genetic material
--What (if any) phenotypic effects could they have?
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Possible phenotypic Effects
--Inversion may break gene into two parts.
--Position effect: the position of many genes on chromosome effects their regulation. Inversions can cause inappropriate expression patterns.
What happens during meiotic prophase I?
X =
Chromosome pairing when heterozygous for a paracentric inversion
This paracentric loop structure is one way to detect invesions.
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dicentric chromatid
acentric chromatid During anaphase I the dicentric chromosome
will break.The acentric chromosome may be lost.
What will happen to inversion heterozygotes showing recombination outside the inversion?
Chromosome pairing when heterozygous for a pericentric inversion
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Summary: Inversions
In heterozygotes, inverted region loops out when homologous chromosomes pair in meiosis I.
Can have phenotypic effects--may break genes in two--may have position effects
Observed recombination is reduced because of the production of nonviable gametes.
Translocations
--Movement of genetic material between nonhomologous chromosomes.
--How is this different from normal crossing over?
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Translocations
Two types:--Nonreciprocal translocation:one way transfer of genetic material.
-- Reciprocal translocation:two way exchange of genetic material (more common).
Phenotypic Effects of Translocations
--New linkage relationships established which may effect gene expression= Position Effect.
--Chromosomal breaks may occur wthin a gene.Can use to map genes.Ex: Neurofibromatosis
Robertsonian Translocation
--Long arms of two acrocentric chromosomes translocated to a produce a metacentric chromosome.
--Smaller chromosome may be lost.
two nonhomologous,nontranslocated chromosomes
two chromosomes with reciprocal translocations
If Heterozygous for a Reciprocal Translocation
What happens during prophase I?
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Cross structure: one way to detect translocations
What happens to homologous chromosomes in anaphase I?
Segregation is normally controlled by the homologous centromeres
Segregation can occur in three ways.
About ½ of segregations occuring give nonviable gametes due to either extra copies, or no copies of some genes
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Summary: Translocations
--Recombination between nonhomologous chromosomes-May involve legitimate recombination or nonlegitimate recombination.
--Cross structure formed in heterozygotes in meiosis I.
--Can have phenotypic effects-break genes in two-position effects
--Nonviable gametes can be produced by some types of segregation
Human Evolution: Inversions and translocations
--Several human chromosomes differ from chimpanzees by only a pericentric inversion.
--Chimps, gorillas, orangutans all have 48 chromosomes, humans: 46.
-May be from a Robertsonian translocation in a human ancestor.
Aneuploidy: change in the number of individual chromosomes.
Polyploidy: change in number of chromosome sets.
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Generation of Aneuploids
--Chromosome lost during mitosis or meiosis.Ex: centromere deleted
--Small chromosomes generated by Robertsonian translocations, then lost.
--Nondisjunction
Nondisjunction at different cell divisions.
Types of Aneuploids
--Nullisomy: loss of both members of a homologous pair
--Monosomy: loss of a single chromosome
--Trisomy: gain of a single chromosome
--Tetrasomy: gain of two homologous chromosomes
Phenotypic Effects of Aneuploidy
--Most are lethal because of changes in gene dosage(or have profound phenotypic effects)
--Exception: sex chromosomes in mammalsWhy less lethal?All but one X in humans is inactivated.Y chromosome is teeny-weeny.
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Aneuploidy in Humans
Trisomy 21: Down Syndrome Incidence of Down syndrome increases drastically with maternal age
Polyploidy:multiples of chromosome sets are present
Whole sets of chromosomes fail to separate in either mitosis or meiosis
Autopolyploidy: extra chromosome sets from the same species.
Allopolyploidy: chromosome sets from two species.
A major mechanism of speciation in plants
http://whyfiles.org/shorties/199wheat/
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Modern bread wheat: a hexaploid with genes from 3 species.
If a chromosome set in a polyploid does not have homologs infertile.
7,000-10,000 years of domestication!