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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!