Chapter 16: Variations in Chromosome Structure and Number Flashcards Preview

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Flashcards in Chapter 16: Variations in Chromosome Structure and Number Deck (46):

Chromosomal Mutation??? (Chromosomal aberrations)

- variations from the normal wild type condition in chromosome structure or chromosome number.


Chromosomal mutations can arise via what two ways?

- spontaneously
- induced experimentally (chemicals / radiation)


What are the four common types of chromosomal mutations involving changes in structure?

1. Deletions
2. Duplications
3. Inversions
4. Translocations


All four common types of chromosomal mutations involve what common origin?

- the structure mutations arise when one or more breaks in the chromosome have occurred.
L> wherever the break occurs, broken ends remain without the specialized sequences found at the end of chromosomes (telomeres) that prevent their degradation.


Chromosomal Deletions?

- mutation in which part of the chromosome is missing
- it starts where breaks occur in the chromosome.


Deletions are mutations in which part of the chromosome is missing and usually occur where there are break-in the chromosome due to possibly what five things?

1. heat
2. radiation
3. viruses
4. chemicals
5. recombination errors


Can deletion mutations be reverted to the wild type state?



If a deletion involves the loss of a centromere, the result is what?

- an acentric chromosome which is usually lost during meiosis.


Pseudodominance? (think deletions)

- the deletion of a dominant allele of a heterozygote results in the appearance of the phenotype of the recessive allele. This unexpected expression of the recessive allele is caused by the absence of the dominant allele.


Consequences of deletions??
L> think homozygote vs heterozygote

- In heterozygote condition, they may function normally but remaining allele may be recessive for deleterious condition and cause severe consequences. If loss of a centromere you get an a centric chromosome which may be lost in meiosis. This could cause fatal consequences. When homologous chromosomes line up after a deletion event there are resulting loops of unpaired DNA.
- not usually noted in homozygotes because they usually die if the deletion is large enough.


Explain Ectrodactyly?
- causes? (3)
- frequency of occurrence?
- chromosome involved ? number of mutations?
-phenotypic expression?

- translocations, inversions and deletions
- 1/90000 babies
- absence of the central digit ray
- five different genetic mutations known all on chromosome 7 concerning homeobox genes( aka ones that have to do with body plan)


- chromosome involved?
- frequency ?
- known cases
- cause?

- rearrangement of chromosome 8
- 1/billion people
- 50 ever recorded since the middle ages
-due to the disruption in the cross talk cell communication between the epidermis and the dermis as hair follicles form in a three month old fetus. and essentially at the hair line to the toes...
- normally: signals from the dermis form follicles at the three month mark as individual follicles form.... a signal is sent to prevent the area around it from developing follicles as well.


What is a duplication mutation??

a chromosomal mutation that results in the doubling of a segment of chromosome


What are the three types of duplication mutations?

1. Tandem Duplication
2. Reverse Tandem Duplication
3. Terminal Tandem Duplication


Tandem duplications??

- when the mutation generates duplicated segments that are adjacent to each other with the order of the genes in both segments the same as the order of the original
ex: original -> ABCDEFGH


Reverse Tandem Duplication??

- when the order of the genes in the duplicated segment is the opposite of the order of the original.
Ex: original : ABCDEFGH


Terminal Tandem Duplication??

- when the duplicated segments are arranged in tandem at the end of a chromosome
ex: original: ABCDEFGH


Heterozygote duplications result in what?

loops of unpaired genes


What are multi gene families?

a set of genes inherited by duplication and variation in some ancestral gene. The duplications can be clustered on the same chromosome or dispersed on to other chromosomes.


Example of multi gene families from lecture?

-hemoglobin and myoglobin families
L>immunoglobulants (antibodies), histocompatibility antigens (gene products on cells that prevent you from being a tissue donor)


Reiterated genes??

completely identical for robustness and since you need a whole lot of them ..hundreds of them on tandem on chromosomes
L>histones, rRNA, tRNA


Phenotypic effects in Drosophila for Duplications
- Bar mutation on X chromosome (explain)
ie gene involved
L> Homozygous
L> hemizygous

- 16Agene: bar gene.... 1 copy=regular eye in drosophila ... but if you have a duplication of this on one of your alleles... more copies of the gene= smaller eye
- inheritance is similar to x linked mutation
- as soon as you have this duplication it is immediately expressed
L> eye is vertical and slit shaped
L> smaller eye than wild type but larger than if homozygous for bar mutation
L> similar to homozygous for bar mutation.


Explain the evolution of the Bar Mutation!

- unequal crossing over due to homologous chromosomes pair improperly or crossing over events in the misfired region results in gametes with a duplication or deletion.


When discussing Bar mutation what does B^D indicate?

double bar mutation
( 3 copies of 16A)
L> two evolutionary pathways?
L> single duplication on one and deletion of other via unequal crossing
L> unequal crossing of chromosomes with double 16A respectively resulting in one chromosome with three and one with one



a chromosomal mutation that results when a segment of chromosome is excised and then reintegrated at an orientation 180 degrees from the original orientation


What are the two types of inversions in regards to the centromere?

1. Paracentric inversion
1. Pericentric inversion


Paracentric inversion?

- does not include the centromere


Perecentric inversion

- does include the centromere
L> can change the lengths of the arms of the chromosome


Position effect??
L> X+/W^w ?

change in gene expression due to location on the chromosome
L> normal heterozygous genotype for red eyes...if inversion takes place, patches of white are expressed (drosophila )


Position effect:
- heterochromatin ??

- euchromatin is expressed ...heterochromatin is tightly packed away ....therefore silenced
- if the gene is near a region that is not normally transcribed the gene product is not going to be seen
- x chromosome inactivation??


Position Effect:
-In terms of drosophila eye colour when is the only time phenotypic expression of a position effect seen?

- when you have the heterozygous genotype for wild eye white allele and an inversion takes place if it is the wild allele involved in the allele you will see white patches because it won't be expressed. The wild allele is now located near heterochromatin which is normally not expressed and therefore the wild allele is now no expressed as well. The pigments for the red type will not e expressed.


What are the two types of inversions?

- Homozygous inversion
-Heterozygous inversion


Homozygous inversion?

- normal meiosis
- no problems related to gene duplications or deletions


Heterozygous inversion?

- inversion loops are produced
- products of meiosis may not be viable
- different outcomes if para or pericentric
- NO meiotic problems if crossing over is not involved.


Pairing of homologous chromosomes issue with inversions?

- require the formation of loops containing the inverted segments called inversion loops


If there is no crossing over within the inversion loops of a paracentric inversion heterozygote what is the result?

- all resulting gametes receive a complete set of genes.....and they are a lll viable.
L> two gametes: normal gene order and two with inverted segments.


Paracentric Heterozygotes?
L> explain whats going on.

-dicentric bridge formation in meiosis 1
L> chromosome with two centromeres via continued migration it breaks.
L> the other product is a chromosome with no centromere (acentric fragment) which is lost in meiosis.
L> gametes from recombined chromatids are inviable if there are genes missing or multiple copies of genes.
L> chromatids not involved in crossing over are viable.
( two viable and two nonviable )


Pericentric Heterozygotes ??
L> explain whats going on here!

- viable gametes:
L> normal gene arrangement
L> inversion gene arrangement
- nonviable gametes
L> due to deletion and duplication of genes
(two viable and two non)



-a chromosomal mutation in which there is a change in position of chromosome segments and the gene sequences they contain to a different location in the genome.


With translocation mutations is there a gain or loss in genetic material?

neither. no gain or loss.


What are the different types of translocations?(3)

1. nonreciporical intrachromosomal
2. nonreciporical interchromosomal
3. reciprocal interchromosomal


Nonreciporical INTRAchromosomal Translocation?

-if a chromosome segment changes position within the same chromosome


Nonreciporical INTERchromosomal Translocation??

-if a chromosome segment is transferred from one chromosome to another. (one way exchange...only one chromosome is receiving )


Reciprocal INTERchromosomal Translocation?

-an exchange of segments between the two chromosomes involved.
(two way exchange)


Translocations and tumours explain.

- they are the most common class of mutation associated with cancer.
- varieties of chromosomal mutations may be observed in tumours.


Specific translocation mutations are observed in some tumours such as what type of cancer examined in lecture?

- Chronic Myelogenous Leukemia (CML)
L> reciprocal translocation between Chr. 9 and Chr. 22
L> converts a proto oncogene to an oncogene