Chapter 8 - Chromosome Variation

Question 1

Which of the following cannot be used to determine the physical location of a gene on a chromosome?
A. In situ hybridization
B. Somatic Cell Hybridization
C. Three-Point Genetic Cross
D. Deletion mapping
E. All of the above choices can be used to determine the chromosomal location of a gene.

Answer 1

C. Three-Point Genetic Cross

Molecular and direct sequencing can determine location; three-point crosses only give relative positions.

Question 2

A hypothetical chromosome has the following segments, where ( • ) represents the centromere: S T U V • W X Y Z
What type of chromosomal mutation would result in the following chromosome rearrangement:
S T X Y U V • W X Y Z
A. Deletion
B. Tandem duplication
C. Displaced duplication
D. Reverse duplication
E. Nondysjoined duplication

Answer 2

C. Displaced duplication

Question 3

The expression of some genes is dependent upon their chromosomal position. Which kind of chromosome rearrangement would most likely cause a position-effect phenotype?
A. deletion
B. tandem duplication 
C. inversion
D. all of the above

Answer 3

C. inversion

Question 4

During meiosis when crossing over occurs in a paracentric inversion loop, what percent of the meiotic products will result in nonviable gametes? In a pericentric loop?

Answer 4

50%

50%

Question 5

Recombination between a wild-type chromosome and an ____ on the homologous chromosome always leads to loss and duplication of DNA on the two respective recombinant chromosome regardless of the position of the _____.

Answer 5

inversion; centromere

Question 6

Which is NOT an effect of chromosomal translocations, and why?
A. Translocated genes may become under the control of different regulatory sequences.
B. Translocations can cause regions of the chromosome to be duplicated.
C. Translocation breaks can disrupt existing gene function.
D. Translocations can affect the number of chromosomes found in species through evolution.
E. All of the above.

Answer 6

B. Translocations can cause regions of the chromosome to be duplicated.

The “duplicated” gene(s) in a translocation aren’t truly the result of a duplication - they were swapped, then separated.

Question 7

A plant species has 2n = 18 chromosomes. How many chromosomes would you expect to find in a somatic cell of a tetraploid individual of this species?

Answer 7

2n = 18, n = 9; therefore
4n = 36

Question 8

The 3 main categories of chromosomal mutations are chromosome _____, where there is a physical change in the structure of the chromosome; an _____, where one or more “___” chromosomes are lost or gained; and ______ where one or more “____” of chromosomes are gained.

Answer 8

rearrangements

aneuploidy; single

polyploidy; sets

Question 9

Name the 4 types of chromosome rearrangements.

Answer 9

duplications
deletions
inversions
translocations

Question 10

There are 3 subcategories of duplications: ___ duplications where duplicated portions immediately proceed the original; ___ duplications where duplicated portion is a given distance from the original (or even on a different chromosome…?); and __ duplication where the duplication is inverted from its original orientation (tandem + inversion). Give examples of each using linked genes AB•SCD.

Answer 10

tandem: AB•SCD → AB•SCDCD
displaced: AB•SCD → AB•SCDSC
reverse: AB•SCD → AB•SCCSD

Question 11

T or F: duplications can usually be seen with a microscope

Answer 11

True

Question 12

The Bar Eye mutation in Drosophila is the result of a small ___ on the ___ chromosome. Because of the chromosome that’s affected, ___ females and males (due to ____) have a more extreme phenotype.

Answer 12

duplication; X

homozygous; hemizygosity

Question 13

The most likely explanation for a mutated phenotype due to a duplication is…?

Answer 13

An imbalance (too much) of protein product.

Question 14

Why would an X-linked duplication create an imbalance of product in Drosophila but not as badly for humans.

Answer 14

Drosophila’s dosage compensation system is such that males with fewer X chromosomes produce extra, so if there is a duplication, the duplicated sequence will produce 2x plus the extra males normally produce. For humans, X inactivation would help mitigate.

Question 15

Duplication arises when chromosomes do not ___ properly resulting in ___ ___ ___ in meiosis 1; this yields one chromosome with a ____ and the other with a ____.

Answer 15

align; unequal crossing over.

duplication; deletion

Question 16

What example of a gene demonstrates that duplication plays an evolutionary role, and what does this do the the functionality of the product (evolutionarily speaking)?

Answer 16

Globin genes

New uses for existing product can be co-opted depending on what new gene regulators are nearby / upstream.

Question 17

In a “testcross” between a WT with a deletion (heterozygous) and a homo. rec., what will the progeny be? Explain.

Answer 17

50% of the progeny will be WT, 50% will be homo. rec. The latter will be due to the pseudodominance of the progeny with one recessive allele and a deletion.

Question 18

What is the typical phenotype of a “homozygous” deletion?

Answer 18

Usually lethal: deletions are usually large, so the probability of losing an essential gene is high.

Question 19

Define pseudodominance.

Answer 19

Pseudodominance is a phenomenon where a recessive mutation is “revealed” due to the deletion of the gene on the corresponding homolog (comparable to hemizygosity).

Question 20

Define haploinsufficient and describe its role in deletions.

Answer 20

A phenomenon where one copy of a WT allele is insufficient to make enough product to produce the expected phenotype. In the case of a deletion, the deletion is a loss-of-function mutation, and the loss-of-function mutation becomes dominant.

Question 21

“Loss-of-function mutations in a ____ gene become _____.”

Answer 21

haploinsufficient; dominant

Question 22

The notch phenotype in Drosophila is an example of a haploinsufficient gene; what does haploinsufficient mean? What chromosome is it found on? What effect does this have on the phenotype? Also, what does homozygosity cause?

Answer 22

The WT allele doesn’t produce enough gene product to produce a normal wing (it’s notched), so it’s haploinsufficient.

It’s a deletion on the X chromosome.

It causes the wing to be notched thereby causing the haploinsufficiency to be dominant.

Homozygous for the deletion is lethal (recessive lethal).

Question 23

Paracentric inversion vs. pericentric inversion?

Answer 23

The first excludes the centromere, the latter includes the centromere.

Question 24

Consequences of a genetic inversion include ____ ____ where the expression of inverted genes is affected by the surrounding chromosomal environment, so if the inverted sequence moves a gene from a _____ active area to a _____ inactive area (an area of heterochromatin), the expression of that gene will be suppressed.

Answer 24

position effects; transcriptionally; transcriptionally

Question 25

What kind of problems do homozygous inversions cause during crossing over? Heterozygous?

Answer 25

None.
Problems come during pairing in prophase 1 - the inversion needs to loop to align itself during synapsis. Furthermore, if crossing over includes the inverted region, 1/2 the gametes will be non-viable.

Question 26

Recombination between heterozygous chromosomes - one is WT and one recessive with a ____ inversion - generates one recombinant gamete that is ____ (it has ___ centromeres), and another recombinant gamete that is ____ (it has ___ centromeres). Both recombinant gametes are __-_____.

Answer 26

paracentric; dicentric (2 centromeres); acentric (no centromeres).

non-viable.

Question 27

Recombination between heterozygous chromosomes - one is WT and one recessive with a ____ inversion - generates two recombinant gametes that have ____ genes and ____ genes. Both recombinant gametes are __-_____.

Answer 27

pericentric; deleted and duplicated; non-viable

Question 28

T or F: recombination with a WT chromosome and a recessive with an inversion (paracentric or pericentric) always leads to half viable and half non-viable gametes.

Answer 28

False: this is true ONLY when crossing over takes place in the region of the inversion. Any region outside the inversion will not impact viability.

Question 29

How do paracentric and pericentric inversions impact anaphase (specify anaphase 1 or 2)?

Answer 29

In a paracentric inversion, one chromosome will have 2 centromeres (dicentric), another will have none (acentric), so at the end of anaphase 1, the acentric chromosome will be lost; and at the end of anaphase 2, the dicentric chromosome will break.
In a pericentric inversion, all chromosomes will have one centromere, so none will be lost in anaphase 1 or be broken in anaphase 2, however, the loss of critical genes and duplications of others will render them non-viable.

Question 30

A ____ is an exchange of genetic material between non-homologous chromosomes and can create new gene _____.

Answer 30

translocation; linkages

Question 31

This type of translocation is strictly unidirectional and is very rare.

Answer 31

nonreciprocal translocation

Question 32

This type of translocation is the most common type.

Answer 32

reciprocal translocation

Question 33

What two mechanisms impact the phenotype of an organism with a translocation?

Answer 33

(1) Gene regulation at the break site can change and therefore can impact the phenotype.
(2) A break within the gene itself can alter the function of the gene and therefore the product it makes.

Question 34

What is a Robertsonian translocation? What might be its role in human evolution (include total chromosomal pairs)?

Answer 34

A type of reciprocal translocation where the long arms of two acrocentric chromosomes translocate becoming one metacentric chromosome. A second chromosome with 2 short arms is produced, but is typically lost.

A Robertsonian translocation may have occurred between LCA of great apes and humans to form human chromosome #2 - we have 23 pairs, they have 24 pairs, and the existence of homologous bands between chromo #2 and two acrocentric primate chromosomes.

Question 35

What causes the inheritable form of Down Syndrome?

Answer 35

A Robertsonian translocation between chromosome 14 and 21.

Question 36

Explain how fertility is impacted when considering a para- or pericentric inversion vs. a reciprocal translocation.

Answer 36

In meiotic cells with para- or pericentric inversions, 1/2 of all recombinant gametes whose recombination INCLUDED the inverted portion of the chromosome are non-viable. In an organism with a reciprocal translocation, 1/2 of all gametes are non-viable.

Question 37

Explain why para- and pericentric inversions impact fertility.

Answer 37

Viability in para- and pericentric inversions is limited only to meiotic cells, so fertility is reduced, but the reduction is limited only to those cells that participated in crossing over with the chromosome that has the inverted sequence - and ONLY if crossing over included the inverted portion of the chromosome (50% of those “qualifying” meiotic cells).

Question 38

Explain why reciprocal translocations impact fertility.

Answer 38

In reciprocal translocation, viability of gametes will depend on chromosomal alignment during meiosis 1. If the “normal” chromosomes are aligned on one side and the translocated chromosomes on the other, then all gametes will be viable. However, there is a 0.5 probability that normal chromosomes will align on opposite sides of the metaphase plate; this will cause all gametes to be non-viable reducing overall fertility by 1/2 (there will be duplicates of some genes and deletions of others - but these are NOT true dups and del - they’re just swapped out).

Question 39

An ____ is a change in the number of individual chromosomes. there are 4 major types: ____ (2n-2); ____ (2n-1); ____ (2n+1); and ____ (2n+2). 3 instances that cause it are: ___ becomes damaged, a _____ translocation, and _____ of a chromosome.

Answer 39

aneuploidy; nullisomy; monosomy; trisomy; tetrasomy.

centromere; Robertsonian; nondisjunction

Question 40

An extra copy of two different non-homologous chromosomes is called double _____.

Answer 40

double trisomic (two homologous pairs having a 3-some)

Question 41

Why are most aneuploids lethal? Why are the exceptions non-lethal?

Answer 41

They change the gene dosage, therefore the product. If the genes are large, this can be disastrous.
The few exceptions that are tolerated are Down (due to the small size, and therefore, a reduced dosage effect); and aneuploids of sex chromosomes (X inactivation / Y is small).

Question 42

Rates of aneuploidy in humans are ___ than many mammals. Some estimates are as high as ___% for spontaneous abortions.

Answer 42

higher; 30%

Question 43

The most common form of aneuploidy in humans is?

Answer 43

Sex chromosome aneuploid

Question 44

Down Syndrome (non-heritable) aka? Cause?

Answer 44

Trisomy 21; nondisjunction during formation of the egg

Question 45

Familial Down Syndrome is caused by a ____ translocation between the ___ arms of chromosome __ and __ (the __ arms translocate, but are usually ____ due to their size). This is different from trisomy 21 because the 3rd copy of 21 is attached to ___ ___.

Answer 45

Robertsonian; long; 14 and 21; short; lost.

chromosome 14.

Question 46

A carrier of heritable Down Syndrome has __ copies of chromosome 21, and ___ copies of chromosome 14; however, one copy is ____ to the other for a total of ___ chromosomes, but they are physiologically ____.

Answer 46

2; 2; attached; 45; normal

Question 47

A very rare phenomenon where both homologs can be inherited from one parent is called _____ _____. One example is children born with cystic fibrosis who has a parent with only one ___ allele on chromosome ___, so the most likely explanation is ____ of one of the gametes. Then after fertilization the embryo was briefly ____, and the third chromosome was later lost.

Answer 47

uniparental disomy

recessive; 7; nondisjunction

trisomic.

Question 48

How many chromosomes would a human zygote with a nullisomy have? A tetrasomy?

How many chromosomes would a triploid organism have if it was normally diploid with 23 pairs? A tetraploid?

Answer 48

nullisomy = 2n-2 = 44; tetrasomy = 2n+2 = 48;

triploid = 3n = 69; tetraploid = 4n = 92

Question 49

What causes polyploidies?

Answer 49

Entire set of chromosomes fail to segregate.