Translocations in the germline
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Translocations in the germline
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[post_date] => 2024-12-23 09:11:10
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[post_content] => Practice Passage (Question 1-7)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
During meiosis, homologous chromosomes line up and exchange information in the process of crossing over. However, if the chromosomes misalign, the resulting gametes may be unequal in chromosome content and, often, nonviable. Chromosomal rearrangements known as translocations are one major source of misalignment. Translocations can be classified into two main categories: reciprocal and nonreciprocal. Reciprocal translocations involve two chromosomes exchanging genetic information, whereas nonreciprocal translocations, also known as unbalanced translocations, involve the transfer of genetic material from one chromosome to another without a reciprocal exchange. In the majority of reciprocal translocations, the chromosome number does not change. As many as 1 in 600 newborns have a reciprocal translocation somewhere in their genome. While this may have no direct effect on the health of the individual carrying the translocation, it may lead to infertility and/or miscarriage due to inviable gametes.
Robertsonian translocations are reciprocal translocations that involve acrocentric chromosomes – these are chromosomes in which the centromere is almost entirely at one end. During the translocation, the longer (q) arms of two acrocentric chromosomes fuse to create one larger chromosome (fusion chromosome) with a centromere in the center, while the smaller (p) arms join to form a mini DNA segment with few genes and no centromere, which is generally lost by the cell.
Figure 1 Robertsonian translocation bringing together the long arms of chromosomes 14 and 21
For the fusion chromosome, the single remaining functional centromere can come from either of the two original chromosomes involved in the translocation. For example, if chromosomes 14 and 21 are involved in a Robertsonian translocation, the fused chromosome can retain the functional centromere from either chromosome 14 or chromosome 21. The centromere can play a significant role in determining which homologous chromosome it aligns with during cell division (e.g. whether it aligns with chromosome 14 or 21).
The karyotype of a parent with a Robertsonian translocation is seen in Figure 2. Although the fusion chromosome is shown paired with chromosome 14, it could line up with either chromosome 14 or 21 during meiosis, depending on its centrosome and other factors.
Figure 2. Karyotype of a Robertsonian translocation. Attribution: modified from National Human Genome Research Institute, CC-BY-SA 3.0
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[question] => According to the reference karyotype in Figure 2, which of the following Robertsonian translocations and resulting genotypes is most likely?
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[description] => Reason for Correct Answer:
A diploid genotype refers to the specific combination of two sets of chromosomes present in the cells of an organism.
According to the passage, a Robertsonian translocation involves the fusion of two acrocentric chromosomes, resulting in a single, large chromosome.
This would effectively reduce the chromosome count from 46 to 45.
Acrocentric chromosomes are chromosomes in which the centromere (the constricted point where the two chromatids of a chromosome are joined) is located very close to one end.
According to the karyotype, this matches chromosomes 13, 14, 15, 21, or 22.
When a Robertsonian translocation occurs, it is usually between these chromosomes (including 15 and 22) and usually results in a total of 45 chromosomes in affected individuals, or genotypes 45 XX or 45 XY.
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[each_answer] => A. Translocation between chromosomes 13 and 22 resulting in 46 XX
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[each_answer] => B. Translocation between chromosomes 3 and 10 resulting in 45 XX
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[each_answer] => C. Translocation between chromosomes 15 and 22 resulting in 45 XX
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[each_answer] => D. Translocation between sex chromosomes resulting in 45 X
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[quiz_unique_key] => 1403770772
[question] => Why are Robertsonian translocations only observed in acrocentric chromosomes?
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[answer] => 3
[description] => Reason for Correct Answer:
Translocations can be seen in every chromosome.
A Robertsonian translocation involves the fusion of two centromeres and the loss of two chromosomal arms.
Any attempt to delete the q arms of a non-acrocentric chromosome would lead to too much gene loss to create a viable fetus.
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[each_answer] => A. Non-acrocentric chromosomes are far larger and thus less likely to mispair
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[each_answer] => B. Centromeric fusion in non-acrocentric chromosomes would create a four-armed chromosome that could never separate properly during meiosis
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[each_answer] => C. Any non-acrocentric participation in a Robertsonian translocation would lead to too much gene loss
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[each_answer] => D. Acrocentric chromosomes have much more accessible centromeres to allow fusion to occur
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[quiz_unique_key] => 1403770772
[question] => How many different gametes can the person whose karyotype is pictured in Figure 2 produce (in regards to chromosome 14 and 21 segregation)?
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[answer] => 3
[description] => Reason for Correct Answer:
The karyotype in Figure 2 shows one normal chromosome 14, one normal chromosome 21, and one fusion chromosome.
Although the karyotype shows the fusion chromosome paired with chromosome 14, the passage discusses that it could line up with either 14 or 21 during meiosis, which produces gametes.
So, consider the outcome of events when the fusion chromosome lines up with 14 during meiosis (and 21 is alone) PLUS the outcome of events if it lines up with 21.
Here are the possible outcomes:
You can see that, in the gametes, the normal chromosomes can end up with each other, the fusion chromosome can end up with either of the normal chromosomes, and the fusion chromosome of either normal chromosome can end up alone. So, this makes six distinct gametes.
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[answers] => Array
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[each_answer] => A. 2
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[1] => Array
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[each_answer] => B. 4
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[each_answer] => C. 6
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[3] => Array
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[each_answer] => D. 8
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[3] => Array
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[question] => Which of the following events could lead to Down syndrome (Trisomy 21) in a newborn?
I. Nondisjunction in the mother in meiosis I
II. Nondisjunction in the father in meiosis II
III. A Robertsonian translocation in a germ line cell
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[answer] => 4
[description] => Reason for Correct Answer:
A Robertsonian translocation involves the fusion of two acrocentric chromosomes (such as chromosomes 14 and 21) at the centromere, whereas a nondisjunction refers to the failure of chromosomes to separate properly during meiosis I or II. The outcomes of nondisjunction events in egg cells are shown below.
https://en.wikipedia.org/wiki/Nondisjunction
Nondisjunction during meiosis I in the mother would result in one daughter cell receiving both copies of chromosome 21, while the other daughter cell receives none. If a sperm with a normal chromosome 21 fertilizes an egg with an extra chromosome 21, the resulting zygote will have Trisomy 21, leading to Down syndrome.
During meiosis II, if the father’s chromosomes do not separate correctly, this would cause one sperm cell to contain an extra chromosome 21 while the other lacks chromosome 21. If this sperm with an extra chromosome 21 fertilizes a normal egg, the resulting zygote will have Trisomy 21, leading to Down syndrome.
If a parent carries a Robertsonian translocation involving chromosome 21 in their germ cells, they can produce gametes with the fusion 14/21 chromosome in addition to another copy of chromosome 21. If such a gamete combines with a normal gamete during fertilization, the resulting zygote can have Trisomy 21 due to the extra genetic material from the translocated chromosome.
Therefore, the answer is I, II, and III.
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[each_answer] => A. I and II only
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[each_answer] => B. I and III only
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[each_answer] => C. I and III only
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[each_answer] => D. I, II, and III
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[quiz_unique_key] => 1403770772
[question] => If reciprocal translocation between nonhomologous chromosomes occurs at the same meiotic phase as crossing over between homologous chromosomes, when does this occur?
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[answer] => 1
[description] => Reason for Correct Answer:
Crossing over occurs when homologous chromosomes pair up and exchange segments of genetic material. If a crossing over event involves nonhomologous chromosomes, it can lead to reciprocal translocations, where segments of one chromosome are exchanged with segments of another nonhomologous chromosome.
Crossing over must occur during meiosis I, as homologous chromosomes are separated after meiosis I.
Anaphase I is too late, because homologous chromosomes have already been separated.Crossing over between homologous chromosomes occurs during Prophase I.
https://en.wikipedia.org/wiki/Meiosis#/media/File:Meiosis_Stages.svg
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[each_answer] => A. Prophase I
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[each_answer] => B. Anaphase I
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[each_answer] => C. Telophase I
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[each_answer] => D. Metaphase II
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[question] => You want to mimic a reciprocal translocation in a lab. Which of the following techniques is most suitable?
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[answer] => 3
[description] => Reason for Correct Answer:
Reciprocal translocations involve the exchange of DNA between chromosomes.
In vitro transcription is a laboratory method for creating RNA from DNA using enzymes; it is not a method for the creation of DNA.
Southern blotting is a laboratory technique used to detect specific DNA sequences in a DNA sample. It does not involve the manipulation or recombination of chromosomal segments to mimic reciprocal translocations.
DNA hybridization is a method used to study sequence similarity between DNA strands from different sources, providing information about genetic relatedness. Although it helps analyze genetic similarities, it does not involve the physical manipulation of chromosomal segments to mimic reciprocal translocations.
Creation of recombinant DNA involves the precise excision and joining of DNA fragments from different chromosomes, resembling the exchange of genetic material between nonhomologous chromosomes in a reciprocal translocation event. Creating recombinant DNA allows researchers to mimic the genetic rearrangements observed in reciprocal translocations.
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[each_answer] => A. In vitro transcription
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[each_answer] => B. Southern blotting
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[each_answer] => C. Creation of recombinant DNA
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[each_answer] => D. DNA hybridization
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[question] => Unlike Robertsonian translocations, crossing over between homologous chromosomes is a normal meiotic process, but it can introduce abnormalities if an error occurs. Which genetic outcome does NOT typically result from defective or abnormal crossing over?
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[answer] => 3
[description] => Reason for Correct Answer:
Defective crossing over could result in an unequal amount of genetic information being shared between homologous chromosomes.
That could lead to certain segments being deleted from one chromosome and duplicated on the other one.
Abnormal crossing over can also result in the inversion of a chromosome segment. In inversion, a segment breaks off, flips, and reattaches in reverse order. Inversions can disrupt genes and cause genetic abnormalities.
Abnormal crossing over would not likely result in a loss of an entire chromosome, such as Monosomy X, where the second sex chromosome is absent.
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[each_answer] => A. Deletion of a chromosomal segment
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[each_answer] => B. Inversion of a chromosomal segment
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[each_answer] => C. Monosomy X (Turner syndrome)
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[each_answer] => D. Duplication of a gene
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Figure 1
