Chapter 7 Questions

A break-apart probe is used to detect a translocation. The results of FISH analysis show two signals in 70% of the nuclei counted and three signals in 30% of the nuclei. Is there a translocation present?

A translocation is present, indicated by the nuclei in which three signals appear. The probe spans the translocation breakpoint, producing two separate signals when a translocation occurs. The third signal is the intact homologous chromosome.

Angelman syndrome is caused by a micro deletion in chromosome 15. Which method, karyotyping or metaphase FISH, is better for accurate detection of this abnormality. Why?

Metaphase FIHS is preferred over karyotyping for the detection of micro deletions. The lower resolution of karyotyping makes the detection of small deletions difficult.

Is 47;XYY a normal karyotype?

No. This karyotype is aneuploid with polysomy Y (one extra Y chromosome).

What FISH technique is most useful for the detection of multiple complex genomic mutations?

Spectral karyotyping labels each chromosome with a different fluorescent color so that multiple complex genomic mutations are more clearly identified.

A FISH test with a centromere 13 probe is ordered for a suspected case of Patau syndrome (trisomy 13). How many signals per nucleus will result if the test is positive for Patau syndrome?

The FISH results will reveal three signals per nucleus with a probe to chromosome 13.

What would be the results if a centromere 13 probe was used on a case of Edward syndrome (trisomy 18)?

The FISH results will reveal two signals per nucleus with a probe to chromosome 13. A probe to centromere 18 would yield three signals per nucleus.

During interphase FISH analysis for the t(9;22) translocation, one nucleus was observed with two normal signals (one red for chromosome 22 and one green for chromosome 9) and one composite red/green signal. Five hundred other nuclei were normal. What is one explanation for this observation?

The composite signal indicates the presence of a translocation between chromosomes 9 and 22; however, because only 1 of 500 nuclei showed the composite signal, the more likely explanation is that this is an artifactual result, possibly due to the overlap of chromosomes on the slide.

What chromosomal location is indicated by 15q21.1?

This location is on the long arm of chromosome 15, region 2, band 1, subbed 1.

The results of a CGH analysis of Cy3 (green)-labeled test DNA with Cy5 (red)-labeled reference DNA on a normal chromosome spread revealed a bright red signal along the short arm of chromosome 3. How is this interpreted? a. 3p deletion b. 3q deletion c. 3p amplification d. 3q amplification

a. 3p deletion The red signal from the reference chromosome region indicates a loss or deletion in the test chromosome at 3p (a). Amplification would yield a green signal at this location. Events at 3p would not affect the signal at 3q.

Cells were harvested from a patient's blood, cultured to obtain chromosomes in metaphase, fixed onto a slide, treated with trypsin, and then stained with Giemsa. The resulting banding pattern is called: a. G banding b. Q banding c. R banding d. C banding

a. G banding Giemsa staining produces G banding (a). When chromosomes are stained with the fluorescent dyes quinacrine and quinacrine mustard, the resulting fluorescence pattern visualized after staining is Q banding. Treatment of chromosomes with acridine orange dye will produce a pattern opposite to the G banding pattern called R banding. Alkali treatment of chromosomes results in centromere staining, or C banding.

A small portion from the end of chromosome 2 has been found on the end of chromosome 15, replacing the ned of chromosome 15, which has moved to the end of chromosome 2. This mutation is called a(n): a. reciprocal translocation b. inversion c. deletion. d. robertsonian translocation

a. reciprocal translocation The exchange of portions of chromosomes with no loss of genetic material is a reciprocal translocation (a). An inversion reverses the orientation of DNA, only involving one chromosome. Deletions are a loss of material, from 1 bp to millions of bp from chromosomes. In robertsonian translocations, chromosomes break at their centromeres, and the long arms fuse to form a single chromosome with one centromere.

Write the numerical and structural chromosomal abnormalities represented by the following genotypes: a. 47, XY, +18 b. 46, XY, del(16) p(14) c. iso(Xq) d. 46, XX, del(22) q(11.2) e. 45, X

a. trisomy 18 b. deletion in the short arm of chromosome 16 at region 1, band 4 c. isochromosome formed by centromeric joining of two long arms of the X chromosome d. deletion in the long arm of chromosome 22 at region 1, band 1, subband 2 e. loss of one X or the Y chromosome

A centromeric probe is used to visualize chromosome 21. Three fluorescent signals are observed in the cell nuclei when stained with this probe. These results would be interpreted as consistent with: a. a normal karyotype b. Down syndrome c. Klinefelter syndrome d. technical error

b. Down syndrome Three centromeric signals instead of two from chromosome 21 is a finding consistent with Down syndrome (trisomy 21; b). The presence of two centromeric signals for each chromosome is normal. Klinefelter syndrome is indicated by an extra X chromosome in men (47;XXY). Technical error can result in aberrant signals, such as chromosome overlap; however, these artifactual signals are usually rare

Phytohemagglutinin is added to a cell culture when preparing cells for karyotyping. The purpose of the phytohemagglutinin treatment is to: a. arrest the cell in metaphase b. spread out the chromosomes c. fix the chromosomes on the slide d. stimulate mitosis in the cells

d. stimulate mitosis in the cells Cells must enter metaphase for karyotyping. Mitosis is stimulated in culture using phytohemagglutinin (d). Once the cells go into the cell cycle, Colcemid is used to arrest the cells in metaphase. Chromosomes are distributed from the nucleus by lysis with hypotonic buffer. Chromosomes are fixed on the slide with methanol.

A chromosome with a centromere located such than one arm of the chromosome is longer than the other arm is called: a. metacentric b. paracentric c. telocentric d. submetacentric

d. submetacentric A chromosome with a long and short arm is submetacentric (d). In a metacentric chromosome, the centromere is near the middle so the the arms are of approximately equal length. One arm is very short in telocentric chromosomes where the centromere is close to one end. Paracentric refers to an inversion within one arm of a chromosome, not involving the centromere.