Cytogenetics

Question 1

What letters represent the short and long arms of a chromosome?

Answer 1

Short arm: p, Long arm: q

Question 2

What is a metacentric chromosome?

Answer 2

The short and long arm are equal in size

Question 3

What is a sub-metacentric chromosome?

Answer 3

The short and long arm are unequal in size.

Question 4

What is an acrocentric chromosome?

Answer 4

The short arm is very small and close to the centromere.

Question 5

What is the most common technique that is used for blood karyotyping? Describe the process.

Answer 5

G-bandin - Lymphocytes (T-cells) from peripheral blood are stimulated with phyohemaglutinin (PHA) and cultured for 72 hours before being placed in a hypotonic solution (KCl) to arrest division. Cells are then fixed on slides with methanol/acetic acid. Banding is achieved by trypsin treatment for G-bands and staining with Giemsa. Results in GC rich light bands and AT rich dark bands

Question 6

What do del, dup, mar, stand for?

Answer 6

Deletion, duplication, marker

Question 7

Describe what the karyotype 46,XY,del(5)(p15.2) indicates.

Answer 7

Male with normal number of chromosomes who has a deletion of part of the short arm of one of his chromosome #5s.

Question 8

What karyotype does a Turner syndrome patient have?

Answer 8

45,X (females) - this is a monosomy X

Question 9

What karyotype does a Klinefelter syndrome patient have?

Answer 9

47, XXY (males)

Question 10

What karyotype does a female with trisomy 21 (Down syndrome) have?

Answer 10

47,XX,+21

Question 11

What is euploidy?

Answer 11

Normal, balanced set of chromosomes

Question 12

What is diploidy?

Answer 12

Normal number of chromosomes in a somatic cell (2n)

Question 13

What is haploid?

Answer 13

Chromosome number of gametes in mature egg or sperm (1n so it can be 23,X or 23, Y in humans)

Question 14

What is aneuploidy?

Answer 14

Unbalanced number of chromosomes, where either one is missing or you have an additional chromosome (2n-1 or 2n+1) Ex: trisomy 21

Question 15

What is triploidy? What are the three ways it can occur?

Answer 15

3 haploid sets (3n)

1) Dispermy - ovum is fertilized by two sperm to yield a triploid zygote (most common cause)
2) Fertilization by diploid sperm
3) Non-disjunction in the egg

Question 16

What is polyploidy?

Answer 16

Having extra sets of chromosomes
Ex: triploidy: 68,XXX or 69,XXY

tetraploidy: 96,XXXX
mixoploidy: 46,XX/69,XXX

Question 17

What are the two potential consequences of triploidy?

Answer 17

1) 2 maternal + 1 paternal = normal fetal development, small placenta
2) 2 paternal + 1 maternal = abnormal fetal growth, increased placental development

Question 18

What is nondisjunction?

Answer 18

The aberrant segregation of homologous chromosomes into the same daughter cell - occurs most often in Meiosis I of oogenesis and is associated w/ advanced maternal age

Question 19

What is mosaicism? What are the two different types?

Answer 19

Mixture of two or more cell lines in an individual (Ex: 45,X/46,XX) May result from error in disjoining of chromatids in mitosis

Constitutional mosaicism - numerical chromosome abnormalities

Gonadal mosaicism - involves testes and ovaries

Question 20

Describe what yellow, red, and green mean in a Comparative Genomic Hybridization Array (aCGH)/chromosome microarray.

Answer 20

Red - patient has less DNA than the control (deletion)

Green - patient has more DNA than the control (duplication)

Yellow - DNA is equal in patient and control

Question 21

Describe what the 0 line is in the Array profile and what marks in the negative and positive direction indicate.

Answer 21

0 line = normal

Left of 0 line/negative = deletions

Right of 0 line/positive = duplication

Question 22

What are some of the limits of microarray methodology?

Answer 22

Cannot detect balanced rearrangements, does not provide information regarding location of additional copy, copy number variations may or may not be pathogenic

Question 23

What is fluorescence in situ hybridization (FISH)?

Answer 23

Complementary stretch of DNA (probe) is labeled and hybridized onto a cell. FISH can then identify specific chromosomal regions that have been deleted or duplicated - location specific. It is used clinically as a confirmation for diagnoses from microarray studies.

Question 24

What are the two types of chromosomal deletions? And given an example of each.

Answer 24

Terminal - at ends of chromosomes (Wolf-Hirschorn Syndrome, deletion on chromosome 4)

Interstitial - in middle of chromosomes (Smith-Magenis syndrome, deletion on chromosome 17)

Question 25

What is a chromosomal duplication and what are the two different types?

Answer 25

Duplication is when a visible chromosomal segment is present twice on the same chromosome.

Direct, Inversion

Question 26

What is Williams Beuren Syndrome? What happens when a duplication occurs in this same chromosomal region?

Answer 26

Interstitial microdeletion of chromosome 7 - CV disease, distinctive face, overfriendliness, ADD

If duplication occurs in same chromosomal region, result is autism

Question 27

What are the two types of inversion duplication?

Answer 27

Pericentric - 2 breaks on either side of centromere - results in duplication deficiency chromosomes

Paracentric - 2 breaks on same chromosome arm, not involving centrome - results in acentric and dicentric chromosomes

Question 28

What are ring chromosomes?

Answer 28

Deletion of terminal short arm and terminal long arm results in the unstable ends rejoining

Question 29

What is a translocation?

Answer 29

Exchange or transfer of chromosomal segments between two ore more chromosomes.

Question 30

What is a Robertsonian translocation?

Answer 30

Originates through joining of long arms of acrocentric (centromere is at end of chromosome instead of the middle) chromosomes 13,14,15,21, and 22

Question 31

What will be the recurrence rate for a woman with monosomic 21?

Answer 31

100% recurrence - Monosomy means she only has one chromosome 21, so all of her children will have trisomy 21 (Down’s syndrome) b/c her 21 is not viable