Cytogenetics

Question 1

At the start of meiosis, the cell has how many chromosomes and chromatids?

Answer 1

46 chromosomes, each with 2 chromatids

Question 2

Genetic diversity is achieved through what two processes?

Answer 2

Crossing Over
Independent Assortment/Segregation

Question 3

At the end or meiosis I, the cell has how many chromosomes and how many chromatids?

Answer 3

23 chromosomes, each with 2 chromatids

Question 4

At the end of meiosis II, the cell has how many chromosomes and how many chromatids?

Answer 4

23 chromosomes, each with 1 chromatid

Question 5

At the end of mitosis, the cell has how many chromosomes and how many chromatids?

Answer 5

46 chromosomes, each with 1 chromatid

Question 6

True or False: Only half of the daughter cells will contain recombinant DNA at the end of meiosis

Answer 6

True; only 2 of 4 chromatids are involved in crossing over

Question 7

Recombination rates are higher in females or males?

Answer 7

Females

Question 8

What are the products and non-dysjunction in mitosis?

Answer 8

50% trisomy, 50% monosomy

Question 9

What are the products and non-dysjunction in meiosis I?

Answer 9

50% trisomy (1 paternal allele, 1 maternal allele, 1 allele from the other parent)
50% monosomy.

Question 10

What are the products and non-dysjunction in meiosis II?

Answer 10

50% normal
25% triosomy (2 paternal or maternal allele and 1 allele from the other parent – important to know for trisomic rescue and UPD)
25% monosomy

Question 11

What percentage of XXY is maternal/paternal in origin?

Answer 11

55% maternal
45% paternal

Question 12

What percentage of monosomy X is maternal/paternal in origin?

Answer 12

80% paternal
20% maternal

Question 13

What are the mechanisms of UPD?

Answer 13

Trisomic rescue
Monosomic rescue/mitotic duplication
Somatic crossing over (generating segmental UPD)
Gamete complementation (least likely – when 1 sperm misses the chromosome that the egg has 2 of)

Question 14

What is Isodisomy?

Answer 14

Isodisomy = inheritance of 2 copies of the same homolog from one parent (meiosis II error or monosomic rescue)

Question 15

What is Heterodisomy?

Answer 15

Heterodisomy = inheritance of 2 different homologs from one parent (meiosis I error or trisomic rescue)

Question 16

What percentage of chr abnormal spontaneous abortions are due to triploidy?

Answer 16

15-20% of all chromosomally abnormal spontaneous abortions are due to triploidy (6% of all spontaneous abortions)

Question 17

What percentage of triploidy is diandric vs digynic?

Answer 17

85% Diandric = 2 paternal sets and 1 maternal sets of chromosomes
15% Digynic = 1 paternal set and 2 maternal sets of chromosomes.

Question 18

What is the phenotype of a diandric triploid fetus?

Answer 18

Diandric: Microcephaly and large placenta, do not survive to term.

¾ syndactyly, incomplete ossification of skull, etc.

Question 19

What is the phenotype of a digynic triploid fetus?

Answer 19

Digynic: growth retarded fetus , macrocephaly, small placenta, can survive to term (usually <1 yr).

¾ syndactyly, incomplete ossification of skull, etc.

Question 20

What is a molar pregnancy?

Answer 20

Molar pregnancy is when tissue that normally becomes a fetus instead becomes an abnormal growth in the uterus, this growth triggers symptoms of pregnancy. The placenta has fluid-filled sacs (1-3cm)

Question 21

What is a complete molar pregnancy?

Answer 21

Complete molar: fetus never present, large hydropic placenta, diploid (80% paternal; 85% 46XX, 15% 46XY).
Can occur via empty egg fertilized by 1 sperm then endoduplicates (75%), or if empty egg is fertilized by 2 sperms.
Risk of choriocarcinoma (15-20%).

Question 22

What is a partial molar pregnancy?

Answer 22

Partial mole: fetus present in early stages, mixture of normal and hydrophic villi, polyploidy.

Question 23

What is a benign ovarian teratoma?

Answer 23

46XX karyotype with chromosomes of all maternal origin, arise from abnormal development of primary oocyte – becomes a teratoma.

Question 24

What are some caveats to CVS?

Answer 24

Approximately 2% maternal cell contamination and 1% confined plancental mosaicism.

Question 25

What percentage of cystic hygromas are associated with a chromosome abnormality?

Answer 25

Cystic Hygroma (60%)

Question 26

What percentage of holoprosencephaly is associated with a chromosome abnormality?

Answer 26

Holoprosencephaly (47%)

Question 27

What percentage of T-E fistulas are associated with a chromosome abnormality?

Answer 27

T-E fistula (40%)

Question 28

What percentage of T-E fistulas are associated with a chromosome abnormality?

Answer 28

VSD (38%)

Question 29

What percentage of Patau syndrome have trisomy 13?

Answer 29

80% has trisomy 13, 20% has 13;14 translocations.

Question 30

What is the most common autosomal translocation in humans?

Answer 30

13;14
Incidence about 1/1,100
Most common are der(13;14)(q10;q10) – about 85% of total
Der(14;21)(q10;q10) – about 10%.

Question 31

What is the likelihood of an inversion being inherited?

Answer 31

Inversions = ~90%;

Question 32

What is the likelihood of an unbalanced translocation being inherited?

Answer 32

Unbalanced translocations = 82%;

Question 33

What is the likelihood of a balanced translocation being inherited?

Answer 33

Balanced translocations = 67%.

Question 34

What are the risks for someone with a de novo reciprocal translocation to have a serious congenital abnormality?

Answer 34

Reciprocal balanced translocation = 6.1%

Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)

Question 35

What are the risks for someone with a de novo Robertsonian translocation to have a serious congenital abnormality?

Answer 35

Robertsonian translocation = 3.7%

Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)

Question 36

What are the risks for someone with a de novo inversions to have a serious congenital abnormality?

Answer 36

Inversions = 9.4%

Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)

Question 37

What percentage of large structural chromosome rearrangements are paternal in origin?

Answer 37

Overall, about 75% of cytogenetically detectable (large) structural rearrangements are paternal in origin.

Exceptions include de novo non-homologous Robertsonians (90% maternal) and many of the interstitial microduplications and deletions (there is no parental bias in DiGeorge and Williams syndrome deletions).

Question 38

What percentage of de novo non-homologous Robertsonians rearrangements are maternal?

Answer 38

90% maternal

Question 39

Are paracentric or pericentric inversions more likely to result in dicentric (2 centromeres) or acentric (no centromere) chromosomes?

Answer 39

Paracentric more likely to result in dicentric or acentric chromosomes

Less likely to survive

Question 40

Which are more likely to survive small or large inversions?

Answer 40

Large inversions have smaller distal segments, therefore smaller chance of duplication/deletion therefore more likely to survive

Question 41

Nondisjunction events leading to aneuploidy most often occur in maternal meiosis I or II?

Answer 41

Meiosis I

Trisomy 18 is meiosis II

Question 42

True/False: Maternal age has no impact on the risk for Klinefelter syndrome

Answer 42

False

Question 43

XYY syndrome caused by a meiotic error must have occurred during?

Answer 43

Paternal meiosis II

Question 44

What percentage of all conceptions are triploid?

Answer 44

1-3%

Question 45

What is the general risk of recurrence ANY other viable trisomy after a miscarriage resulting from a non-viable trisomy?

Answer 45

2X