Sheet 5

Question 1

Robertsonian translocation is a special case for which chromosomes?

Answer 1

Acrocentric chromosomes

Question 2

What are the causes of triploidy?

Answer 2

1) 66% due to 2 sperms fertilizing 1 egg by mistake (1n sperm + 1n sperm + 1n egg)
2) 10% due to 1 egg carrying 2 sets of chromosomes with 1 set of chromosomes being contributed by the sperm. (2n egg + 1n sperm)
3) 25% due to a sperm carrying two sets of chromosomes fertilizing an egg carrying a haploid (1n) (2n sperm + 1n egg)

Question 3

What causes tetraploidy?

Answer 3

Endomitosis: When DNA replicates (instead of 2n we have 2n+2n) without subsequent cell division (cytoplasm, in this case, does not divide), so we end up with one cell carrying 4n.

Question 4

Which one is viable in humans: Triploidy or tetraploidy?

Answer 4

Neither case is viable

Question 5

What is maternal tripoidy?

Answer 5

Triploidy caused by the maternal egg (egg is 2n)

Question 6

What is paternal tripoidy (or diandry/dispermy)?

Answer 6

Triploidy caused by the paternal sperm (sperm is 2n)

Question 7

What are the most common clinical signs of triploidy?

Answer 7

1) Hydatidiform mole (characteristic, >90% cases)
2) Severe intrauterine growth retardation
3) Macrocephaly
4) Total syndactyly of third and fourth fingers
5) CNS, heart, and renal defects

Question 8

What characterizes a molar pregnancy?

Answer 8

1) The embryo lacks all embryonic tissues
2) The embryo has an abnormal benign growth (it is not cancerous).
3) The uterus is full of abnormal tissue.

Question 9

What is the characteristic appearance of a Molar pregnancy?

Answer 9

Abnormal uterine tissue with a characteristic grape-like appearance called “Bunches of grapes” appearance.

Question 10

What happens if the sperm accidentally throws out the egg’s chromosomes?

Answer 10

The cell (23 chromosomes) undergoes chromosomal rescue

Question 11

What is chromosomal rescue?

Answer 11

A mechanism that takes place when there are 23 chromosomes instead of
46 chromosomes. To cope with this situation, the cell replicates its DNA to end up with 46 chromosomes (all of which are paternal), with no maternal genes. This is considered a complete
molar pregnancy, since we only have paternal genes.

Question 12

Is there a fetus and is it viable after chromosomal rescue?

Answer 12

There is no fetus (not viable)

Question 13

If 2 sperm fertilize 1 egg, is there a fetus and is it viable?

Answer 13

There is a fetus, but it’s not viable (will die during pregnancy)

Question 14

What is a complete mole?

Answer 14

The oocyte only carries the paternal DNA, while the maternal DNA is missing. So, we say it has somehow lost its DNA.

Question 15

How can a complete mole happen?

Answer 15

#1: The oocyte is fertilized by 1 sperm and the paternal DNA reduplicates itself (homozygous)
#2: The oocyte is fertilized by 2 separate sperms. (heterozygous)

Question 16

What is the karyotype for a complete mole?

Answer 16

Diploid – 46 chromosomes (46XY or 46XX – the 46YYs are not viable)

Question 17

Who does the DNA belong to in a complete mole?

Answer 17

All paternal

Question 18

How can a partial mole happen?

Answer 18

#1: The oocyte is fertilized by 1 sperm and the paternal DNA reduplicates itself.
#2: The oocyte is fertilized by 2 separate sperms.

Question 19

What is a partial mole?

Answer 19

The oocyte has an intact set of maternal DNA.

Question 20

What is the karyotype for a partial mole?

Answer 20

Triploid - 69 chromosomes (69 XXY- an extra set of paternal DNA)

Question 21

Two distinct phenotypes of human triploid fetuses have been recognized according to what?

Answer 21

The parental origin of the extra haploid set.

Question 22

What is triploidy of diandric type?

Answer 22

The extra haploid set of chromosomes arises from the father

Question 23

What is triploidy of digynic type?

Answer 23

The extra haploid set of chromosomes arises from the mother.

Question 24

What is the difference between a diandric fetus and a digynic fetus?

Answer 24

Diandric fetuses appear relatively well grown with a large placenta, while digynic fetuses show intrauterine growth retardation with a
small placenta.

Question 25

Uniparental diploidy changes the balance between:

Answer 25

The embryo/fetus and its supporting membranes.

Question 26

What does paternal uniparental diploidy produce?

Answer 26

Hydatidiform moles: abnormal conspectuses that develop to show widespread hyperplasia (overgrowth) of the trophoblast but no fetal parts

Question 27

What can a hydatidiform mole transform into?

Answer 27

Choriocarcinoma

Question 28

What does maternal uniparental diploidy produce?

Answer 28

Ovarian teratomas: rare benign tumors of the ovary which consist of disorganized embryonic tissue but are lacking in vital extra-embryonic membranes.

Question 29

What are the findings in triploidy?

Answer 29

1) CHD
2) Kidney anomalies
3) Low-set, malformed ears
4) Hypertelorism
5) Foot deformities
6) Abdominal wall defects

Question 30

What are the findings in diandric triploidy?

Answer 30

1) Enlarged placenta
2) Cyst-like placenta
3) Well-formed fetus with or without microcephaly.

Question 31

What are the findings in digynic triploidy?

Answer 31

1) Macrocephaly

2) Severe intrauterine growth retardation

Question 32

What is mosaicism?

Answer 32

Two or more distinct cell lines from a single zygote differing because of a mutation or nondisjunction (normal cells + mutated cells)

Question 33

What abnormal genetic changes might occur as a zygote undergoes mitosis?

Answer 33

1) Mistakes made by DNA polymerase during the S phase causing a sequence mutation
2) Nondisjunction taking place during anaphase

Question 34

What is a chimera?

Answer 34

An individual made up of 2 zygotes (zygote twin absorbing its other twin)

Question 35

How does a chimera happen?

Answer 35

#1: 2 zygotes fuse during early development
#2: 2 zygotes exchange cells