Exam 3 - Preimplantation Genetic Testing

Question 1

What is PGD?

Answer 1

preimplantation genetic diagnosis

used to identify genetic abnormalities in embryos created through IVF before implantation

• one or both parents has a known genetic abnormality
• embryos are screened for a specific mutation

Question 2

What is PGS?

Answer 2

preimplantation genetic screening

• embryos from normal genetic parents are screened for chromosomal disorders

Question 3

What is PGT?

Answer 3

preimplantation genetic testing

PGT-A (aneuploidy, same as PGS, screening for number of chromosomes)

PGT-M (monosomic/single gene defects, same as PGD, screening for mutations)

Question 4

When is PGT done in the IVF process?

Answer 4

after IVF embryos are created, they are biopsied, then frozen while waiting for results

once results come back the best embryo can be transferred

Question 5

What do they take during the biopsy for PGT?

Answer 5

5-10 cells from the trophectoderm with minimal trauma to the remaining embryo

Question 6

What are the two groups of diseases that PGT can screen for?

Answer 6

chromosomal disorders, and single gene disorders

Question 7

How is a karyotype typically prepared?

Answer 7

• lymphocytes from blood samples are grown in culture
• phytohemagglutinin stimulates mitosis
• colcemid stops mitosis in metaphase
• cells are centrifuged and concentrated, than added to a fixative and put on microscope slides
• stained to enhance chromosomes, which are photographed on a microscope and put together into the karyotype

Question 8

What is euploidy, aneuploidy, trisomy, and triploidy?

Answer 8

Euploidy: normal compliment of chromosomes, exact multiple
of haploid number
Aneuploidy: abnormal number of chromosomes
Trisomy: presence of additional (third) chromosome of one type in an otherwise diploid cell
Triploidy: 3 complete sets of chromosomes (3n, 69 in humans)

Question 9

If an individual was a female with trisomy 21, how would you denote that?

Answer 9

47XX+21

Question 10

If an individual was a male with monosomy 18, how would you denote that?

Answer 10

45XY-18

Question 11

What is a single gene disorder? What are some of the general categories of single gene disorders?

Answer 11

diseases that are caused by a single gene, but different mutations of genes can result in the same disease with varying degrees of severity and phenotype

ex:
- autosomal dominant
- autosomal recessive
- x linked
- codominance

Question 12

If a disease is recessive, what happens when someone has no copies, one copy, or two copies of the mutation?

Answer 12

none - unaffected
one - carrier
two - affected

Question 13

If a disease is dominant, what happens when someone has no copies, one copy, or two copies of the mutation?

Answer 13

none - unaffected
one - affected
two - affected

Question 14

If a disease is x linked recessive, what happens when someone has no copies, one copy, or two copies of the mutation?

Answer 14

males:
can only have 1 copy, but if they have one copy they are effected since they dont have a second x chromosome to compensate

females:
1 copy is carrier, 2 is affected

Question 15

If a disease is x linked dominant, what happens when someone has no copies, one copy, or two copies of the mutation?

Answer 15

males:
can only have 1 copy, but if they have one copy they are affected

females:
one or two copies are affected

Question 16

If a man has an x linked recessive disease and the woman is not a carrier, which children of his will have the disease?

Answer 16

His sons will be completely unaffected (because the man will pass on his y chromosome) but his daughters will all be carriers (not affected because the mother is not a carrier)

Question 17

If a woman carries an x linked recessive disease and the man is not affected/carrier, which children of hers will have the disease?

Answer 17

her sons will have a 50/50 chance of having the disease (if she passes on the mutated X, that is their only x chromosome)

her daughters have a 50/50 chance of being a carrier but they wont be affected, because the father will pass on the other X that will compensate for it

Question 18

What is an example of codominance?

Answer 18

AB blood - if an A and B are both passed down, neither is dominant

Question 19

What is multifactorial inheritance?

Answer 19

multiple genes can impact a trait, or environmental factors can impact the trait

Ex: height is impacted by multiple genes and can also be impacted by things like malnutrition, fetal alcohol syndrome, etc.

Question 20

What are non-inheritable genetic disorders?

Answer 20

changes in the number of chromosomes (they result from random events during gamete formation)

De novo mutations (genetic alterations present for the first time in one family member, resulting from a mutation in germ cell or that arises in the zygote

Question 21

What is mosaicism?

Answer 21

presence of two or more genetically different cell lines in an individual who has developed from a single zygote (single fertilized egg)

Question 22

How is chimerism different from mosaicism?

Answer 22

chimerism is a single organism from two different zygotes (and therefore two genetically distinct cell lines)

Question 23

How does mosaicism occur?

Answer 23

error during meiotic or mitotic cell division, such as non-disjunction or anaphase lag with trisomy rescue

Question 24

What is meiotic non-disjunction?

Answer 24

the chromosomes fail to separate correctly resulting in one extra copy of a chromosome in the egg, which then ends up as 3 copies after fertilization in the zygote

Question 25

What is anaphase lag? How does it result in mosaicism?

Answer 25

if there is delayed movement of chromosomes during anaphase, the extra copy of the chromosome is excluded from the nucleus

creates some cells that are trisomy and others that are euploid

Question 26

How does the frequency of maternal and paternal trisomies compare?

Answer 26

10X more commonly from maternal origin than paternal origin

Question 27

What is mitotic non-disjunction? How does it create a mosaicism?

Answer 27

during mitosis the chromosomes fail to separate correctly, resulting in one extra copy of a chromosome in one cell and a monosomy in the other cell (which is lethal)

some cells undergo normal mitosis and are euploid, others that undergo non-disjunction end up trisomy (and some monosomy, but they die)

Question 28

What different “forms” can mosaicism take? How do they occur?

Answer 28

if it happens in the early 4 cell stage, the mosaicism will be in the embryo and placenta (because the mutation occurs before there is a separate inner cell mass and trophectoderm)

if it happens after the blastocyst stage when there are separate embryo and placenta cells, the mosaicism can be confined to the embryo or placenta

Question 29

If mosaicism is only in the embryo or placenta cells, what implications can this have regarding PGT?

Answer 29

if it is only in the embryo - PGT will be normal but embryo actually has a mutation

if it is only the placenta - mutation will be detected but the embryo is actually completely normal

Question 30

Why does the rate of mosaicism decrease during embryonic and fetal development?

Answer 30

embryos can self correct:
- euploid cells proliferate faster than aneuploid cells
- mosaic embryos may half proliferation of abnormal cells or exclude abnormal cells

Question 31

What is NGS?

Answer 31

Next Generation Sequencing

samples undergo whole genome amplification, fragmentation, and sequencing

data is compared to a reference genome to determine the number of chromosomal copies

easily shows the number of each chromosome which can help determine chromosomal abnormalities and sex

Question 32

Can NGS detect mosaicism?

Answer 32

yes, if >20% of biopsied cells are abnormal