Exam 3 - Preimplantation Genetic Testing Flashcards
What is PGD?
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
What is PGS?
preimplantation genetic screening
- embryos from normal genetic parents are screened for chromosomal disorders
What is PGT?
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)
When is PGT done in the IVF process?
after IVF embryos are created, they are biopsied, then frozen while waiting for results
once results come back the best embryo can be transferred
What do they take during the biopsy for PGT?
5-10 cells from the trophectoderm with minimal trauma to the remaining embryo
What are the two groups of diseases that PGT can screen for?
chromosomal disorders, and single gene disorders
How is a karyotype typically prepared?
- 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
What is euploidy, aneuploidy, trisomy, and triploidy?
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)
If an individual was a female with trisomy 21, how would you denote that?
47XX+21
If an individual was a male with monosomy 18, how would you denote that?
45XY-18
What is a single gene disorder? What are some of the general categories of single gene disorders?
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
If a disease is recessive, what happens when someone has no copies, one copy, or two copies of the mutation?
none - unaffected
one - carrier
two - affected
If a disease is dominant, what happens when someone has no copies, one copy, or two copies of the mutation?
none - unaffected
one - affected
two - affected
If a disease is x linked recessive, what happens when someone has no copies, one copy, or two copies of the mutation?
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
If a disease is x linked dominant, what happens when someone has no copies, one copy, or two copies of the mutation?
males:
can only have 1 copy, but if they have one copy they are affected
females:
one or two copies are affected
If a man has an x linked recessive disease and the woman is not a carrier, which children of his will have the disease?
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)
If a woman carries an x linked recessive disease and the man is not affected/carrier, which children of hers will have the disease?
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
What is an example of codominance?
AB blood - if an A and B are both passed down, neither is dominant
What is multifactorial inheritance?
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.
What are non-inheritable genetic disorders?
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
What is mosaicism?
presence of two or more genetically different cell lines in an individual who has developed from a single zygote (single fertilized egg)
How is chimerism different from mosaicism?
chimerism is a single organism from two different zygotes (and therefore two genetically distinct cell lines)
How does mosaicism occur?
error during meiotic or mitotic cell division, such as non-disjunction or anaphase lag with trisomy rescue
What is meiotic non-disjunction?
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
What is anaphase lag? How does it result in mosaicism?
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
How does the frequency of maternal and paternal trisomies compare?
10X more commonly from maternal origin than paternal origin
What is mitotic non-disjunction? How does it create a mosaicism?
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)
What different “forms” can mosaicism take? How do they occur?
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
If mosaicism is only in the embryo or placenta cells, what implications can this have regarding PGT?
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
Why does the rate of mosaicism decrease during embryonic and fetal development?
embryos can self correct:
- euploid cells proliferate faster than aneuploid cells
- mosaic embryos may half proliferation of abnormal cells or exclude abnormal cells
What is NGS?
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
Can NGS detect mosaicism?
yes, if >20% of biopsied cells are abnormal
