Sheet 5 Flashcards
Robertsonian translocation is a special case for which chromosomes?
Acrocentric chromosomes
What are the causes of triploidy?
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)
What causes tetraploidy?
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.
Which one is viable in humans: Triploidy or tetraploidy?
Neither case is viable
What is maternal tripoidy?
Triploidy caused by the maternal egg (egg is 2n)
What is paternal tripoidy (or diandry/dispermy)?
Triploidy caused by the paternal sperm (sperm is 2n)
What are the most common clinical signs of triploidy?
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
What characterizes a molar pregnancy?
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.
What is the characteristic appearance of a Molar pregnancy?
Abnormal uterine tissue with a characteristic grape-like appearance called “Bunches of grapes” appearance.
What happens if the sperm accidentally throws out the egg’s chromosomes?
The cell (23 chromosomes) undergoes chromosomal rescue
What is chromosomal rescue?
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.
Is there a fetus and is it viable after chromosomal rescue?
There is no fetus (not viable)
If 2 sperm fertilize 1 egg, is there a fetus and is it viable?
There is a fetus, but it’s not viable (will die during pregnancy)
What is a complete mole?
The oocyte only carries the paternal DNA, while the maternal DNA is missing. So, we say it has somehow lost its DNA.
How can a complete mole happen?
#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)
What is the karyotype for a complete mole?
Diploid – 46 chromosomes (46XY or 46XX – the 46YYs are not viable)
Who does the DNA belong to in a complete mole?
All paternal
How can a partial mole happen?
#1: The oocyte is fertilized by 1 sperm and the paternal DNA reduplicates itself. #2: The oocyte is fertilized by 2 separate sperms.
What is a partial mole?
The oocyte has an intact set of maternal DNA.
What is the karyotype for a partial mole?
Triploid - 69 chromosomes (69 XXY- an extra set of paternal DNA)
Two distinct phenotypes of human triploid fetuses have been recognized according to what?
The parental origin of the extra haploid set.
What is triploidy of diandric type?
The extra haploid set of chromosomes arises from the father
What is triploidy of digynic type?
The extra haploid set of chromosomes arises from the mother.
What is the difference between a diandric fetus and a digynic fetus?
Diandric fetuses appear relatively well grown with a large placenta, while digynic fetuses show intrauterine growth retardation with a
small placenta.
