Cytogenetics Flashcards
At the start of meiosis, the cell has how many chromosomes and chromatids?
46 chromosomes, each with 2 chromatids
Genetic diversity is achieved through what two processes?
Crossing Over
Independent Assortment/Segregation
At the end or meiosis I, the cell has how many chromosomes and how many chromatids?
23 chromosomes, each with 2 chromatids
At the end of meiosis II, the cell has how many chromosomes and how many chromatids?
23 chromosomes, each with 1 chromatid
At the end of mitosis, the cell has how many chromosomes and how many chromatids?
46 chromosomes, each with 1 chromatid
True or False: Only half of the daughter cells will contain recombinant DNA at the end of meiosis
True; only 2 of 4 chromatids are involved in crossing over
Recombination rates are higher in females or males?
Females
What are the products and non-dysjunction in mitosis?
50% trisomy, 50% monosomy
What are the products and non-dysjunction in meiosis I?
50% trisomy (1 paternal allele, 1 maternal allele, 1 allele from the other parent)
50% monosomy.
What are the products and non-dysjunction in meiosis II?
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
What percentage of XXY is maternal/paternal in origin?
55% maternal
45% paternal
What percentage of monosomy X is maternal/paternal in origin?
80% paternal
20% maternal
What are the mechanisms of UPD?
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)
What is Isodisomy?
Isodisomy = inheritance of 2 copies of the same homolog from one parent (meiosis II error or monosomic rescue)
What is Heterodisomy?
Heterodisomy = inheritance of 2 different homologs from one parent (meiosis I error or trisomic rescue)
What percentage of chr abnormal spontaneous abortions are due to triploidy?
15-20% of all chromosomally abnormal spontaneous abortions are due to triploidy (6% of all spontaneous abortions)
What percentage of triploidy is diandric vs digynic?
85% Diandric = 2 paternal sets and 1 maternal sets of chromosomes
15% Digynic = 1 paternal set and 2 maternal sets of chromosomes.
What is the phenotype of a diandric triploid fetus?
Diandric: Microcephaly and large placenta, do not survive to term.
¾ syndactyly, incomplete ossification of skull, etc.
What is the phenotype of a digynic triploid fetus?
Digynic: growth retarded fetus , macrocephaly, small placenta, can survive to term (usually <1 yr).
¾ syndactyly, incomplete ossification of skull, etc.
What is a molar pregnancy?
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)
What is a complete molar pregnancy?
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%).
What is a partial molar pregnancy?
Partial mole: fetus present in early stages, mixture of normal and hydrophic villi, polyploidy.
What is a benign ovarian teratoma?
46XX karyotype with chromosomes of all maternal origin, arise from abnormal development of primary oocyte – becomes a teratoma.
What are some caveats to CVS?
Approximately 2% maternal cell contamination and 1% confined plancental mosaicism.
What percentage of cystic hygromas are associated with a chromosome abnormality?
Cystic Hygroma (60%)
What percentage of holoprosencephaly is associated with a chromosome abnormality?
Holoprosencephaly (47%)
What percentage of T-E fistulas are associated with a chromosome abnormality?
T-E fistula (40%)
What percentage of T-E fistulas are associated with a chromosome abnormality?
VSD (38%)
What percentage of Patau syndrome have trisomy 13?
80% has trisomy 13, 20% has 13;14 translocations.
What is the most common autosomal translocation in humans?
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%.
What is the likelihood of an inversion being inherited?
Inversions = ~90%;
What is the likelihood of an unbalanced translocation being inherited?
Unbalanced translocations = 82%;
What is the likelihood of a balanced translocation being inherited?
Balanced translocations = 67%.
What are the risks for someone with a de novo reciprocal translocation to have a serious congenital abnormality?
Reciprocal balanced translocation = 6.1%
Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)
What are the risks for someone with a de novo Robertsonian translocation to have a serious congenital abnormality?
Robertsonian translocation = 3.7%
Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)
What are the risks for someone with a de novo inversions to have a serious congenital abnormality?
Inversions = 9.4%
Robertsonian translocation = 3.7%
reciprocal balanced translocation = 6.1%
inversions = 9.4%
(R-B-I = 3-6-9)
What percentage of large structural chromosome rearrangements are paternal in origin?
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).
What percentage of de novo non-homologous Robertsonians rearrangements are maternal?
90% maternal
Are paracentric or pericentric inversions more likely to result in dicentric (2 centromeres) or acentric (no centromere) chromosomes?
Paracentric more likely to result in dicentric or acentric chromosomes
Less likely to survive
Which are more likely to survive small or large inversions?
Large inversions have smaller distal segments, therefore smaller chance of duplication/deletion therefore more likely to survive
Nondisjunction events leading to aneuploidy most often occur in maternal meiosis I or II?
Meiosis I
Trisomy 18 is meiosis II
True/False: Maternal age has no impact on the risk for Klinefelter syndrome
False
XYY syndrome caused by a meiotic error must have occurred during?
Paternal meiosis II
What percentage of all conceptions are triploid?
1-3%
What is the general risk of recurrence ANY other viable trisomy after a miscarriage resulting from a non-viable trisomy?
2X
