Numerical Chromosomal Abnormalities Flashcards
Chromosomal abnormalities occurring during mitosis result in what type of mutation?
Cause mosaicism
Somatic mutations
Germ line mutations if they arise in germ line predecessors
Chromosomal abnormalities occurring during meiosis result in what type of mutations?
Germ line mutations
What is the synaptonemal complex?
Proteins that link the homologous chromosomes to each other during crossover
Homologous chromosomes separate during which meiotic division?
Meiosis I
Sister chromatids separate during which meiotic division?
Meiosis II
Female gametes are arrested in which stage until ovulation?
Meiosis I
What are chiasmata and what critical role to they play?
Chiasmata are the points of connection between homologous chromosomes during crossing over.
This physical link is critical for normal chromosomal disjunction.
Two processes that result in genetic variability between gametes.
Independent assortment of parental chromosomes. ~8x10^6
Crossing over of homologous chromosomes.
Approximate number of possible variants ~80^23
What is the ploidy of normal somatic cells during G2?
2n
What is the ploidy of normal gametes after meiosis I?
2n
A non disjunction during what phase of meiosis results in 100% abnormal cells?
Meiosis I
A non disjunction during what phase of meiosis results in 50% abnormal cells?
Meiosis II
What do Gimsa dark regions in a chromosome indicate?
High concentrations of A-T pairs
What do p and q signify?
p denotes loci on the short (petite) arm of a chromosome
q denotes loci on the long arm of a chromosome
What is the difference between daughter and mother cells in mitosis and meiosis?
In mitosis, the daughter and mother cells are genetically identical.
In meiosis, the daughter cells have wide genetic variation from the mother cells and 1/2 the total chromosomes (n vs. 2n)
What is G-Banding?
The resulting striated coloration of chromosomes after dyeing with Gimsa dye.
What are the three classifications of chromosomes, based on centromere location?
Metacentric - centered centromere approximately equal p & q lengths
Sub-metacentric - off-center centromere
Acrocentric - centromere nearly at end of chromosome, only stalk and satellite region beyond it
How are genetic abnormalities denoted in short hand?
Total number of chromosomes
Sex chromosomes
Abnormality type
Chromosome on which the abnormality occurs
Specific locus of abnormality (p/q numbers)
Extra copies of all chromosomes is termed:
Polyploidy
E.g.: triploidy (3n), tetraploidy (4n)
Gain or loss of one chromosome is termed:
Aneuploidy
E.g.: Trisomy 2, monosomy X
What is the most commonly occurring pathway that results in triploidy?
Dispermy - simultaneous fertilization of the egg by two sperm or by a single 2n sperm
What is the most commonly occurring pathway that results in tetraploidy?
Endomitosis, wherein the cell undergoes DNA replication, but fails to separate the replicated strands during cell division.
In mothers over age 35, what is the frequency of abnormal chromosomes in fetuses, and what percentage of those abnormalities are numerical?
1:50 fetuses have abnormal chromosomes.
85% are numerical abnormalities.
What is the most common mechanism leading to aneuploidy?
Meiotic chromosome nondisjunction
Aberrations in the frequency or location, or both, of recombination events in meiosis I are associated with what?
Increased rates of meiosis I non disjunction
What does “terminalization” refer to?
The loss of cohesion between homologous chromosomes and the movement of chiasmata to the ends of the chromosomes. Causes precocious separation of chromosomes, known as meiosis I nondisjunction.
Trisomy 21 name, incidence & characteristics.
Down Syndrome
1/830 live births
Intellectual disabilities, characteristic features, short stature, gastrointestinal abnormalities, early onset Alzheimer’s
Trisomy 13 name, incidence, and characteristics.
Patau syndrome
1/22700
CNS abnormalities, facial clefts, renal hyperplasia, congenital heart disease
Trisomy 18 name, incidence, and characteristics.
Edwards syndrome
1/7500
Hypertonicity, CNS abnormalities, growth retardation, congenital heart disease
What is the overall incidence of trisomy disorders in live births?
Approximately 1:700
Why are phenotypes associated with sex chromosome aneuploidy often less severe than those associated with autosomal aneuploidy?
Due to X chromosome inactivation and the relatively low number of genes that reside on the Y chromosome.
Defects in sex chromosome number can often be traced to what common factor?
Errors in maternal meiosis as a result of increased maternal age.
Klinefelter Syndrome abbreviation, incidence, and common cause.
(47, XXY)
1:1000 live male births
50% from errors in MALE meiosis I due to failure of recombination in pseudoautosomal regions
15% result from mosaicism, commonly (46, XY/47, XXY)
47, XYY syndrome incidence, characteristics, and cause.
Incidence is 1:1000 live male births
Caused by paternal meiosis II errors, producing YY sperm
Indistinguishable physically or mentally from unaffected males, increased risk of behavioral and educational problems, delayed language ability
Turner syndrome abbreviation, incidence, characteristics and cause.
45, X
1:4000 live female births, 25% mosaic
>99% abort spontaneously
Short stature, webbed neck, edema of hands and feet, renal and cardiovascular anomalies
What is mosaicism and what causes it?
Mosaicism is the presence of two or more chromosome compliments within one individual (ie. some cells 46, XY/ others 47,XY,+21)
Commonly caused by nondisjunction in an early post-zygotic mitotic division.
What are the two types of mosaicism?
Polyploidy (2n/4n)
Aneuploidy (normal/trisomy 21)
