Chromosome Number Prestudy Flashcards
What is a nondisjunction?
error in cell division whereby improper segregation leads to abnormal chromosome number (monosomy or trisomy).
When can nondisjunction occur?
in any cell division during Meiosis I, Meiosis II, or mitosis
What will nondisjunction during mitosis cause?
mosaicism: mixture of normal cells with mutated cells with chromosomal abnormalities
What is the main risk factor for meiosis I nondisjunction?
age
What is euploidy?
normal chromosome number (haploids = 23, diploids = 46)
What is aneuploidy?
any abnormality of chromosome number that is not a multiple of 23.
Example: monosomy (n=45)
trisomy: n = 47
What is polypoidy?
abnormalities in number of chromosomes with multiples of 23.
Ex: tripoidy n = 69, tetraploidy n=92
When is triploidly most common?
at fertilization if two sperm fertilize one egg
What is the most common mechanism of tetraploidy?
after DNA duplicates, but cells fail to divide
What is ISCN? Give an example of normal genotype?
international system for human cytogenetic nomenclature.
Normal: 46,XX or 46, XY
How will a monosomy affect an autosome? Trisomies?
all monosomies are lethal in autosomes. Most trisomies are lethal. (potential for live birth- 13, 18, 21)
What happens with aneuploidy in sex chromosomes?
tolerate aneuploidy better than autosomes do, but at least one X is required for survival
Which is usually the least severe disjunction?
mosaicism, because the phenotype is less severe due to normal cell lines mixed with abnormal instead of all cells being abnormal
How common is aneuploidy in female eggs? Sperm?
about 20% in eggs
about 1-2% in sperm
What is most likely outcome of aneuploidy in a pregnancy?
failure of the pregnancy
How common are spontaneous abortions/miscarriages?
15% of recognized pregnancies end in miscarriage
How often do fertilized eggs fail to come to term?
about 50% of fertilized eggs will fail
What is the most common cause of miscarriage?
chromosome imbalance
What is the most common chromosomal abnormality involved in miscarriage? What is the most common class of chromosomal abnormality?
45, X is single most common at 20% (turner’s syndrome)
trisomies together make up most common class. (nearly 55%)
What are the viable trisomies?
21, 18, 13
What are the common clinical findings for trisomy 21?
down syndrome usually has mental retardation, flat facial profile, prominent epicanthal folds, simian crease, duodenal atresia (intestinal obsturction) and congential heart defects
What is the most common cause of trisomy 21?
nondisjunction during maternal meiosis I.
Usually only 2-4% mosaic and 5% translocation
What is Edwards syndrome?
Trisomy 18. Severe mental retardation, prenatal growth deficiency, heart defects, rocker bottom feet, clenched hands, usually die before age 1.
Usually from maternal nondisjunction
What is the ISCN nomeclature for trisomy 18 in a female?
47,XX,+18
What is Patau Syndrome?
Trisomy 13. Abnormal midface, eye and forebrain. Polydactyly and midline defects. rapidly fatal
What is the Positive Exposure project?
project that tries to promote genetic diversity as a good thing, not something to hide away. Photographs children with abnormalities and tries to make them feel good about themselves
What will happen to an individual with no X chromosomes?
they will die it is lethal. Usually sex chromosome abnormalities are not as serious.
What is turner syndrome?
Only have single X chromosome. short stature, ovariag dysgenesis, congenital heart defect, lymphatic problems. normal intelligence.
What is the karyotype for Turner Syndrome?
45,X.
What is the only monosomy associated with human live birth?
45,X turner syndrome
What is the most common cause of turner syndrome?
PATERNAL nondisjunction (50% of the time it is meiotic nondisjunct and 80% of that is paternal) Mosaicism is also pretty common (40%)
Will advanced maternal age significantly increase risk for Turner Syndrome?
no, turner syndrome is mostly paternal
Is turner syndrome often fatal for fetuses?
Yes. over 99% of fetuses die before birth. After birth they have high chance of living.
What is Klinefelter Syndrome?
47, XXY
What is the most common cause of Klinefelter syndrome?
half of the time it is paternal nondisjunct
half of the time it is maternal and has a maternal age risk effect
Does Klinefelter syndrome produce mental retardation?
no, but it does typically drop IQ by about 10-15 points
When is Klinefelter usually diagnosed?
when puberty doesnt occur normally or the man is infertile later in life
What is triple X syndrome?
47, XXX. Essentially normal phenotype, mild learning problems
What usually causes triple X syndrome?
maternal nondisjunction.
What is 47,XYY syndrome and what are its phenotypes?
essentially normal phenotype, but tall and has educational and behavioral problems
What is the most common cause of 47,XYY syndrome?
always from paternal meiosis II nondisjunction
