17 Questions
What is the term for any chromosome number that is an exact multiple of the number in a haploid gamete (n)?
Euploidy
What condition is considered lethal in humans with chromosome numbers of 69 and 92?
Polyploidy
Which type of cells contain an extra set of the entire genome, with chromosome numbers of 3n and 4n?
Tetraploid cells
What is the term for any chromosome number that is not an exact multiple of the number in a haploid gamete (n)?
Aneuploidy
Which type of abnormality involves a number of chromosomes that is not a multiple of the basic set of chromosomes?
Aneuploidy
What term is used to describe the failure of chromosomes to separate normally during meiotic or mitotic cell divisions?
Nondisjunction
Which syndrome is associated with the karyotype 45,X and is embryonic or fetal lethal in humans, except for the X chromosome?
Turner Syndrome
What type of rearrangement involves no loss or gain of genetic information and usually has no phenotypic effect?
Balanced rearrangements
Which type of translocation involves the exchange of broken segments between nonhomologous chromosomes?
Reciprocal translocation
What is the risk of producing a child with Down Syndrome in a carrier of a Robertsonian translocation?
1/3
Which type of non-reciprocal translocation involves a segment removed from one chromosome being inserted into a different chromosome?
Insertions
In chromosome segregation in an insertion carrier, what can offspring be produced with?
Duplication or deletion of the inserted segment
What kind of mutation involves the production of one or more copies of any piece of DNA, including a gene or even an entire chromosome?
Duplication
Which mechanism of formation involves misdivision through the centromere in meiosis II?
Misdivision through the centromere in meiosis II
What is the most common isochromosome?
Isochromosome of the long arm of the X chromosome
What happens in an abnormal chromosome with two centromeres?
Two chromosome segments fuse end to end, with loss of their acentric fragments
What do marker chromosomes refer to?
-Unidentified chromosomes, called ”marker” chromosomes
Study Notes
- Mixploidy: two or more genetically different cell lineages within one individual, can be mosaicism (from one zygote) or chimerism (from different zygotes)
- Triploidy: three sets of chromosomes, can result from dispermy (two sperm) or failure of meiotic division
- Euploidy tetraploidy: four sets of chromosomes, rare in live births, can be seen at tumors, mosaics can occur from failure of cell division in the first mitotic division or dispermic fertilization
- Aneuploidy: number of chromosomes not a multiple of the basic chromosomes, most common and clinically significant type of human chromosome disorders
- Nondisjunction: failure of chromosomes to separate normally during meiotic or mitotic cell divisions, causes monosomy (one chromosome missing) or trisomy (one extra chromosome)
- Monosomy: Turner Syndrome (45,X), embryonic or fetal lethal in humans except for X chromosome
- Trisomy: Trisomy 21 (Down Syndrome, 95% of cases free trisomy 21, 4% Robertsonian translocations, 1% mosaic), Trisomy 18 (Edwards Syndrome), Trisomy 13 (Patau Syndrome), 47,XXY (Klinefelter Syndrome), 47,XYY, 47,XXX
- Abnormalities of Chromosome Structure: two kinds of rearrangements, balanced and unbalanced
- Balanced rearrangements: no loss or gain of genetic information, change in chromosomal gene order, inversions, translocations, insertions, usually have no phenotypic effect
- Unbalanced rearrangements: net gain or loss of DNA with change in gene dosage, deletions, duplications, larger deletions or duplications have greater chance of phenotypic consequences
- Balanced Rearrangements: carrier of a balanced chromosome rearrangement is usually phenotypically normal, but there is a risk of unbalanced gametes and therefore of unbalanced offspring
- Inversions: single chromosome undergoes two breaks and is reconstituted with the segment between the breaks inverted, pericentric (including centromere) and paracentric (not including centromere)
- Translocations: reciprocal translocation (nonhomologous chromosomes exchange broken segments, present in 1 of every 600 newborn, familial or de novo), Robertsonian translocation (centric fusion, loss of short arms of acrocentric chromosomes, estimated risk 1%)
- Robertsonian Translocation: short arms of acrocentric chromosomes consist largely of satellite DNA and hundreds of copies of ribosomal RNA genes, loss of short arms is not deleterious, karyotype considered balanced despite having only 45 chromosomes.
- Nondisjunction during mitosis can also occur after formation of the zygote, causing mosaicism.
- Anaphase lag: one homologous chromosome or one chromatid fails to join to the daughter cell, causing one normal and one monosomic cell.
- Kinds of Aneuploidy: monosomy (Turner Syndrome, 45,X), trisomy (Trisomy 21, Down Syndrome, Trisomy 18, Edwards Syndrome, Trisomy 13, Patau Syndrome, 47,XXY, Klinefelter Syndrome, 47,XYY, 47,XXX)
- Structural rearrangements: result from chromosome breakage and reconstitution in an abnormal combination, balanced rearrangements (no loss or gain of genetic information, change in chromosomal gene order, inversions, translocations, insertions) and unbalanced rearrangements (net gain or loss of DNA with change in gene dosage, deletions, duplications)
- Balanced rearrangements: carrier is usually phenotypically normal but there is a risk of unbalanced gametes and offspring
- Inversions: single chromosome undergoes two breaks and is reconstituted with the segment between the breaks inverted, pericentric and paracentric
- Translocations: reciprocal translocation (exchange of broken segments between nonhomologous chromosomes), Robertsonian translocation (centric fusion)
- Robertsonian translocation involves loss of short arms of two acrocentric chromosomes, considered balanced despite having only 45 chromosomes.
- Offspring: gametes that can be produced by a carrier of a Robertsonian translocation (t(14;21))
Test your knowledge of chromosome abnormalities and disorders that are responsible for genetic diseases, reproductive wastage, congenital malformations, mental retardation, and the pathogenesis of malignancy. Learn about the specific chromosome abnormalities responsible for over 100 identifiable syndromes and their prevalence in live births and pregnancies.
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