Chapter 9 - Terms (Tom's): Chromosome Variation
fragile-X syndrome
A form of X-linked mental retardation that appears primarily in males; results from an increase in the number of repeats of a CGG trinucleotide.
nullisomy
Absence of both chromosomes of a homologous pair (2n - 2).
monosomy
Absence of one of the chromosomes of a homologous pair.
Rearrangements
Alters the structures of chromosomes. Change from the wild type in the structure of one or more chromosomes.
dicentric bridge
Area in dicentric chromatid indicated, resulting from paracentric inversion with crossing over.
Yes
Can problems with translocations can be similar to problems with inversions?
primary Down syndrome
Caused by the presence of three copies of chromosome 21.
aneuploidy
Change from the wild type in the number of chromosomes; most often an increase or decrease of one or two chromosomes.
Patau syndrome (trisomy 13)
Characterized by severe mental retardation, a small head, sloping forehead, small eyes, cleft lip and palate, extra fingers and toes, and other disabilities; results from the presence of three copies of chromosome 13.
Edward syndrome (trisomy 18)
Characterized by severe retardation, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities; results from the presence of three copies of chromosome 18.
Down syndrome (trisomy 21)
Characterized by variable degrees of mental retardation, characteristic facial features, some retardation of growth and development, and an increased incidence of heart defects, leukemia, and other abnormalities; caused by the duplication of all or part of chromosome 21.
acentric chromatid
Chromatid indicated resulting from paracentric inversion with crossing over.
dicentric chromatid
Chromatid indicated resulting from paracentric inversion with crossing over.
telocentric chromosome
Chromosome in which the centromere is at or very near one end.
submetacentric chromosome
Chromosome in which the centromere is displaced toward one end, producing a short arm and a long arm.
acrocentric chromosome
Chromosome in which the centromere is near one end, producing a long arm at one end and a knob, or satellite, at the other end.
metacentric chromosome
Chromosome in which the two chromosome arms are approximately the same length.
autopolyploidy
Condition in which all the sets of chromsomes of a polyploid individual possessing more than two haploid sets are derived from a single species.
mosaicism
Condition in which regions of tissue within a single individual have different chromosome constitutions.
allopolyploidy
Condition in which the sets of chromosomes of a polyploid individual possessing more than two haploid sets are derived from two or more species.
fragile site
Constriction or gap that appears at a particular location on a chromosome when cells are cultured under special conditions. One fragile site on the human X chromosome is associated with mental retardation (fragile-X syndrome) and results from an expanding trinucleotide repeat.
Deletions are all lengths
Deletions are short, long, extremely long encompassing numerous genes or all of the above.
one nonrecombinant gamete from original / one acentric chromatid (dissintegrates) / two nonviable recombinant gametes resulting from tearing apart of dicentric chromatid / one nonrecombinant gamete with paracentric inversion from original.
Describe gamete products of paracentric inversion with crossover.
one nonrecombinant gamete from original / two nonviable recombinant gametes where segments are missing from one chromatid and appear on the other (ends are mirrors of each other) / one nonrecombinant gamete with pericentric inversion from original.
Describe gamete products of pericentric inversion with crossover.
q
Designation of the long arm of a human submetacentric chromosome (letter).
p
Designation of the short arm of a human submetacentric chromosome (letter).
copy-number variation (CNV)
Difference among individual organisms in the number of copies of any large DNA sequence (larger than 1000 bp).
chromosome mutation
Difference from the wild type in the number or structure of one or more chromosomes; often affects many genes and has large phenotypic effects.
2n
Diploid number. (?n)
Yes
Do deletions frequently accompany translocations?
reverse duplication
Duplication of a chromosome segment in which the sequence of the duplicated segment is inverted relative to the sequence of the original segment.
Duplications are all lengths
Duplications are short, long, extremely long encompassing numerous genes or all of the above.
deletions and duplications (more?)
During meiosis, both ________ and _______ cause a "bump" to form when homologous chromosomes pair.
imbalances in genetic pathways "abnormal gene dosage"
Effects of duplications:
source of new genes for evolution
Effects of duplications:
If inversions occur within a gene, you can lose gene function (bad news if that is an important gene).
Effects of inversions:
Pericentric inversion with crossover - reduced fertility due to nonviable recombinant gametes.
Effects of inversions:
Can link genes formerly located on different chromosomes.
Effects of translocations:
Chromosomal breaks that bring about translocation may take place within a gene and disrupt its function.
Effects of translocations:
Genes translocated may come under control of different regulatory sequences or other genes that affect their expression.
Effects of translocations:
Pseudodominance
Expression of a recessive gene when the homologous copy is missing.
Duplications, Deletions, Inversions, Translocations & Fragile sites*
Five major types of Rearrangement mutations.
unbalanced gamete
Gamete that has a variable number of chromosomes; some chromosomes may be missing and others may be present in more than one copy.
reciprocal translocation
Genetic information is exchanged between non-homologous chromosomes (more common).
nonreciprocal translocation
Genetic information moves from one chromosome to a non-homologous chromosome.
1n
Haploid number. (?n)
Not rare at all - humans contain average of 1000 copy number variations (deletions, duplications)
How rare are mutations in nature?
By a Robertsonian translocation between two chromosomes in our common ancestors with apes.
How was chromosome 2 in humans likely formed?
Yes
If inversions happen near a gene, can it affect function?
translocation carrier
Individual organism heterozygous for a chromosome translocation.
gynandromorph
Individual organism that is a mosaic for the sex chromosomes, possessing tissues with different sex-chromosome constitutions.
uniparental disomy
Inheritance of both chromosomes of a homologous pair from a single parent.
deletion
Is it more likely that a deletion or duplication would be lethal?
chromosome deletion
Loss of a chromosome segment.
translocation
Movement of genetic material between non-homologous chromosomes.
deletion
Mutation shown here.
duplication (tandem duplication)
Mutation shown here.
inversion (paracentric inversion)
Mutation shown here.
paracentric inversion with crossing over
Mutation shown here.
pericentric inversion with crossover
Mutation shown here.
translocation (reciprocal translocation)
Mutation shown here.
chromosome duplication
Mutation that doubles a segment of a chromosome.
occurs on acrocentric chromosomes
On what type of chromosome does Robertsonian translocation occur?
Polyploidy
One or more complete sets of chromosomes are added.
3n, 4n, 5n or more
Polyploid number. (?n)
polyploidy
Possession of more than two haploid sets of chromosomes.
trisomy
Presence of an additional copy of a chromosome (2n + 1).
trisomy 8
Presence of three copies of chromosome 8; in humans, results in mental retardation, contracted fingers and toes, low-set malformed ears, and a prominent forehead.
tetrasomy
Presence of two extra copies of a chromosome (2n + 2).
chromosome inversion
Rearrangement in which a segment of a chromosome has been inverted 180 degrees.
segmental duplications
Regions larger than 1000 bp that are almost identical in sequence in eukaryotic genomes.
familial Down syndrome
Syndrome in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families.
karyotype
The complete set of chromosomes possessed by an organism. Usually presented as a picture of metaphase chromosomes lined up in descending order of their size.
tandem duplication
The duplicated region of a duplication mutation "sits" next to the original copy.
displaced duplication
The duplicated region of a duplication mutation is in a different location (or on a different chromosome).
Aneuploidy
The number of chromosomes is altered: one or more complete chromosomes are added or deleted.
Rearrangements, Aneuploidy & Polyploidy
Three main classes of chromosome mutations.
Robertsonian translocation
Translocation in which the long arms of two acrocentric chromosomes become joined to a common centromere, resulting in a chromosome with two long arms and usually another chromosome with two short arms.
amphidiploidy
Type of allopolyploidy in which two different diploid genomes are combined such that every chromosome has one and only one homologous partner and the genome is functionally diploid.
paracentric inversion - centromere not included in inversion
Type of inversion shown here.
pericentric inversion - centromere is included in inversion
Type of inversion shown here.
aneuploidy (trisomy)
Type of mutation shown here.
polyploidy (autotriploid)
Type of mutation shown here.
rearrangement (duplication)
Type of mutation shown here.
adjacent-1 segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-1 segregation takes place when N1 and T2 move toward one pole and T1 and N2 move toward the opposite pole. (nonviable gametes)
adjacent-2 segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then adjacent-2 segregation takes place when N1 and T1 move toward one pole and T2 and N2 move toward the opposite pole. (nonviable gametes)
alternate segregation
Type of segregation that takes place in a heterozygote for a translocation. If the original, nontranslocated chromosomes are N1 and N2 and the chromosomes containing the translocated segments are T1 and T2, then alternate segregation takes place when N1 and N2 move toward one pole and T1 and T2 move toward the opposite pole. (viable gametes)
reduces chromosome number
What is effect of Robertsonian translocation on chromosome number?
Robertsonian translocations
What is the cause of familial Down Syndrome in which the long arm of chromosome 21 is translocated to another chromosome; tends to run in families (seen here)?
Haploinsufficiency.
When one copy of a gene is not enough to provide wild type function (look at the Notch example in your book)
deletions - missing components in biological pathways - "gene dosage" is missing
Which is generally more problematic, duplications or deletions - why?
position effect
if gene positions are altered by an inversion, they may be expressed at inappropriate times or in inappropriate tissues, an outcome referred to as a ______ _______.
