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 ______ _______.