Chapter 8 Chromosome mutations: variation in chromosome number and arrangement

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Answer

What are chromosome mutations (aberrations)?

Variations from the normal diploid chromosome content, including: a change in the total number of chromosomes, the deletion or duplication of genes or segments of a chromosome, and rearrangements of the genetic material either within or among chromosomes.

What is aneuploidy?

A condition in which an organism gains or loses one or more chromosomes and has other than an exact multiple of the haploid set (2n ± x chromosomes).

What is euploidy?

A condition in which complete haploid sets of chromosomes are present (multiples of n).

What is monosomy?

The loss of one chromosome to produce a 2n − 1 complement.

What is disomy?

A normal complement of 2n chromosomes (no gain or loss).

What is trisomy?

The presence of three copies of one chromosome (2n + 1 chromosomes).

What is tetrasomy and pentasomy?

Tetrasomy = 2n + 2; pentasomy = 2n + 3.

What is polyploidy?

The presence of more than two haploid sets of chromosomes: triploidy (3n), tetraploidy (4n), pentaploidy (5n), etc.

What is autopolyploidy?

Polyploidy in which multiple sets of chromosomes are identical to the haploid complement of the same species.

What is allopolyploidy?

Polyploidy resulting from the combination of chromosome sets from different species as a consequence of interspecific matings.

What is an amphidiploid?

A fertile polyploid produced when a sterile hybrid undergoes a natural chromosomal doubling.

What is nondisjunction?

The failure of chromosomes or chromatids to disjoin and move to opposite poles during meiosis I or II, producing chromosomal variation.

What is the difference between first-division and second-division nondisjunction?

First-division nondisjunction: homologous chromosomes fail to separate in meiosis I. Second-division nondisjunction: sister chromatids fail to separate in meiosis II. Both produce disomic and haploid gametes, but with different chromosome compositions.

Why is monosomy for autosomes usually not tolerated?

Monosomy for the X chromosome occurs in humans, but monosomy for any of the autosomes is usually not tolerated in humans and other animals. Trisomy for sex chromosomes has a less dramatic phenotype than trisomies for autosomes, which are often lethal.

Why are trisomies often found in spontaneously aborted fetuses but monosomies are not?

This suggests that monosomic gametes may be very functionally impaired.

What happens during synapsis in a trisomy?

Three copies of one chromosome are present, so pairing configurations are usually irregular. At any particular region along the chromosome length, only two of the three homologs may synapse, though different regions of the trio may be paired (trivalent).

What are the three trisomy disorders?

Down syndrome (trisomy 21), Patau syndrome (trisomy 13), and Edwards syndrome (trisomy 18).

How does polyploidy originate?

Polyploidy can originate by: (1) the addition of one or more sets of chromosomes identical to the haploid complement of the same species (autopolyploidy), or (2) the combination of chromosome sets from different species as a consequence of interspecific matings (allopolyploidy).

How does autopolyploidy arise?

Tetraploids are more common than triploids. May arise if chromosomes replicate but the cell fails to divide (e.g., colchicine treatment prevents spindle formation, resulting in a tetraploid cell).

How does allopolyploidy arise?

An allopolyploid arises from hybridization of two related species. If the sterile hybrid undergoes a natural chromosomal doubling, a fertile amphidiploid is produced.

How can amphidiploid plants be produced artificially?

By somatic cell hybridization: cells are isolated, protoplasts are fused to form somatic cell hybrids, division and differentiation of hybrids is stimulated, and hybrid leaves form the amphidiploid.

What is the role of polyploidy in plant evolution?

Changes in ploidy can create reproductive isolation from diploid ancestors → rapid speciation. Polyploidy can increase flower, fruit, or leaf size. Duplicate genes can evolve new functions or divide ancestral functions. Allopolyploidy brings together genomes from different species → new combinations of traits and rapid adaptation to novel environments.

What are examples of polyploidy in agriculture?

Strawberry (8x): combines genomes from two octoploid species. Seedless watermelon (3x): a 4x × 2x cross produces largely sterile, seedless plants. Bread wheat (6x): combines three ancestral genomes AABBDD. Banana (often 3x): seedless varieties propagate through suckers or tissue culture.

How did bread wheat originate?

Triticum tauschii (DD) × Triticum urartu (AA) → Triticum turgidum (AABB, pasta wheat) ~0.5–0.8 million years ago. Triticum turgidum (AABB) × Aegilops speltoides (BB) → Triticum aestivum (AABBDD, bread wheat) ~10,000 years ago.

What are the types of chromosomal rearrangements?

Deletions, duplications, inversions, nonreciprocal translocations, and reciprocal translocations.

What genetic mechanisms produce chromosomal rearrangements?

Homologous recombination and nonhomologous end joining.

What is a deletion (deficiency)?

When a chromosome breaks in one or more places and a portion of it is lost.

What is a terminal deletion?

A deletion that occurs near one end of the chromosome.

What is an intercalary deletion?

A deletion that occurs from the interior of the chromosome.

What is a deletion (deficiency) loop?

For synapsis to occur between a chromosome with a large intercalary deletion and a normal complete homolog, the unpaired region of the normal homolog must loop out of the linear structure into a deletion or compensation loop.

What is a duplication?

A repeated segment of the genetic material.

How do duplications arise?

Duplications arise as the result of unequal crossing over during meiosis or through a replication error prior to meiosis.

What is the role of gene duplication in evolution?

Gene duplication may play a role in evolution. This hypothesis is supported by the discovery of genes that have a substantial amount of their DNA sequence in common, but whose gene products are distinct. Other support includes the presence of gene families—regional groups of genes whose products perform the same general function.

What is neofunctionalization?

The process by which a duplicated gene evolves a new function. Example: The SVMP complex encodes proteins (metalloproteinases) forming a venom cocktail in vipers; these venom genes evolved from the nonvenom gene ADAM28 through gene duplication. Repeated gene duplication supplied copies that diversified into several toxin classes.

What is an inversion?

A rearrangement of the linear gene sequence rather than the loss of genetic information. It requires two breaks in the chromosome and subsequent reinsertion of the inverted segment.

What is a paracentric inversion?

An inversion that does not include the centromere. The arm ratio is unchanged.

What is a pericentric inversion?

An inversion that includes the centromere. The arm ratio is changed.

What is an inversion loop?

A structure formed during synapsis of inverted chromosomes in an inversion heterozygote. The chromosomes must form a loop to align homologous regions.

What happens when crossing over occurs within a paracentric inversion loop?

Resultant gametes: 2 NCO (normal sequence and inverted sequence) and 2 SCO (one dicentric with duplication and deletion, one acentric with duplication and deletion).

What happens when crossing over occurs within a pericentric inversion loop?

Resultant gametes: 2 NCO (normal sequence and inverted sequence) and 2 SCO (both with duplication and deletion).

What is a translocation?

The movement of a chromosomal segment to a new location in the genome.

What is a reciprocal translocation?

A translocation that involves the exchange of segments between two nonhomologous chromosomes. It has an unusual synapsis configuration during meiosis.

What is a nonreciprocal translocation?

A translocation in which a segment moves from one chromosome to another without an exchange (one chromosome donates, the other receives).

What is alternate segregation?

A segregation pattern in a reciprocal translocation heterozygote that leads to a normal and a balanced gamete (forms balanced gametes).

What is adjacent segregation?

A segregation pattern in a reciprocal translocation heterozygote that leads to gametes containing duplications and deficiencies.

What are common chromosome aberrations in cancer?

Chromosome gains or losses, deletions, amplifications, translocations, and whole-genome doubling.

How do chromosome aberrations contribute to cancer?

Altered gene dosage: extra oncogene copies or loss of tumor-suppressor genes can favor uncontrolled growth. New gene functions: rearrangements can create cancer-driving fusion genes. Tumor evolution: chromosomal instability generates diversity that can support progression and treatment resistance. Clinical importance: specific abnormalities help classify cancers, predict outcomes, and identify treatment targets.

What is the Philadelphia chromosome?

The altered chromosome 22 resulting from a translocation between chromosomes 9 and 22. It contains the BCR-ABL1 fusion gene.

What is the BCR-ABL1 fusion gene?

The main oncogenic driver of chronic myelogenous leukemia (CML). It produces a tyrosine kinase that is continuously active. BCR-ABL1 provides continuous growth signals that promote G1-to-S progression and suppress apoptosis, leading to excessive production of myeloid cells, particularly granulocytes.

How is CML treated?

Drugs such as imatinib inhibit the fusion kinase, directly targeting the molecular driver.