Chromosome Mutations: Variation in Number and Arrangement

Chapter 8: Chromosome Mutations—Variation in Number and Arrangement

Introduction

Chromosome mutations, also known as chromosome aberrations, are large-scale changes in the structure or number of chromosomes. These modifications can result in significant phenotypic variations and are distinct from mutations affecting individual genes. Chromosome mutations include changes in chromosome number (aneuploidy, polyploidy) and structural rearrangements (deletions, duplications, inversions, and translocations).

• Chromosome mutations and aberrations:

  • Variation in total number of chromosomes

  • Deletions

  • Duplications

  • Rearrangements

8.1 Variation in Chromosome Number: Terminology and Origin

Chromosome Number Variations

Chromosome number can vary due to mutations that alter the total number of chromosomes or the arrangement of genetic material within or among chromosomes.

• Aneuploidy: Variation in chromosome number where an organism gains or loses one or more chromosomes (not entire sets).

• Monosomy: Loss of a single chromosome in a diploid genome ($2n-1$).

• Trisomy: Gain of a single chromosome ($2n+1$).

• Euploidy: Complete haploid sets of chromosomes are present.

• Polyploidy: More than two sets of chromosomes are present (e.g., triploid $3n$, tetraploid $4n$, pentaploid $5n$).

Table 8.1: Terminology for Variation in Chromosome Numbers

Term	Explanation
Aneuploidy	$2n \pm x$ chromosomes
Monosomy	$2n-1$
Disomy	$2n$
Trisomy	$2n+1$
Tetrasomy, Pentasomy, etc.	$2n+2$, $2n+3$, etc.
Euploidy	Multiples of $n$
Diploidy	$2n$
Polyploidy	$3n$, $4n$, $5n$, ...
Triploidy	$3n$
Tetraploidy, Pentaploidy, etc.	$4n$, $5n$, etc.
Autopolyploidy	Multiples of the same genome
Allopolyploidy	Multiples of closely related genomes

Nondisjunction

Nondisjunction is the failure of paired homologous chromosomes to separate properly during meiosis I or II, resulting in gametes with abnormal chromosome numbers. This process is a primary cause of aneuploidy.

• Can occur during either meiotic division

• Results in gametes with $n+1$ or $n-1$ chromosomes

8.2 Monosomy and Trisomy Result in a Variety of Phenotypic Effects

Monosomy

• Loss of one chromosome ($2n-1$)

• Unmasks recessive lethal alleles

• Haploinsufficiency: When one copy is not sufficient for survival

Trisomy

• Gain of one chromosome ($2n+1$)

• More viable than monosomy, but often lethal in autosomes

• Plant trisomies are often viable but show altered phenotypes

Trisomy 21—Down Syndrome

• Caused by an extra chromosome 21 (three copies)

• Individuals express 6 to 8 out of 12–14 possible characteristics

Down Syndrome Critical Region (DSCR)

• Critical region on chromosome 21

• Genes are dosage sensitive and responsible for many phenotypic features

Origin of Extra 21st Chromosome

• Usually due to nondisjunction during maternal meiosis

• 95% of cases originate from the ovum

• Incidence increases with maternal age

Diagnostic Testing

• Amniocentesis or chorionic villus sampling (CVS): Fetal cells obtained from amniotic fluid or placenta

• NIPGD (Noninvasive prenatal genetic diagnosis): Fetal cells and DNA obtained from maternal blood

Familial Down Syndrome

• Down syndrome can run in families due to translocation of chromosome 21

Other Human Aneuploidies

• Patau syndrome: Trisomy 13

• Edwards syndrome: Trisomy 18

• Both syndromes result in severe malformations and early lethality

Aneuploid Conditions

• Trisomies are found in about 20% of spontaneously aborted fetuses

• Autosomal monosomies are rare, suggesting functional impairment

8.3 Polyploidy in Plants

Polyploidy

• More than two multiples of the haploid chromosome set

• Triploid ($3n$), tetraploid ($4n$), pentaploid ($5n$)

Origin of Polyploidy

• Autopolyploidy: Addition of chromosome sets identical to the same species

• Allopolyploidy: Combination of chromosome sets from different species via hybridization

8.4 Variation in Chromosome Composition and Arrangement

Chromosome Rearrangements

• Deletions

• Duplications

• Inversions

• Nonreciprocal translocations

• Reciprocal translocations

Alteration of Chromosomes

• Deletions and duplications: Change the total amount of genetic information

• Inversions and translocations: Rearrange genetic material without changing the total amount

8.5 A Deletion is a Missing Region of a Chromosome

Deletions (Deficiency)

• Missing regions due to chromosome breaks

• Terminal deletion: Near one end

• Intercalary deletion: Interior of the chromosome

Buckling Out (Compensation Loop)

• Occurs during synapsis between a chromosome with a large intercalary deletion and a normal homolog

• Unpaired region of the normal homolog forms a loop (deletion or compensation loop)

Cri du Chat Syndrome

• Deletion of a small terminal part of chromosome 5

• Segmental deletion leads to anatomical malformations in infants

8.6 A Duplication is a Repeated Segment of a Chromosome

Duplications

• Repeated segment of a chromosome

• Single locus present more than once in the genome

• Can produce a compensation loop

• Arise from unequal crossing over during meiosis

Summary Table: Chromosome Mutations

Type	Description	Example/Effect
Aneuploidy	Gain/loss of one or more chromosomes	Down syndrome (Trisomy 21)
Polyploidy	More than two sets of chromosomes	Common in plants
Deletion	Loss of chromosome segment	Cri du chat syndrome
Duplication	Repeated chromosome segment	Gene redundancy, Bar mutation in Drosophila
Inversion	Reversal of chromosome segment	May affect gene expression
Translocation	Movement of segment to new location	Familial Down syndrome

Additional info: Chromosome mutations are a major source of genetic diversity and can have profound effects on development, viability, and evolution. Understanding these mutations is essential for fields such as genetics, medicine, and agriculture.