Chromosomal Mutations - Practice Questions & MCQ

• A chromosomal mutation is any change or error that occurs within the chromosome.
• Such errors can occur due to any mistakes or problems during cell processes like mitosis and meiosis.
• Unlike gene mutations that involve the alteration of a gene or a segment of DNA in the chromosome, chromosomal mutations change the chromosome itself.
• Three types of chromosomal mutations exist: 
  • mutations on the structure of chromosomes 
  • mutations on the chromosome number
  • mutations on the sex chromosomes

Structural Chromosomal Mutation:

• This happens when homologous chromosomes paired up, genes in chromosomes broke apart, genes inserted in the wrong chromosome, or genes or set of genes are completely lost in the chromosome.
• Structural chromosomal mutations are classified into four: 
  • Deletion:
    • This type of mutation occurs when a part of the DNA is not duplicated or is lost during DNA replication. 
    • The size of this region can either be a mere nucleotide or can be large as an entire chromosome. 
    • Common disorders due to deletion mutation in humans are: Cri du chat, Duchenne muscular dystrophy, Digeorge syndrome, etc.
  • Duplication:
    • It occurs when an extra copy of a region (or regions) in the DNA is produced. 
    • Common disorder due to duplication mutation in humans is Charcot-Marie-Tooth disease type I. 
  • Inversion:
    • A portion in the chromosome is reversed and gets inserted back into the chromosome. 
    • When centromere is involved in inversion, it is called pericentric inversion.
    • When centromere is not involved and either the short or long arm of the chromosome is involved, it is called paracentric inversion.
  • Translocation:
    • When a fragmented chromosome tends to join with a nonhomologous chromosome.  
    • Most common type is Reciprocal translocation in which two non-homologous chromosomes exchange parts.
      

Aneuploidy:

• Aneuploidy is a type of mutation in the chromosome number wherein the ploidy (chromosome number) of the new individual is different from its wild type. 
• This is typically a result of the nondisjunction of chromosomes during mitosis or meiosis, hence producing offspring with either extra or lost chromosomes.
• The naming of aneuploid conditions is generally based on the number of chromosomes added or deleted:
  • An aneuploid in which one chromosome is devoid of its homologue is called monosomic (2n-1). 
  • An aneuploid in which a complete pair of homologous chromosomes is absent is called nullisomic (2n-2).
  • An aneuploid in which an extra chromosome is present is called trisomic (2n+1).

Polyploidy:

• Polyploidy is a type of mutation that occurs when an individual bears more than one haploid set of chromosomes. 
• If the individual with polyploidy bears three sets of haploid chromosomes, the condition is said to be triploidy whereas if it has four haploid sets, the condition is said to be tetraploidy.
• It is of following types:
  • Autopolyploidy: When there is a numerical increase in the same genome.
  • Allopolyploidy: It has developed through hybridization between two species followed by a doubling of chromosomes.