7.3 Chromosomal Mutation

7.3 Chromosomal Mutation

Chromosomal Mutation

• Definition: A change occurs in the structure or number of chromosome

• If the change occurs in the structure of chromosome: chromosomal aberration

Chromosomal Aberration

• 4 types:

• Deletion

• Inversion

• Translocation

• Duplication

Deletion 

•The loss of one segment containing one or more genes.

E.g.: Cri du chat syndrome

"Cry of the cat" in French

is a genetic, chromosomal disease that resulted from the deletion of variable sizes in the short arm of chromosome 5.

Inversion  

•A region of a chromosome breaks off and rotates through 180° before rejoining the chromosome.

Translocation

•Involves a region of a chromosome breaking off and rejoining either the other end of the same chromosome or another non-homologous chromosome.

Duplication

•A region of a chromosome becomes duplicated; a set of genes will be repeated 

Multiple Choice

Cri du chat syndrome occur due to

• Duplication

• Translocation

• Inversion

• Deletion

Alteration of Chromosomal Number

1.Changes that involve either individual chromosome number or set of chromosome number (all genome)

2. May result in abnormalities

3. Error occurs during meiosis: chromosomes fail to segregate properly during meiosis due to nondisjunction during meiosis I or meiosis II

4. Disjunction: Homologous chromosomes or sister chromatids segregate normally to the opposite poles during meiosis.

Alteration of Chromosomal Number

5. Nondisjunction: Homologous chromosomes / sister chromatids fail to segregate properly & move to the same pole of the cell.

6.Occurs during anaphase I or anaphase II in meiosis.

7.There are 2 types of chromosomal number alteration:

i. Aneuploidy

ii. Euploidy / Polyploidy

Aneuploidy

Condition where the diploid cell (2n) gains or losses one or more chromosomes

• Involves nondisjunction on a specific individual chromosome only

• Caused by abnormalities in the autosome number OR sex chromosome number

• Can occur in male (during spermatogenesis) or female (during oogenesis)

• 2 types of aneuploidy

i. Monosomy

ii. Trisomy

Multiple Choice

Non-disjunction in meiosis I occur during

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

Monosomy

One chromosome is missing from a diploid cell / organism/ chromosome set

• Occurs when abnormal gamete (n-1) fertilises with normal gamete (n)

• Produces abnormal zygote (2n – 1)

• E.g: no. of chromosomes in human = 46 – 1 = 45

Trisomy

One chromosome is gained / added for a diploid cell / organism / chromosome set

• Occurs when abnormal gamete (n+1)  fertilises with normal gamete (n)

• Produces abnormal zygote (2n + 1)

• E.g: no. of chromosomes in human = 46 + 1  = 47

Abnormalities in the sex chromosome number 

During spermatogenesis

• If nondisjunction occurs during meiosis  I & II

 – sperm will have abnormal sex  chromosome

 • XY, XX @ YY

• Abnormal sperm (XY) fertilise with  normal ovum (X)

 – Klinefelter syndrome (XXY)

Multiple Choice

When abnormal sperm (XY) fertilise with normal ovum (X), it will result in

• Klinefailter syndrome

• Klinefelter syndrome

• Turner syndrome

• Tarner syndrome

Abnormalities in the sex chromosome number 

If non disjunction happened

i. Some ovum might not carry any chromosome X and some others might carry 2 chromosome X

• If abnormal ovum without X chromosome (O) fertilizes with normal sperm (X)

i. Turner syndrome (XO)

Klinefelter syndrome

 If abnormal ovum without X  chromosome (O) fertilises with normal  sperm (Y)

 – YO: dead

• If abnormal ovum (XX) fertilizes with  normal sperm (Y)

 – Klinefelter syndrome (XXY)

Klinefelter Syndrome (XXY): 2n+1 (Trisomy)

Male individual, has an extra X chromosome (47 chromosomes).

• Has abnormally small / undeveloped testes

• Sterile, failed to produce sperm

• Enlarged breast, feminized voice, knocked knees, long limbs

Turner Syndrome (XO): 2n-1 (Monosomy)

Female individual, has only one X chromosome (45 chromosomes).

• Sterile because of undeveloped ovary.

• Short stature, poorly developed breasts, webbed neck

Abnormalities in the autosome number 

During oogenesis / spermatogenesis

Down syndrome / Trisomy 21

• Non disjunction of chromosome 21  during meiosis I or II

• Produce abnormal gamete with extra  chromosome 21

• If the abnormal gamete (n+1=24) is  fertilized with normal gamete (n=23),

•   Produces abnormal zygote with 3  chromosomes 21, (2n+1=47)

Phenotype:

• Characteristic facial features

• Short stature (about 4 feet tall)

• Large, protruding tongue with  furrowing

• Correctable heart defects

• Developmental delays

Karyotype of Down Syndrome

Euploidy/ Polyploidy

• Changes of the chromosome number which involved the whole chromosome set

  • Occurred when a set of chromosome did not  separate during gametogenesis

  • Common in plants than in animals

  • Gametes fusion will produce cell @ organism  which have more than 2 sets of chromosome

  • Polyploidy: Cell with 3 or more sets of  chromosome

  • 2 types of polyploidy

   • Autopolyploidy

   • Allopolyploidy

Autopolyploidy

• Increase in number of chromosomes within  the same species

  • The chromosomes set are homologous with  the parent cell

  • Can be induced using colchicines (inhibit  spindle fibre formation)

  • Examples in plants: strawberries (octaploid,  8n), potato (tetraploid, 4n) & bananas  (triploid, 3n)

Allopolyploidy

• Chromosome number in a sterile hybrid becomes doubled and produces fertile hybrids

  • F1 hybrids produced from different species are usually sterile

Examples of Polyploid Crops

Triploid crops: banana, apple, ginger, watermelon, citrus.

• Tetraploid crops: potato, durun, wheat,  maize, cotton, tobacco.

• Hexaploid crops: chrysanthemum,  bread wheat, oat.

• Octaploid crops: strawberry, dahlia,  sugar cane.