7.3 Chromosomal Mutation

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.

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

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

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

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

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)

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)

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)

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.