BIOL110 - Module 3: Lecture 8 - Meiotic non-disjunction & chromosomal abnormalities

Illustration of Non-Disjunction at Meiosis I and Meiosis II

Non-disjunction occurs when chromosomes fail to separate during cell division, leading to gametes with missing or extra chromosomes.

Meiosis I Non-disjunction

In Meiosis I, the failure involves homologous chromosomes. Both members of a homologous pair migrate to the same pole.

      [ Normal 2n ]
           |
     ( Meiosis I )
    /             \
 [n+1]           [n-1]   <-- Non-disjunction here
  / \             / \
(n+1)(n+1)      (n-1)(n-1)

• Result: 100% of gametes are abnormal. Two gametes have an extra chromosome ($n+1$) and two are missing one ($n-1$).

Meiosis II Non-disjunction

In Meiosis II, the failure involves sister chromatids. The initial separation in Meiosis I is successful, but one of the two daughter cells fails to separate its chromatids.

      [ Normal 2n ]
           |
     ( Meiosis I )
    /             \
 [ n ]           [ n ]   <-- Normal Disjunction
  / \             / \
(n+1)(n-1)       (n) (n)  <-- Non-disjunction in left cell

• Result: 50% of gametes are abnormal ($n+1$ and $n-1$) and 50% are normal ($n$).

Meaning of the Terms Monosomy and Trisomy

• Trisomy ($2n+1$): A condition where an individual has three copies of a particular chromosome instead of the normal pair.

  • Trisomy 21 (Down Syndrome): Characterized by mental retardation and specific physical features.

  • Trisomy 13 (Patau Syndrome) and Trisomy 18 (Edwards Syndrome): Severe defects and typically very short life expectancy.

• Monosomy ($2n-1$): A condition where an individual is missing one chromosome from a pair.

  • Turner Syndrome (XO): The only viable human monosomy, occurring in females who possess only one $X$ chromosome.

Diagram: Behaviour of the X and Y Chromosomes During Meiosis

Despite being different sizes and carrying different genes, the $X$ and $Y$ chromosomes behave as a pair because they share small regions of homology called pseudoautosomal regions (PARs).

     [ X Chromosome ]      [ Y Chromosome ]
     |      (PAR)---|------|---(PAR)      |
     |              |      |              |
     |              |      |              |
     |      (PAR)---|------|---(PAR)      |
             (Synapsis occurs at PARs)

1. Prophase I: The PARs at the tips of the $X$ and $Y$ chromosomes pair up and undergo crossover.

2. Metaphase I: The $X$ and $Y$ pair aligns at the metaphase plate together.

3. Anaphase I: They segregate to opposite poles, ensuring each secondary spermatocyte receives either an $X$ or a $Y$.

Human Sex Determination

The sex of a zygote is determined by the specific sex chromosome carried by the male gamete (sperm) that fertilizes the female gamete (ovum).

1. Gamete Contribution:

   • Female Gametes (Ova): Always carry one $X$ chromosome ($23, X$).

   • Male Gametes (Sperm): 50% carry an $X$ chromosome ($23, X$) and 50% carry a $Y$ chromosome ($23, Y$).

2. Zygote Formation:

   • XX Zygote: Created when an $X$-bearing sperm fertilizes the ovum, resulting in a female.

   • XY Zygote: Created when a $Y$-bearing sperm fertilizes the ovum, resulting in a male.

3. Summary Table:

   • Mother ($XX$) x Father ($XY$)

   • Gametes: Mother ($X$) | Father ($X$ or $Y$)

   • Offspring: 50% $XX$ | 50% $XY$