GAMETOGENESIS ERRORS

What is the most common error in gametogenesis?

Meiotic non-disjunction, especially in oogenesis.

During which meiotic stage does non-disjunction most commonly occur?

Primarily in meiosis I, but can also occur in meiosis II
or mitosis.

What is non-disjunction?

Failure of homologous chromosomes (meiosis I) or
sister chromatids (meiosis II) to separate properly during division.

What is anaphasic delay and its consequence?

Delayed movement of a chromosome in anaphase II
Can result in a nulisomic gamete (n – 1)
Fertilization of this gamete → monosomic zygote (2n – 1)

What syndrome results from anaphasic delay involving sex chromosomes?

Turner syndrome (45,Xo)
→ monosomy of the X chromosome.

What is aneuploidy?

Presence of an abnormal number of chromosomes, either
extra or missing.

What usually happens to embryos with autosomal aneuploidy?

Most are not viable and are
spontaneously aborted.

What is the most common autosomal aneuploidy in humans?

Trisomy 16

What is the most common autosomal aneuploidy compatible with life?

Trisomy 21 (Down syndrome)

What are the three major viable trisomies?

1. Trisomy 21 → Down syndrome

2. Trisomy 18 → Edwards syndrome

3. Trisomy 13 → Patau syndrome

What is the survival prognosis for Trisomy 18 and 13?

Only
~10% of newborns survive beyond 1 year.

What are the two most common sex chromosome aneuploidies?

Turner syndrome
(45,Xo) → missing X chromosome
Klinefelter syndrome
(47,XXY) → extra X chromosome in males