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Genetics for the
Obstetrician
Chapter 21
Amelia McLennan, MD and
Jennifer McClarren, MS, CGC❍ Meiosis begins with 46 chromosomes each consisting of two chromatids. At the end of meiosis I, how many
chromosomes and chromatids are present? At the end of meiosis II, how may chromosomes and how many
chromatids are present?
Meiosis I—23 chromosomes and 46 chromatids.
Meiosis II—23 chromosomes and 23 chromatids.
❍ What is crossing over and when specifically does it occur?
The exchange of genetic material between homologous chromosomes, a mechanism for increasing genetic
variation. Occurs during Meiosis I (during Pachytene of Prophase I).
❍ When does female meiosis begin?
At about 4 months’ gestation.
❍ When is Meiosis I completed?
At ovulation.
❍ When is Meiosis II completed?
At fertilization.
❍ What is the most common trisomy in liveborn infants?
Trisomy 21 (Down syndrome).
❍ What percentage of first trimester pregnancy losses have a chromosome abnormality?
50%.
❍ Monosomy for an entire chromosome is typically incompatible with life. What condition is an exception to this?
45, X (Turner syndrome).