Clinical Considerations: Meiotic Nondisjunction
- Nondisjunction is failure of chromosomes or chromatids to separate properly during cell division.
- Prophase I normally allows chromosome pairs to align and exchange genetic material.
- Anaphase I normally separates homologous chromosome pairs toward opposite poles.
- Meiotic nondisjunction can occur when members of a homologous pair fail to separate.
- A gamete with n plus 1 chromosomes contains 24 chromosomes in humans.
- A gamete with n minus 1 chromosomes contains 22 chromosomes in humans.
- Fertilization with a normal 23-chromosome gamete can produce a zygote with an abnormal chromosome number after nondisjunction.
- Trisomy is the presence of one extra chromosome, producing 47 chromosomes in a human zygote.
- Monosomy is the absence of one chromosome, producing 45 chromosomes in a human zygote.
- Chromosomes 8, 9, 13, 18, and 21 are frequently affected by nondisjunction.
- Aneuploidy is an abnormal number of chromosomes.
- Karyotyping can detect aneuploidy by showing chromosome number and morphology.
- Down syndrome is trisomy 21.
- Down syndrome can include intellectual disability, short stature, stubby appendages, congenital heart malformations, and other defects.
- Klinefelter syndrome is sex-chromosome aneuploidy classically associated with an XXY chromosome complement.
- Klinefelter syndrome can include infertility, variable degrees of masculinization, and small testes.
- Turner syndrome is sex-chromosome monosomy classically associated with an XO chromosome complement.
- Turner syndrome can include short stature, sterility, and other abnormalities.
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