Embryology · General embryology: weeks 1 to 8

Meiosis and nondisjunction: where trisomies come from !

Process

When chromosomes fail to separate while eggs or sperm form, the embryo gets one too many or one too few.

Updated 2026-11-01

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What it is

Meiosis cuts chromosomes from 46 to 23 in two divisions: meiosis I separates homologous chromosomes (after crossing over) and meiosis II separates sister chromatids. Nondisjunction is a failure of that separation: one gamete ends up with 24 chromosomes and another with 22. After fertilization this gives trisomies (47) or monosomies (45), the commonest chromosomal cause of miscarriage and of syndromes such as Down, Edwards, Patau, Turner and Klinefelter.

Key facts

Meiosis I: separates homologues; the reductional division, where crossing over happens.
Meiosis II: separates sister chromatids, like a mitosis.
Free trisomy 21: most cases; the error is usually maternal and in meiosis I.
Robertsonian translocation (often 14;21): a minority of Down cases; can be inherited from a balanced carrier.
Mosaicism: nondisjunction in a mitosis after fertilization; only some cells are abnormal.
Complete autosomal monosomy is not compatible with life; 45,X is the only viable monosomy.

Step by step

Homologues pair and cross over in prophase I.
If they fail to separate at anaphase I, both go to the same daughter cell.
The gamete has 24 chromosomes (disomic) or 22 (nullisomic).
Joined with a normal gamete, the zygote has 47 (trisomy) or 45 (monosomy).
Many aneuploidies end in first-trimester miscarriage; some reach term.

Aneuploidies the exams ask about

Karyotype	Syndrome	Key features
47,+21	Down	Hypotonia, AV canal defect, duodenal atresia, intellectual disability
47,+18	Edwards	Clenched fists with overlapping fingers, rocker-bottom feet, heart defects
47,+13	Patau	Holoprosencephaly, cleft lip and palate, polydactyly
45,X	Turner	Short stature, gonadal dysgenesis, aortic coarctation, cystic hygroma
47,XXY	Klinefelter	Small testes, infertility, tall stature, gynecomastia

Markers

Trisomy 21
45,X
47,XXY
t(14;21)

Clinical notes

Trisomy risk rises with maternal age; prenatal screening is offered to every pregnant woman, with more emphasis after 35.
If a child has translocation Down syndrome, the parents are karyotyped: a balanced carrier has a high recurrence risk.
Uniparental disomy: trisomy rescue can leave both copies from one parent (Prader-Willi, Angelman).

Test yourself

Think of the answer, then tap the question to see it.

In which division and which parent does the nondisjunction behind trisomy 21 most often happen?

Maternal meiosis I. The risk rises with the mother's age.

Which type of Down syndrome calls for karyotyping the parents?

Robertsonian translocation Down syndrome, because a parent may be a balanced carrier with a high recurrence risk.

What explains a person having both 46,XX and 45,X cells?

Mosaicism: the nondisjunction or chromosome loss happened in a mitosis after fertilization.

What does meiosis I separate, and what does meiosis II separate?

Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 2: Gametogenesis: Conversion of Germ Cells into Male and Female Gametes (Wolters Kluwer, 2015); Down syndrome: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026); Turner syndrome: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026); Klinefelter syndrome: MedlinePlus Genetics (MedlinePlus (U.S. National Library of Medicine), 2026).

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