[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Embryology](https://durgical.com/en/embryology/) › [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general) › Meiosis and nondisjunction: where trisomies come from

[Embryology](https://durgical.com/en/embryology/) · [General embryology: weeks 1 to 8](https://durgical.com/en/embryology/#a-general)

# 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

**See it in 3D:** [Ovaries](https://durgical.com/en/anatomy/ovary/) · [Testes](https://durgical.com/en/anatomy/testis/)

## Key points

- Commonest cause of Down syndrome: maternal nondisjunction in meiosis I.
- Translocation Down: the only type with an important inherited risk; test the parents.
- Turner 45,X: the only monosomy compatible with life.
- Mosaicism = a post-zygotic mitotic error; often a milder phenotype.
- Aneuploidy is the main cause of early miscarriage.

## 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](https://durgical.com/en/conditions/down-syndrome/) | 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](https://durgical.com/en/conditions/turner-syndrome/) | Turner | Short stature, gonadal dysgenesis, aortic coarctation, cystic hygroma |
| [47,XXY](https://durgical.com/en/conditions/klinefelter-syndrome/) | 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).

## Memory aid

Meiosis I splits 'couples', meiosis II splits 'twins': if a couple refuses to split, a chromosome is left over.

## 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.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/)

## See it in 3D

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

- [Ovaries](https://durgical.com/en/anatomy/ovary/)
- [Testes](https://durgical.com/en/anatomy/testis/)

## Related

- [Gametogenesis: spermatogenesis and oogenesis](https://durgical.com/en/embryology/gametogenesis-spermatogenesis-and-oogenesis/)
- [Fertilization: capacitation, acrosome reaction and zona reaction](https://durgical.com/en/embryology/fertilization-capacitation-acrosome-reaction-and-zona-reaction/)
- [Prenatal diagnosis: ultrasound, serum screening, cell-free DNA, CVS, amniocentesis](https://durgical.com/en/embryology/prenatal-diagnosis-ultrasound-serum-screening-cell-free-dna-cvs-amniocentesis/)
- [Differences of sex development: CAH, AIS and 5α-reductase](https://durgical.com/en/embryology/differences-of-sex-development-cah-ais-and-5-reductase/)

## Related conditions

- [Down syndrome (trisomy 21)](https://durgical.com/en/conditions/down-syndrome/)
- [Turner syndrome](https://durgical.com/en/conditions/turner-syndrome/)
- [Klinefelter syndrome (47,XXY)](https://durgical.com/en/conditions/klinefelter-syndrome/)
- [Molar pregnancy (hydatidiform mole)](https://durgical.com/en/conditions/molar-pregnancy/)

[← All of embryology](https://durgical.com/en/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](https://medlineplus.gov/genetics/condition/down-syndrome/) (MedlinePlus (U.S. National Library of Medicine), 2026); [Turner syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/turner-syndrome/) (MedlinePlus (U.S. National Library of Medicine), 2026); [Klinefelter syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/klinefelter-syndrome/) (MedlinePlus (U.S. National Library of Medicine), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/embryology/meiosis-and-nondisjunction-where-trisomies-come-from/