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Related Concept Videos

Meiosis I01:49

Meiosis I

Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by a...
Nondisjunction01:21

Nondisjunction

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate correctly and move to the opposite poles of the cells. This produces daughter cells with abnormal chromosome numbers.  Nondisjunction is common during anaphase I or anaphase II of meiosis.  Mutations in synaptonemal complex proteins that attach homologous chromosomes increase the chances of nondisjunction in anaphase I of meiosis I. In contrast, mutations in topoisomerases and condensins that hold sister...
Nondisjunction01:29

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During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.
Nondisjunction01:29

Nondisjunction

During meiosis, chromosomes occasionally separate improperly. This occurs due to failure of homologous chromosome separation during meiosis I or failed sister chromatid separation during meiosis II. In some species, notably plants, nondisjunction can result in an organism with an entire additional set of chromosomes, which is called polyploidy. In humans, nondisjunction can occur during male or female gametogenesis and the resulting gametes possess one too many or one too few chromosomes.
Karyotyping01:17

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Overview
Meiosis vs. Mitosis02:57

Meiosis vs. Mitosis

Cell division is necessary for growth and reproduction in organisms. Mitosis aids cell growth and development by dividing somatic cells. In contrast, meiosis causes the division of germ cells and plays an essential role in sexual reproduction. Due to their unique functional requirements, mitosis and meiosis differ from each other in multiple aspects.
Before the start of mitosis and meiosis I, the cell synthesizes DNA, resulting in two homologous copies of each chromosome. DNA synthesis is...

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Related Experiment Video

Updated: May 13, 2026

Midface Hypoplasia and Cranial Base Morphology in Syndromic Craniosynostosis: A Comparative Analysis Study Using a Predictive Regression Model
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Trisomy 21 and facial developmental instability.

John M Starbuck1, Theodore M Cole, Roger H Reeves

  • 1Department of Anthropology, The Pennsylvania State University, University Park, PA 16802, USA.

American Journal of Physical Anthropology
|March 19, 2013
PubMed
Summary

Down syndrome (DS) faces show increased developmental instability, particularly in structures from the mandibular prominence. This suggests genetic imbalance from trisomy 21 disrupts craniofacial development.

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Area of Science:

  • Genetics
  • Developmental Biology
  • Anthropology

Background:

  • Down syndrome (DS), caused by trisomy 21, is the most common live-born aneuploidy.
  • Gene dosage imbalance on chromosome 21 alters development, leading to characteristic phenotypes like facial dysmorphology.
  • The mechanisms underlying DS craniofacial alterations are not fully understood.

Purpose of the Study:

  • To test the hypothesis that Down syndrome faces exhibit increased developmental instability compared to euploid individuals.
  • To investigate if trisomy 21 disrupts developmental homeostasis and precision in craniofacial morphogenesis.
  • To identify specific facial regions with altered developmental instability in DS.

Main Methods:

  • Assessed developmental instability using fluctuating asymmetry analysis.
  • Compared asymmetry patterns in 3D facial landmarks from DS individuals, their siblings, and euploid controls (ages 4-12).
  • Analyzed four age-matched samples.

Main Results:

  • DS individuals exhibited increased fluctuating asymmetry in facial structures compared to euploid controls.
  • Facial prominences derived from the mandibular prominence showed the highest developmental instability.
  • Facial regions derived from the frontal prominence displayed the lowest developmental instability.

Conclusions:

  • Findings support the "amplified developmental instability" hypothesis for Down syndrome.
  • Trisomy 21 appears to decrease developmental homeostasis, impacting craniofacial morphogenesis.
  • Specific facial structures show differential sensitivity to developmental instability in DS.