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Related Concept Videos

Genetic Variation01:25

Genetic Variation

Genetic variation is the diversity in DNA sequences found among individuals of the same species. This diversity is crucial for a species' survival because it helps organisms adapt to environmental changes. Genetic variation begins with fertilization, where an egg and sperm cell merge. Each of these cells carries 23 chromosomes, up to 46 in the fertilized egg. Chromosomes are long DNA strands that contain genes, the basic units of heredity.
Genes exist in different versions called alleles, which...
Understanding Species and Reproductive Barriers01:17

Understanding Species and Reproductive Barriers

A species is a group of organisms that interbreed and produce fertile offspring. Typically, individuals of the same species appear similar and share common characteristics due to their highly similar genomes. However, not all organisms that look alike are members of the same species. Various mechanisms keep most species discrete. While some mechanisms prevent reproductive behavior and fertilization (pre-zygotic isolation), others prevent the production of fertile offspring after mating has...
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.
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...
X and Y Chromosomes02:32

X and Y Chromosomes

Among mammals, the gender of an organism is determined by the sex chromosomes. Humans have two sex chromosomes, X and Y. Every human diploid cell has 22 pairs of autosomes and one pair of sex chromosomes. A human female has two X chromosomes, while a male has one X chromosome and one Y chromosome.
The germline cells such as egg and sperm cells carry only half the number of chromosomes, i.e., 22 autosomes and one sex chromosome. All eggs have an X chromosome, while sperm cells can carry an X or...
Monohybrid Crosses01:20

Monohybrid Crosses

Overview

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Related Experiment Video

Updated: Jun 6, 2026

Model Surgical Training: Skills Acquisition in Fetoscopic Laser Photocoagulation of Monochorionic Diamniotic Twin Placenta Using Realistic Simulators
09:51

Model Surgical Training: Skills Acquisition in Fetoscopic Laser Photocoagulation of Monochorionic Diamniotic Twin Placenta Using Realistic Simulators

Published on: March 21, 2018

Why are monozygotic twins different?

Sara Silva1, Yolanda Martins, Alexandra Matias

  • 1Department of Obstetrics and Gynecology, University Hospital of S. Jo;atao, Medical Faculty of Porto, Porto, Portugal.

Journal of Perinatal Medicine
|December 15, 2010
PubMed
Summary
This summary is machine-generated.

Monozygotic (MZ) twins, often called identical, show genetic and epigenetic differences due to post-fertilization events. These variations impact disease susceptibility and twin study interpretations.

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Published on: September 5, 2011

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Last Updated: Jun 6, 2026

Model Surgical Training: Skills Acquisition in Fetoscopic Laser Photocoagulation of Monochorionic Diamniotic Twin Placenta Using Realistic Simulators
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Model Surgical Training: Skills Acquisition in Fetoscopic Laser Photocoagulation of Monochorionic Diamniotic Twin Placenta Using Realistic Simulators

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Guide Wire Assisted Catheterization and Colored Dye Injection for Vascular Mapping of Monochorionic Twin Placentas
09:04

Guide Wire Assisted Catheterization and Colored Dye Injection for Vascular Mapping of Monochorionic Twin Placentas

Published on: September 5, 2011

Area of Science:

  • Genetics
  • Epigenetics
  • Developmental Biology

Background:

  • Monozygotic (MZ) twins are commonly perceived as genetically identical.
  • However, significant phenotypic differences are observed between MZ twins.
  • The origins and implications of these discrepancies are not fully understood.

Purpose of the Study:

  • To review the genotypic and phenotypic differences observed in monozygotic twins.
  • To discuss the underlying causes of these variations.
  • To highlight the implications of these differences in various fields.

Main Methods:

  • Literature review of studies reporting genotypic and phenotypic discordance in MZ twins.
  • Analysis of proposed mechanisms contributing to MZ twin differences.
  • Synthesis of findings related to epigenetic and post-fertilization events.

Main Results:

  • MZ twins exhibit discordance in areas such as lateral asymmetry, major malformations, growth, and intrauterine development.
  • Key contributing factors include chromosomal mosaicism, skewed X-inactivation, and imprinting mechanisms.
  • Epigenetic modifications play a crucial role in establishing these differences.

Conclusions:

  • Post-fertilization events lead to significant genotypic and phenotypic variations in MZ twins.
  • These differences have critical implications for understanding complex diseases, organ transplantation, and the validity of twin studies.
  • Further research is needed to fully elucidate the impact of these variations.