MBG lecture 3

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Questions and Answers

What is a key difference between meiotic nondisjunction in the first half versus the second half of meiosis?

Nondisjunction in the first half affects autosomes only.

First-half nondisjunction affects homologous chromosomes, while second-half affects sister chromatids. (correct)

First-half nondisjunction leads to a full set of trisomy as opposed to second-half nondisjunction.

Nondisjunction in the second half cannot lead to aneuploidies.

Which type of chromosomal anomaly is most likely to yield a viable organism?

Trisomy 18

Trisomy 16

Trisomy 13

Trisomy 21 (correct)

What is a common outcome of Turner syndrome?

Increased fertility rates.

Most individuals are short-statured. (correct)

Intellectual disability is unavoidable.

Mosaicism is a common feature.

In terms of phenotypic consequences, Klinefelter syndrome is associated with which of the following?

Tall stature and reduced fertility. Signup and view all the answers

Compared to chimerism, mosaicism generally results from which process?

Early mitotic divisions leading to differing genetic components. Signup and view all the answers

Which chromosomal abnormality is least likely to occur in a viable human being?

Trisomy 16 Signup and view all the answers

Patients with which condition are most likely to experience severe organ malformations?

Trisomy 18 Signup and view all the answers

Individuals with mosaic forms of Trisomy 16 may experience which of the following?

Mild health complications. Signup and view all the answers

Which condition generally leads to significant learning difficulties?

Klinefelter syndrome Signup and view all the answers

What is a direct result of non-disjunction during meiosis?

Trisomy or monosomy in daughter cells Signup and view all the answers

Which of the following best describes a lagging chromosome during cell division?

A chromosome that does not move to either pole Signup and view all the answers

What condition is characterized by both chromosome copies being derived from the same parent?

Isodisomy Signup and view all the answers

Which type of uniparental disomy involves both chromosome copies being different?

Heterodisomy Signup and view all the answers

What is the main consequence of errors occurring during mitosis regarding cell populations?

Mosaicism or chimerism in the resulting cells Signup and view all the answers

Which of the following statements accurately describes the difference between mosaicism and chimerism?

Mosaicism results from somatic mutations in a single zygote; chimerism results from the fusion of two zygotes. Signup and view all the answers

How do autosomal aneuploidies generally impact phenotypic consequences compared to sex chromosome aneuploidies?

Sex chromosome aneuploidies have compensatory mechanisms like X-inactivation. Signup and view all the answers

What is a significant cause of uniparental disomy?

Nondisjunction followed by a rescue mechanism Signup and view all the answers

What mechanism can lead to segmental uniparental disomy?

Postzygotic somatic recombination between homologs Signup and view all the answers

What is the primary mechanism of meiotic nondisjunction?

Failure to split homologous chromosomes during Meiosis I Signup and view all the answers

How does the Limited Oocyte Pool Model explain the relationship between maternal age and nondisjunction?

Reduction in the number of higher quality follicles with age Signup and view all the answers

What effect do early mitotic divisions have on the occurrence of aneuploidy?

They increase the likelihood of aneuploidy due to rapid progression Signup and view all the answers

Which type of mosaicism occurs when only the fetus is affected and the placenta is normal?

Fetal mosaicism without placental involvement Signup and view all the answers

What is a potential complication of confined placental mosaicism (CPM)?

Miscarriage and complications in pregnancy Signup and view all the answers

What determines the clinical impact of mosaicism on an individual?

Proportion of affected cells and tissue specificity Signup and view all the answers

Which of the following best describes the relationship between timing of a nondisjunction event and aneuploidy prevalence?

Early events contribute to wider tissue abnormalities and symptoms Signup and view all the answers

What distinguishes mosaicism from chimerism?

Chimerism occurs due to the fusion of two different embryos Signup and view all the answers

Which of the following statements about maternal age and nondisjunction is true?

Increased maternal age is linked to higher rates of nondisjunction Signup and view all the answers

What outcome is most likely to occur due to nondisjunction during mitosis?

An abnormal distribution of chromosomes leading to aneuploidy Signup and view all the answers

How do earlier mutations during embryonic development primarily affect an individual?

They result in widespread mosaicism across multiple tissues. Signup and view all the answers

Which statement accurately distinguishes chimerism from mosaicism?

Mosaicism involves genetically distinct cells from one zygote, while chimerism comes from two separate zygotes. Signup and view all the answers

What is the clinical implication of 45,X/69,XXY fetus condition related to chromosomal abnormalities?

It may lead to varying degrees of Turner syndrome and Klinefelter syndrome symptoms. Signup and view all the answers

What defines confined chimerism in an individual?

Distinct genetic populations limited to specific tissues without phenotype manifestations. Signup and view all the answers

What process is primarily responsible for ensuring equal division of sister chromatids during mitosis?

Proper attachment and function of the mitotic spindle Signup and view all the answers

Which characteristic is indicative of mosaicism within an individual?

Varied genetic make-up resulting from mutations that occur post-fertilization. Signup and view all the answers

What is a major consequence of defective spindle assembly during cell division?

Unequal distribution of chromosomes leading to aneuploidy Signup and view all the answers

What impacts do late mutations have in an individual compared to early mutations?

They narrowly affect specific cells or tissues rather than the entire organism. Signup and view all the answers

In terms of chromosomal abnormalities, what is a direct result of nondisjunction during the anaphase of mitosis?

An unequal number of chromosomes in daughter cells Signup and view all the answers

What is a significant consequence of mitotic nondisjunction occurring early in development?

It can result in widespread tissue specificity affecting nearly all tissues. Signup and view all the answers

Which statement accurately describes the difference between chimerism and mosaicism?

Chimerism involves genetic contributions from two different zygotes, whereas mosaicism arises from a single zygote. Signup and view all the answers

What is the primary clinical implication of chromosomal aneuploidy caused by nondisjunction in maternal meiosis I?

Embryonic lethality or failure to implant is common in cases involving gene-rich chromosomes. Signup and view all the answers

What mechanism primarily results in maternal meiotic nondisjunction of chromosome 13?

Both homologous chromosomes moving to one pole during anaphase. Signup and view all the answers

Which factor is correlated with an increased incidence of nondisjunction during maternal meiosis I?

Advanced maternal age. Signup and view all the answers

Study Notes

Autosomal and Sex Chromosome Aneuploidies

Most autosomal aneuploidies lead to miscarriage or severe developmental disorders, while sex chromosome aneuploidies are more viable.
X-inactivation and limited Y chromosome gene content help compensate for sex chromosome aneuploidies.

Key Conditions and Viability

Trisomy 21: Extra chromosome 21; high viability with average life expectancy of ~60 years; phenotypes include intellectual disabilities and heart defects.
Trisomy 18: Extra chromosome 18; low viability, most die within the first year; associated with severe intellectual disability and organ malformations.
Trisomy 13: Extra chromosome 13; low viability, most die within the first few weeks; linked to severe intellectual disability and cleft lip.
Trisomy 16: Nonviable in full form; can present as mosaic form with rare growth and developmental delays.
Turner Syndrome (45,X): Missing an X chromosome; high viability with normal life expectancy; presents with short stature and ovarian insufficiency.
Klinefelter Syndrome (47,XXY): Extra X chromosome; high viability with normal life expectancy; characterized by tall stature and reduced fertility.

Mechanisms of Aneuploidy

Nondisjunction: Major cause of aneuploidy, occurring during meiosis or mitosis.
- Meiosis I nondisjunction leads to one daughter cell with trisomy and another with monosomy.
- Meiosis II nondisjunction results in unequal chromatid segregation.
Lagging Chromosomes: Failure of chromosomes to move to the poles during anaphase can result in missing chromosomes in daughter cells.
Multipolar Spindle Formation: Can lead to uneven chromosomal distribution among more than two daughter cells.
Cohesin Dysregulation: Improper breakdown of cohesin proteins can cause unequal chromosome distribution.

Uniparental Disomy (UPD)

UPD occurs when both chromosome copies come from one parent, leading to either isodisomy (identical) or heterodisomy (different).
Segmental UPD affects only parts of a chromosome and can have significant impacts, especially for imprinted genes.

Errors in Gametogenesis and Development

Meiosis errors can result in monosomies or trisomies.
Mitosis errors may cause mosaicism, with potential "rescue" of trisomic cells.
Fertilization errors may lead to triploidy, with all chromosomes from one parent.

Maternal Age and Nondisjunction

Two hypotheses explain the correlation between maternal age and nondisjunction:
- Production Line Hypothesis: Oocyte maturation occurs sequentially from fetal development.
- Limited Oocyte Pool Model: Reduction in the number of follicles increases the likelihood of selecting a lower-quality oocyte.

Impact of Mosaicism

Mosaicism: Presence of genetically distinct cell populations in one organism resulting from mutations post-fertilization.
Confined Placental Mosaicism (CPM): Mutation affects placental cells but not the fetus, often without clinical abnormalities.
Fetal Mosaicism: Can involve both placenta and fetus or only the fetus, leading to varying clinical consequences.

Chimerism vs. Mosaicism

Chimerism: Cells originate from two different fertilized eggs, resulting in two separate genetic lineages.
Mosaicism: All cells derive from a single zygote with genetic diversity due to mutations.

Mechanisms of Chromosome Abnormalities in Mitosis

Defective spindle assembly can lead to unequal chromosome distribution.
Non-disjunction during mitosis can affect multiple tissues, depending on the timing of the error.

Summary of Key Definitions

Aneuploidy: Abnormal chromosome number.
Euploidy: Balanced chromosome content.
Nondisjunction: Failure to separate chromosomes.
Isodisomy/Heterodisomy: Types of uniparental disomy based on chromosome identity.

Consequences of Meiotic vs. Mitotic Nondisjunction

Meiotic nondisjunction affects gametes and can impact entire organisms if errors are not corrected.
Mitotic nondisjunction can affect tissue-specific growth and lead to mosaicism within the organism.

Factors Impacting Nondisjunction

Maternal age significantly increases the risk of nondisjunction during meiosis I.
Chromosome-specific errors, such as those for chromosomes 7, 13, and 18, exhibit unique patterns of nondisjunction events.

Conclusion

Nondisjunction leads to various genetic conditions with differing degrees of viability and phenotypic consequences, influenced by the specifics of gametogenesis and cell division mechanisms.

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Description

This quiz explores the differences between autosomal and sex chromosome aneuploidies, focusing on their causes, viability, and associated conditions like Trisomy 21. It delves into how X-inactivation and gene content impact the severity of phenotypes in individuals with sex chromosome aneuploidies. Test your knowledge on these critical genetic concepts.