Sex Chromosomes and Lyon Hypothesis
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Questions and Answers

What is the role of a Barr body in relation to X chromosomes?

  • It is only found in cases of Turner syndrome.
  • It indicates the presence of an additional Y chromosome.
  • It represents the active X chromosome in males.
  • It is the result of inactivating one of the X chromosomes. (correct)
  • Which of the following statements about males and Barr bodies is correct?

  • Males do not have Barr bodies because they only carry Y chromosomes. (correct)
  • Males inactivate both X chromosomes resulting in Barr bodies.
  • Males can sometimes have more than one Barr body.
  • Males have one Barr body due to having one X chromosome.
  • What can be inferred about the genetic material when a Barr body is present?

  • The organism is male with an extra X chromosome.
  • One of the X chromosomes is inactivated and converted to heterochromatin. (correct)
  • Barr bodies are primarily found in mitochondrial DNA.
  • Both X chromosomes are active and functioning.
  • In a normal female with two X chromosomes, what will happen to the X chromosomes?

    <p>One X chromosome will be randomly inactivated.</p> Signup and view all the answers

    Which statement correctly describes the inactivation process of X chromosomes?

    <p>Inactivation is a random process that can affect either X chromosome.</p> Signup and view all the answers

    How is a Barr body best classified within cellular structures?

    <p>As an inactive X chromosome represented by heterochromatin.</p> Signup and view all the answers

    What is typically absent in male individuals that affects Barr body formation?

    <p>A second X chromosome.</p> Signup and view all the answers

    What happens to the genetic state of the inactivated X chromosome shown as a Barr body?

    <p>It is condensed into heterochromatin.</p> Signup and view all the answers

    Study Notes

    Sex Chromosomes, Barr Body and Lyon Hypothesis

    • Sex is determined at fertilization
    • Sexual differences emerge during the 7th week
    • Sex is influenced by genetic and environmental factors
    • Females (generally XX) lack a Y chromosome
    • Males (generally XY) possess a Y chromosome

    Objectives

    • Understand X inactivation
    • Determine the number of Barr bodies in a human cell
    • Explain the Lyon hypothesis
    • Recognize the advantages of Barr body determination

    Matters of Sex

    • Aneuploidy: Having too many or too few chromosomes compared to normal
    • Monosomy: Possessing only one copy of a chromosome
    • Monosomy (with the exception of the X chromosome) is incompatible with life

    X Chromosome Dosage

    • Females (XX) have twice the X-linked genes of males (XY)
    • X-inactivation: A mechanism to create equal amounts of X gene products in both sexes
    • One X chromosome is inactivated to equalize gene products

    X Chromosomes

    • Demonstrated by culturing cells with different karyotypes (e.g., XY, XX, XXX, XXXX)
    • Enzyme levels encoded by the X chromosome are observed

    X-linked Gene (G6PD)

    • Located on the X chromosome
    • Codes for an enzyme (glucose-6-phosphate dehydrogenase) that breaks down sugar
    • Females produce the same amount of G6PD enzyme as males
    • XXY and XXX individuals have a normal amount of G6PD

    Conclusion

    • Only one X chromosome is active in a given cell; the other is inactive
    • In some cells, the paternal allele is expressed, while in others, the maternal allele is expressed
    • In XXX and XXXX females and XXY males, only one X chromosome is activated; the rest are inactivated
    • X inactivation is random

    Conclusion (cont.)

    • XXX embryos survive due to inactivation of two X chromosomes, resulting in one active X
    • Trisomy 21 (Down syndrome) cannot inactivate the extra chromosome 21
    • Only the X chromosome can be inactivated

    Three Types of Chromatin

    • Euchromatin: True chromatin; possesses active genes
    • Heterochromatin: Stains darker than euchromatin; contains highly repetitive DNA with few active genes; usually found around centromeres and telomeres
    • Facultative Heterochromatin: Active in some cells like euchromatin but inactive like heterochromatin in others

    Facultative Heterochromatin

    • A mechanism to shut off a portion or entire chromosome, preventing gene expression
    • Active X chromosomes behave like euchromatin, with active genes transcribed
    • Inactive X chromosomes behave like heterochromatin

    The cell inactivates one X chromosome by converting it to heterochromatin (inactive DNA). This inactive DNA creates the Barr body, which appears as a dark staining structure.

    Lyon Hypothesis

    • Proposed by Mary Lyon in 1961 to describe X inactivation
    • Consists of five tenants:
    • Condensed X chromosome is genetically inactive
    • X inactivation occurs early in embryo development (around 32 cells/ 1-2 days post-fertilization)
    • At this stage, one of the X chromosomes is randomly inactivated in each cell
    • Inactivation is mitotically stable (remains inactivated during cell divisions)
    • The net effect equalizes phenotypes in males and females for X-linked genes

    Human Chromosomes (Additional Detail)

    • One X chromosome in females is inactivated early in embryonic development.
    • The inactivated X chromosome can be visually identified as a Barr body.

    Barr Body

    • Present in somatic XX nuclei but not in XY nuclei
    • One X chromosome is converted to heterochromatin (Barr body) in females to make the amounts equal
    • Females always have one active X and one inactive X in a given cell
    • Visible in epithelial cells and blood leucocytes as darkly stained or drum-stick-shaped structures, respectively

    X Inactivation

    • Random inactivation of one X chromosome in each cell
    • Results in males and females having similar levels of expression for X-linked genes

    Demonstration of Barr Body

    • Visible in epithelial cells (buccal smear)
    • Visible in blood leukocytes (sex chromatin appears as drum-stick shaped)

    Significance of Barr Body

    • Used to diagnose genetic sex in doubtful cases (e.g., hermaphroditism)
    • Used to study sex chromosome abnormalities
    • Used to diagnose abnormal sexual development and maturation
    • Used to diagnose infertility, primary amenorrhea, and spontaneous abortion
    • Used to identify fetal sex by staining cells from amniotic fluid

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    Description

    This quiz explores the concepts of sex determination, X inactivation, and the Lyon hypothesis. You'll learn about Barr bodies, aneuploidy, and X chromosome dosage. Test your understanding of genetic and environmental influences on sexual differences.

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