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Questions and Answers
What is the role of a Barr body in relation to X chromosomes?
What is the role of a Barr body in relation to X chromosomes?
Which of the following statements about males and Barr bodies is correct?
Which of the following statements about males and Barr bodies is correct?
What can be inferred about the genetic material when a Barr body is present?
What can be inferred about the genetic material when a Barr body is present?
In a normal female with two X chromosomes, what will happen to the X chromosomes?
In a normal female with two X chromosomes, what will happen to the X chromosomes?
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Which statement correctly describes the inactivation process of X chromosomes?
Which statement correctly describes the inactivation process of X chromosomes?
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How is a Barr body best classified within cellular structures?
How is a Barr body best classified within cellular structures?
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What is typically absent in male individuals that affects Barr body formation?
What is typically absent in male individuals that affects Barr body formation?
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What happens to the genetic state of the inactivated X chromosome shown as a Barr body?
What happens to the genetic state of the inactivated X chromosome shown as a Barr body?
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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.