Genetics of Sex Chromosomes

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Questions and Answers

In mammals, what chromosomal combination defines a female individual?

47,XXY

45,X

46,XX (correct)

46,XY

Which region of the X and Y chromosomes is most similar in terms of DNA sequence?

Centromeric region

Distal region of the short arms (correct)

Heterochromatic region

Telomeric region of the long arms

What is the primary function of the SRY gene located on the Y chromosome?

To promote bone growth in males

To initiate the production of estrogen

To code for the testis determining factor (TDF), leading to testes formation (correct)

To determine femaleness by triggering ovarian development

Which of the following describes the gamete production of human males?

They produce gametes with either an X or a Y chromosome. Signup and view all the answers

Why is at least one copy of the X chromosome necessary for both males and females?

The X chromosome possesses necessary genetic information for both genders. Signup and view all the answers

What is the primary purpose of chromosome counting in X-inactivation?

To ensure that only cells with two X chromosomes undergo inactivation. Signup and view all the answers

What is the role of the X-inactivation center (XIC) in X-inactivation?

It ensures that inactivation occurs in cells with two X chromosomes. Signup and view all the answers

Which element within the XIC region is responsible for selecting the X chromosome to be inactivated?

The X-controlling element (XCE) Signup and view all the answers

How does the XIST gene mediate X-inactivation?

It codes for Xist RNA, which coats the chromosome for inactivation. Signup and view all the answers

What prevents the transcription of Xist in active X chromosomes?

The production of a blocking factor. Signup and view all the answers

Approximately what percentage of genes on the X chromosome escape inactivation and remain active?

15% Signup and view all the answers

Which region of the X chromosome has more genes that escape inactivation?

Xp Signup and view all the answers

What is the primary genetic cause of sex chromosome aneuploidy?

Meiotic nondisjunction Signup and view all the answers

What is the most common chromosomal abnormality in spontaneous abortions?

45,X Signup and view all the answers

What is the expected karyotype of an individual with Turner syndrome?

45,X Signup and view all the answers

What is the characteristic chromosomal abnormality seen in individuals with Klinefelter syndrome?

Presence of an extra X chromosome Signup and view all the answers

What is the primary reason the Y chromosome leads to sex linkage?

It lacks many genes found on the X chromosome Signup and view all the answers

In females, how does a recessive allele on one X chromosome interact with an allele on the other X?

It can be masked by a dominant allele Signup and view all the answers

Why do males typically show the effects of any recessive allele on the X chromosome?

Because the Y chromosome has no homologous region to the X Signup and view all the answers

Which of the following is an example of an X-linked disorder?

Red-green color blindness Signup and view all the answers

Which of the following is the most common cause of hypogonadism and infertility in males?

47,XXY Signup and view all the answers

What is the primary characteristic of chromosome instability (CIN)?

A higher-than-normal rate of chromosome missegregation during cell division Signup and view all the answers

What is the function of the spindle assembly checkpoint (SAC)?

To maintain genome stability by delaying cell division until all kinetochores are properly bound Signup and view all the answers

What is amphitelic attachment during mitosis?

Attachment of sister chromatids to opposite spindle poles Signup and view all the answers

What happens when chromosomes are not correctly attached to the spindle?

Kinetochores activate the SAC network, which inhibits the initiation of anaphase. Signup and view all the answers

Which of the following is NOT mentioned as a potential mechanism underlying CIN?

Normal spindle assembly checkpoint function Signup and view all the answers

What is the typical error rate of chromosome segregation in normal cells during cell division?

1% Signup and view all the answers

During mitosis, what must occur before the start of anaphase?

The kinetochores of sister chromatids must connect to the poles of the opposite spindle (amphitelic attachment). Signup and view all the answers

In cells with CIN, approximately what percentage of cell divisions results in errors in chromosome segregation?

20% Signup and view all the answers

What is the consequence of a failure in the spindle assembly checkpoint?

Premature initiation of anaphase, which can lead to missegregation of chromosomes Signup and view all the answers

Study Notes

Molecular Diagnostics & Cytogenetics - Lecture 10

Lecture topics: Sex chromosomes, X-chromosome inactivation, sex chromosome abnormalities, chromosome breakage, and instability syndromes.
Learning outcomes: Students should be able to describe the structure and functions of sex chromosomes, explain mechanisms of X-chromosome inactivation, describe the types of sex chromosome abnormality, define chromosome instability and explain mechanisms of chromosome instability.

Sex Chromosomes

Mammalian sex determination: Determined by the combination of sex chromosomes.
Female: 46,XX (homogametic)
Male: 46,XY (heterogametic)

Sex Chromosomes - detail

Pseudoautosomal regions (PAR): Similar DNA sequences on the short arms of the X and Y chromosomes that resemble crossing-over between autosomes.
X chromosome: Larger than the Y chromosome, containing approximately 1,400 genes.
Y chromosome: Smaller than the X chromosome, containing approximately 200 genes, including the SRY gene.
- SRY gene: Encodes the testis-determining factor (TDF) which triggers testes formation.
- Testosterone: Produced by testes to control the formation of male secondary traits.
- Absence of SRY gene: Triggers the development of ovaries and female characteristics.

X-Chromosome Inactivation

Females: Have two X chromosomes.
Males: Have one X chromosome.
Dosage compensation: Balances the dose of X-chromosome gene expression in females and males.
X-inactivation: One X chromosome is inactivated to prevent cells from having twice (double-dose) of gene products.
Random process: X-inactivation occurs randomly during early embryonic development in one cell, inactivating one X chromosome from either parent.
Barr body: The inactivated X chromosome is visible as a Barr body, a highly condensed chromatin in the cell nucleus.
XIST gene: A gene that controls X-inactivation, located on the X-inactivation center.
- XIST RNA: Coats the chromosome to be inactivated, silencing most of its genes
In active X: A blocking factor is produced in active X preventing the transcription of Xist RNA

Mechanism of X-Inactivation

Chromosome counting: Determining the number of Xs in the cell.
Selection of an X: X-controlling element (XCE) in the XIC region selects an X for inactivation.
Inactivation process: The inactivated X chromosome is coated with Xist RNA which blocks most of its gene expression.

Numerical Abnormalities of Sex Chromosomes

Sex chromosome aneuploidy: Caused by meiotic nondisjunction resulting in an abnormal number of sex chromosomes.
- Examples:
  - 45,X (Turner syndrome): Missing or partially missing X chromosome.
  - 47,XXY (Klinefelter syndrome): Extra X chromosome.

Sex Chromosome Disorders

45,X (Turner syndrome): One of the X chromosomes is missing or partially missing, leading to characteristics like short stature, webbed neck and low hairline.
47,XXY (Klinefelter syndrome): Presence of an extra X chromosome in males, leading to characteristics like tall stature and poor muscle tone.

Sex Linkage

X-linked recessive: The Y chromosome lacks many genes found on the X chromosome. In XX females a recessive allele on one X can be masked by a dominant allele on the other X. In XY males a recessive allele on X has no second copy to mask its effects.
Examples: Red-green color blindness, haemophilia, Duchenne muscular dystrophy, Vitamin D resistant rickets

Chromosome Instability (CIN)

CIN: Higher-than-normal rate of missegregation of chromosomes due to changes in chromosome number or structure (numerical and structural chromosome abnormalities) during cell divisions.
Mechanism: Errors in chromosome segregation, kinetochore-microtubule attachment errors, aberrant sister chromatid cohesion, abnormal centrosome replication, telomere attrition, and spindle assembly checkpoint (SAC) abnormalities.

Chromosome Instability Syndromes

Chromosome breakage syndromes: Number of rare but distinct clinical entities associated with an increased risk of malignancies, defective DNA repair, cell cycle control, or apoptosis.
- Examples:
  - Fanconi anaemia (FA): Rare genetic condition characterized by diverse congenital anomalies, bone marrow failure and malignancy.
  - Ataxia-telangiectasia (AT): Autosomal recessive condition characterized by progressive cerebellar degeneration, oculocutaneous telangiectasias, immunodeficiency, chromosome instability, radiosensitivity and cancer predisposition.

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Description

This quiz explores the genetic basis of sex determination in mammals, focusing on the roles of the X and Y chromosomes. It includes questions about chromosome combinations, gene functions, and mechanisms such as X-inactivation and the X-inactivation center. Test your understanding of these vital concepts in mammalian genetics.