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Sex Chromosomes and Sex Linkage

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36 Questions

What is the primary function of the SRY gene?

Initiating male development

Why are X-linked recessive diseases more common in males?

Because males are hemizygous for the X chromosome

What is the purpose of dosage compensation in mammals?

To normalise the expression levels of X-linked genes

What is the characteristic pattern of inheritance for X-linked recessive diseases?

Affected males do not transmit the disease to their sons

What is the effect of X inactivation on females who are carriers of X-linked recessive diseases?

They may present with a mild form of the phenotype

What is the frequency of X-linked recessive traits in humans?

Around 515 known traits

What is the effect of X inactivation on X-linked dominant traits?

The severity of the disease in females is variable

What is the significance of the observation of a 1:1 ratio in the segregation of particular 'sex' chromosomes to offspring?

It indicates that the chromosomes are sex determining

What is the characteristic pattern of inheritance for X-linked dominant traits?

Affected males can only transmit the disease to their daughters

What is the primary function of the genes on the Y chromosome?

They are involved in spermatogenesis

What is the purpose of Mary Lyon's work on X inactivation?

To establish the mechanism of dosage compensation

What is the name of the gene that encodes a transcription factor that activates a testis-forming pathway?

SRY

What is the effect of inbreeding on X-linked and autosomal recessive diseases?

It increases the frequency of the diseases

What is the characteristic of the distal half of the long arm of the Y chromosome?

It is highly repetitive and heterochromatic

What is the term for the small regions at the tips of the Y chromosome that are homologous with the X chromosome?

Pseudoautosomal regions

Who is credited with mapping the first sex-linked trait to the X chromosome in Drosophila melanogaster?

Thomas Hunt Morgan

What is the state of the embryonic gonad before the testis-forming pathway is triggered?

It is indifferent, meaning it can develop into either a testis or ovary

What is the characteristic of the Y chromosome compared to the X chromosome?

It is smaller and gene poor

What occurs in females as a result of X inactivation?

Some cells express alleles from the paternally inherited X and other cells express alleles from the maternally inherited X

What type of genes often have copies on the Y chromosome?

Genes that escape X inactivation

What is required for X inactivation to occur?

Expression of the non-coding RNA gene Xist

What is found along the length of the inactive X chromosome?

The repressive histone modifications H3K27me3 and H3K9me3

What is the function of DNA methylation in mammalian cells?

To silence genes

What is the result of the deamination of 5'-methylcytosine?

Conversion to thymine

What is the name given to the category of CpG-rich sequences found at gene promoters?

CpG islands

What is the name given to the enzymes that establish histone modifications?

Writers

What is the consequence of losing heterochromatic marks H3K9me3 and H3K9me2?

Chromosomes cannot segregate properly during mitosis

What is one of the essential processes regulated by epigenetic modifications?

Chromosome architecture

What is the primary role of epigenetic modifications in the genome?

To contribute to dynamic and heritable changes in gene expression

What is an example of epigenetic modifications acting in a hierarchical fashion?

H3K27Ac and H3K27me3

Where can epigenetic modifications be found within genes?

Within the body of both active and repressed genes

What is the potential significance of DNA methylation found at repressed promoters?

It suggests the methylation is a consequence, not a cause, of gene repression

What role do epigenetic modifications play at regulatory elements?

They are often at some distance from the genes they control

Which combination of epigenetic modifications frequently occur together?

DNA methylation and H3K9me3

What is suggested by the presence of much descriptive epigenomic data from various sources?

Functional correlates are becoming more established over time

What potential benefit does an integrated understanding of the epigenome provide?

It provides functional information in normal and diseased states

Study Notes

Sex Chromosomes and Sex Determination

  • In the late 19th and early 20th centuries, the difference in chromosome constitutions between males and females was noted, leading to the identification of sex chromosomes as sex-determining.
  • The XY sex-determining system evolved independently from the ZW sex-determining system.
  • Thomas Hunt Morgan mapped the first sex-linked trait (the white locus for eye color) to the X chromosome in Drosophila melanogaster and determined the genetics of sex in flies.

The Mammalian Y Chromosome and Sex Determination

  • The Y chromosome is smaller than the X chromosome and is gene-poor.
  • Small regions at the tips of the Y chromosome are homologous with the X chromosome, known as pseudoautosomal regions (PAR), which allow the X and Y chromosomes to pair and recombine during meiosis in the male.
  • The gene SRY (Sex-determining Region Y gene) is necessary and sufficient to cause male development, encoding a transcription factor that activates a testis-forming pathway early in development.
  • SRY is necessary for spermatogenesis, but other genes on the Y chromosome are also required.

X-Linked Traits

  • In humans and other mammals, mutations in genes on the X chromosome result in abnormalities that are manifested differently between the sexes because males are hemizygous for the X.
  • Dosage compensation between the mammalian sexes is achieved by inactivating one of the two X chromosomes.
  • In males, the sole X remains active in every cell, and each daughter must receive her father's sole X chromosome.
  • For X-linked recessive diseases, males manifest the disease, and rarely females, with no father-to-son transmission.
  • Carrier females are usually asymptomatic, but a minority of carrier females may present with a mild form of the phenotype due to X inactivation.
  • Around 515 X-linked recessive traits are known in humans, with varying frequencies in different ethnic groups.

X Inactivation and Dosage Compensation

  • X inactivation is an epigenetic paradigm that normalizes the expression levels of X-linked genes between males and females.
  • In abnormal cells with multiple X chromosomes, all but one is inactivated, and this applies to both male and female cells.
  • Work by Mary Lyon established that the condensed chromosome observed in 46XX female cells was the inactive one, and that the cells retained a memory of which X was inactivated.
  • Females are mosaic for genes on the X, with some cells expressing alleles from the paternally inherited X and others expressing alleles from the maternally inherited X.
  • Some genes escape X inactivation, being expressed from both X chromosomes, often having copies on the Y chromosome.

Epigenetic Modifications and Genome Function

  • Epigenetic modifications regulate a range of processes in mammalian cells, including chromosome architecture, silencing of repetitive transposable elements, and dynamic and somatically heritable changes in gene expression.
  • Epigenetic modifications are chemical marks on the DNA (especially DNA methylation) and post-translational modifications to chromatin-associated histone proteins.
  • DNA methylation occurs on CG dinucleotides and is symmetrical on the two DNA strands.
  • Histone modifications are established by 'writers', interpreted by 'readers', and can be reversed by 'erasers'.
  • Epigenetic modifications contribute to the architecture and function of the genome, and can be over short regulatory elements or over longer regions of DNA.

Learn about the discovery of sex chromosomes, their importance, and sex linkage. Understand the XY and ZW sex determining systems and their evolution.

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