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26 Cards in this Set
- Front
- Back
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Define the terms:
a. Barr bodies b. Pseudoautosomal region c. Sex determining region |
a. The Barr body represents an inactive X chromosome. The number of Barrbodies corresponds to the number of X chromosomes minus one.
b. pseudoautosomal regions - on the tip of the short arm and long arm c. SRY gene (sex determining region of the Y chromosome) |
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Define the terms:
d. X-inactivation e. Lyonization f. Clone |
d. X-inactivation - the X chromosome is made genetically inactive
e. Lyonization - The process by which all X chromosomes in excess of one are made genetically inactive and heterochromatic. f. A clone is a cell line derived by mitosis from a single ancestral diploid cell |
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Four key features of x-inactivation
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1. Embryonic timing - occurs during the blastocyst stage
2. The inactivation of one X chromosome is a random process - females are a mix or “mosaic” of information from maternal and paternal genes 3. Clonal stability - Once an X chromosome is inactivated, it remains inactive in all progeny cells, thus demonstrating clonal stability 4. X-Chromosome regions subject to inactivation - it was previously thought that the entire complement of genes on the inactive X were "silenced". Not so. |
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X-inactivation center
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(XIC) Several individuals who had deletions of this region had changes in their X-inactivation capabilities, implying that a gene controlling inactivation was located at this site.
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X-Inactive Specific Transcript gene.
What does it do and how does it do it? |
XIST gene causes inactivation. The gene is expressed from the inactive X and localizes to the inactive X chromosome.
Inactivation is thought to spread cis (on the same chromosome as the gene is expressed) |
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Translocations between the X-chromosome and an autosome can result in ...
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The inactivation has the potential (which is thought to be dependent on the underlying DNA repeat sequences present) to extend to the autosomal regions/genes thereby resulting in the functioning of only one dose of a gene that should be expressed in two doses
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Exceptions to Randomness of X-inactivation
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If a translocation occurs between an X chromosome and an autosome, the normal X chromosome is preferentially inactivated
Thus the inactivation does not spread into the autosome resulting in an unbalanced complement |
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Incidence of Sex Chromosome Abnormalities for Male and Female
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Male 1/400:
Klinefelter (47, XXY) = XYY > Other X or Y abnormalities > 48, XXYY, 49, XXXYY, mosaics > XX males > 48 XXXY (Klinefelters) Female: 1/650 Trisomy X > other chromosome abnormalities > Turner > angrogen insensitivity |
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Klinefelter Syndrome (47,XXY)
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Findings include: male or female?
-Tall eunuchoid (appears to lack characteristics indicating male virility) habitus -Intellectual development fairgood (usually less than sibs) -Frequent gynecomastia (excessive development of the breasts in males) -Small testes in adolescents and adults |
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XYY
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Finding include:
-Tall stature -Intellectual development normal-mod. impaired -Acne common -Frequent hypogonadism; undescended testes -Ascertained in prison populations with frequency that exceeds expectation |
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Turner syndrome (45,X)
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Findings include:
-Small stature -Intellectual development good (may be below siblings) -Perceptive hearing loss -Frequent pigmented nevi -Narrow maxilla and small mandible -Low posterior hairline; webbed in 50% of patients -Shield shaped chest with widely spaced nipples -Cardiovascular anomalies in 35% -Lymphedema -Short fourth and fifth metacarpals -Short fifth finger with clinodactyly -Ovarian dysgenesis with infertility |
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Cytogenetic Heterogeneity in Turner syndrome
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In addition to monosomy X (58%), other chromosomal anomalies are also seen in cases with Turner syndrome, including: isochromosome Xq (16%), deletions of X (3%), mosaicism (14%), ring (X) chromosome (5%), or other anomalies (4%).
Females who have Turner syndrome due to a structural chromosome abnormality that results in a deletion of the XIST gene (especially ring chromosomes) have a more severe phenotype, including mental retardation. |
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Trisomy X (47,XXX)
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Findings include:
-Above average height -Possible learning disabilities |
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Individuals having 4 or more sex chromosomes (48,XXXX; 49,XXXXX; 48,XXXY; 49,XYYY; 49,XXXXY)
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Tend to have increased severity of symptoms, with both growth and mental deficiencies being present.
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Define sex determination
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Sex determination refers to the process(es) which determine what sex an embryo will become.
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Sexual differentiation
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Sexual differentiation refers to the actual cellular and morphogenetic changes that take place that distinguish one sex from the other. Differentiation takes place after the sex is determined.
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Barr Bodies
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The Barr body, as seen in female cells, represents an inactive X chromosome and is an area of dark stain. The number of Barr bodies corresponds to the number of X chromosomes minus one.
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Pseudoautosomal region of the sex chromosomes
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Alleles located in the short region (near the telomeres) of human X and Y chromosome at which meiotic pairing occurs, pseudoautosomal alleles undergo reciprocal exchange by crossing over and therefore do not show strict sex-linked inheritance.
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Possible Mechanism (s) for Etiology of XX Male/XY Female Phenotype
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(1) a male with two X chromosomes that appear normal using microscopy, but that contain Y specific sequences
(2) a female with an X and a Y chromosome that appear normal using microscopy, but that contain no Y specific sex determining sequences. |
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Regions on the Y Chromosome that play a role in Sex Determination and Differentiation
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-Yp pseudoautosomal region
-Yq pseudoautosomal region -DAZ genes -USP9Y (by the centromere) |
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Sexual Differentiation:
-SOX9. Localized to 17q |
Target for SRY and is an important early factor in the sex determination cascade. Mutations of this gene, resulting in a condition called camptomelic dysplasia [autosomal dominant]) results in chromosomal males (46,XY) being a phenotypic female
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Sexual Differentiation:
-DAX1. Localized to Xp What does it do? |
Interact with SRY. Duplications of this gene (or region of Xp) result in a chromosomal male (46,XY) being a phenotypic female.
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Other genetic changes that have been ID'ed to play a role in the process of sex differentiation.
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-WT1 (Wilms tumor gene)
-deletions of 9p. |
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Embryology of the Reproductive System
Rates of sexual differentiation vs sex determination |
Abnormalities of sexual differentiation are more common than those of sex determination.
Also, The primordial gonad develops into an ovary when no Y chromosome is present. Testicular development occurs if a Y chromosome is present. |
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Camptomelic Dysplasia
Abnormality of sexual differentiation |
-Rare, autosomal dominant, mutation of SOX9 gene.
-Phenotypic female, with a male XY chromosomal complement -large head, a short trunk, thickened, bowed limbs (especially the femurs and tibia), hip dislocations and abnormalities; a bellshaped chest; and a narrowed trachea (which can lead to respiratory distress). -Remember the x-ray showed in class |
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Androgen insensitivity syndrome (also called testicular feminization)
Sexual Differentiation |
-defect in the androgen receptor results in failure of response to testosterone
-XY individual develops externally as a female -sparse pubic hair and absence of axillary hair -testes are present (in abdomen or inguinal canal) and are sometimes mistaken for a hernia |
