Questions and Answers
What are the sex chromosomes in humans?
X and Y chromosomes
Which sex chromosome influences the immature gonads to become testis?
Y chromosome
Who developed the Lyon hypothesis?
What is the mechanism of dosage compensation in females?
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What happens to a female's X chromosome after it is inactivated?
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What is Turner Syndrome?
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What is another term for 47,XXX?
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What causes hypogonadism and infertility in males?
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What is the extra chromosome in 47,XYY?
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Structural abnormalities of sex chromosomes are common.
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What is an isochromosome?
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Study Notes
Sex Determination and Chromosomes
- Sex chromosomes are the last pair of human chromosomes, designated as X and Y.
- Humans typically have 23 pairs of chromosomes; females have two X chromosomes (XX) while males have one X and one Y (XY).
- Genetic sex is established at fertilization, depending on whether an X or Y sperm fertilizes an X-bearing egg.
Sexual Differentiation
- Gonads appear identical before the seventh week of embryonic life.
- The presence of the Y chromosome drives differentiation of the gonad into testes, while the absence leads to ovary formation.
- Pseudoautosomal regions (PAR1 and PAR2) on X and Y chromosomes are crucial for normal meiotic crossing-over.
X Chromosome Inactivation
- The Lyon hypothesis states that one X chromosome in females is randomly inactivated, creating Barr bodies.
- X inactivation ensures equal gene dosage between males (one X) and females (one active X).
- XIST (X-inactive specific transcript) is the gene responsible for X inactivation and is expressed only from the inactive X chromosome.
Y Chromosome Characteristics
- Composed of pseudoautosomal regions, heterochromatic regions, and Male Specific Region of the Y (MSY).
- MSY contains protein-coding genes and plays a crucial role in male fertility.
- The SRY gene (Sex-determining Region Y) initiates male gonad differentiation.
Sex Chromosome Abnormalities
- Abnormalities can be numerical (aneuploidy) or structural.
- Common numerical abnormalities include Turner Syndrome (45,X) affecting female development and Klinefelter Syndrome (47,XXY) influencing males.
- Turner Syndrome occurs in approximately 1 in 2,500 live-born females, often leading to features like short stature and ovarian failure.
Turner Syndrome (45,X)
- Characterized by clinical features such as webbed neck, posteriorly rotated ears, and edema.
- Increased risks of developmental delays, feeding difficulties, and visual-spatial skill issues.
- Majority of cases have a maternal origin for the missing X chromosome.
Trisomy X (47,XXX)
- The most common sex chromosome disorder in females, occurring in about 1 in 1,000 live births.
- Often undetected; pubertal development is normal with variable ovarian function.
- Linked to risks of learning disabilities and lower IQ compared to siblings.
Klinefelter Syndrome (47,XXY)
- First identified sex chromosome disorder; common cause of infertility in males.
- Features include tall stature, small testes, and possible gynecomastia.
- Developmental concerns include language delays and learning difficulties.
Additional Y Chromosome Variants
- The 47,XYY karyotype occurs once in every 800-1,000 males; features include taller stature and a higher rate of learning disabilities.
- Variants with additional X or Y chromosomes can affect fertility and developmental patterns.
Structural Abnormalities
- Isochromosome Xq often seen in Turner syndrome contributes to fertility issues.
- Structural abnormalities can include deletions and duplications and may increase the risk of conditions like gonadoblastoma.
Summary
- Sex determination relies on the X and Y chromosomes, where the Y chromosome drives male differentiation.
- Chromosomal aberrations can lead to distinct syndromes with varied clinical implications, impacting growth, development, and fertility.
- Research in genetic mechanisms continues to reveal complexities in sex chromosome roles and disorders.
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Description
Explore the fascinating mechanisms of sex determination in humans, focusing on the roles of X and Y chromosomes. Understand how genetic sex is established at fertilization and the initial stages of sexual differentiation. This quiz delves into the biological underpinnings that distinguish male and female development.
