Down Syndrome Diagnosis and Characteristics

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Questions and Answers

What is a common physical characteristic of individuals with XXY syndrome?

Shortened torso

Tall stature (correct)

High hairline

Webbed neck

Which symptom is NOT typically associated with Turner Syndrome?

Heart defect

Failure of ovaries to develop

Tall stature (correct)

Low hairline at neck

What percentage of girls with Turner Syndrome have hearing disorders?

90-95%

20-40%

50-90% (correct)

25-65%

What is the common chromosomal notation for individuals with Turner Syndrome?

45, XO Signup and view all the answers

Which of the following symptoms is common in Turner Syndrome but not in XXY syndrome?

Webbed neck Signup and view all the answers

Which mechanism contributes to chromosomal genetic variation?

Independent assortment Signup and view all the answers

How many possible combinations of chromosomes result from independent assortment in humans?

2^23 Signup and view all the answers

How many possible offspring combinations are there in humans due to random fertilization, excluding crossover?

70 trillion Signup and view all the answers

What is the presence of 2 or more genetically different sets of cells in an individual called?

Mosaicism Signup and view all the answers

Which phase of meiosis does crossing over occur?

Anaphase I Signup and view all the answers

What does aneuploidy refer to?

Deviation from the normal number of chromosomes Signup and view all the answers

How many genetically unique daughter cells result from a single crossing over event?

4 Signup and view all the answers

What is trisomy?

Gain of an extra chromosome Signup and view all the answers

Which of the following is a symptom of Down Syndrome?

Looseness of joints Signup and view all the answers

What is the chromosomal condition associated with Klinefelter's syndrome?

Nondisjunction of the X Chromosome Signup and view all the answers

Which additional feature is always present in individuals with Down Syndrome?

Some degree of mental retardation Signup and view all the answers

What is the main cause of Klinefelter's syndrome?

Nondisjunction of the X chromosome Signup and view all the answers

Which syndrome is characterized by low birth weight and cardiac/renal malformations?

Edwards Syndrome Signup and view all the answers

A child with polydactyly and neurological problems is likely to have which syndrome?

Patau Syndrome Signup and view all the answers

What chromosomal anomaly causes Down Syndrome?

Trisomy 21 Signup and view all the answers

Which of the following is not a symptom of Patau Syndrome?

Prominent occiput Signup and view all the answers

Study Notes

Down Syndrome

Cell cycle is arrested at metaphase, and chromatids are purified, Giemsa stained, and analyzed for number and rearrangements.
Symptoms include low muscle tone, single deep crease across the center of the palm, looseness of joints, small skin folds at the inner corners of the eyes, and excessive space between the first and second toe.
Mental retardation is always present, ranging from mild to severe, with most cases being mild to moderate.

Trisomies

Patau Syndrome (Trisomy 13): characterized by polydactyly, deformed/rocker-bottom feet, neurological problems, facial defects, and cardiac/renal defects.
Edwards Syndrome (Trisomy 18): characterized by low birth weight, prominent occiput, clubbed hands, neurological problems, facial defects, and cardiac/renal malformations.

Klinefelter's Syndrome

Results from nondisjunction of the 21st chromosome (Trisomy 21), leading to characteristic facial and body patterns, and varying mental abilities.
Occurs when a sperm containing both X and Y combines with an egg containing the X, resulting in a male child with an extra X chromosome.
Leads to sterile males with female characteristics.

Genetic Variation

Three mechanisms contribute to chromosomal genetic variation: independent assortment of chromosomes, crossing over, and random fertilization.
Independent assortment reshuffles alleles and chromosomes every generation, resulting in 2^n possible combinations (8,388,608 for humans).
Random fertilization leads to 70 trillion possible offspring (excluding cross over).
Mosaicism, the presence of 2 or more genetically different sets of cells, can affect the expression of genetic disorders.

Mosaicism

All females are mosaics due to X-chromosome inactivation (lyonization).
Can be caused by a mitotic error soon after fertilization.
5% of Down Syndrome patients are a mosaic variant, with only some cells having a trisomy 21.

Independent Assortment

With n=2, there are 4 possibilities (A and a, B and b, C and c) and 8 possible combinations.
The "2" rule states that the number of possible chromosome sorting combinations is 2^n.

Cross Over

A single crossing over event leads to 4 genetically unique daughter cells.
Crossing over occurs during Meiosis I, resulting in recombinant chromosomes.

When Things Go Wrong

Aneuploidy: any deviation from the normal number of chromosomes, usually meaning a cell nucleus possessing too many or too few chromosomes.
Somatic aneuploidy: non-heritable and has a localized effect.
Meiosis can lead to trisomy, monosomy, or tetrasomy.

Karyotype

XXY (Klinefelter's Syndrome): males with some development of breast tissue, little body hair, and small testes, leading to infertility and possible mental retardation.
Turner Syndrome (XO): sterile females with webbed necks, shortened torso, and a range of physical and developmental abnormalities, affecting 1 in 2,000 live births.

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Description

This quiz covers the diagnosis and symptoms of Down Syndrome, including cell cycle arrest, chromatid analysis, and physical characteristics such as low muscle tone and joint looseness.