Genetics of Down Syndrome and Turner Syndrome

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Questions and Answers

What is the primary reason proposed for the increase in the incidence of Down Syndrome with maternal age?

Decrease in the number of available eggs

Increased non-disjunction in older eggs (correct)

Increased fertility treatments in older women

Higher rates of chromosomal anomalies in sperm

In the context of familial Down Syndrome, what is the estimated occurrence rate at birth?

1 in 100,000 births

1 in 31,000 births (correct)

1 in 50,000 births

1 in 10,000 births

What type of mosaicism is defined as being limited exclusively to the gonads?

Gonadal mosaicism (correct)

Somatic mosaicism

Embryonic mosaicism

Lethal mosaicism

Which karyotype indicates the presence of an additional chromosome associated with Down Syndrome?

47,XX,+21 Signup and view all the answers

What percentage of Turner Syndrome cases is associated with reduced fertility and delayed or absent periods?

30% Signup and view all the answers

What is the chromosomal makeup of Klinefelter syndrome?

47,XXY Signup and view all the answers

Which of the following characteristics is NOT associated with Turner syndrome?

Normal breast development Signup and view all the answers

Which syndrome is characterized by the presence of an extra Y chromosome?

Jacob’s syndrome Signup and view all the answers

In which syndrome do most individuals not survive to birth?

Turner syndrome Signup and view all the answers

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

Enlarged breasts Signup and view all the answers

Which syndrome is associated with only 45 chromosomes?

Turner syndrome Signup and view all the answers

What is a potential cognitive aspect associated with Klinefelter syndrome?

Learning disabilities Signup and view all the answers

Which syndrome can include menstrual irregularities but is considered physically normal?

Triple X syndrome Signup and view all the answers

What is the primary cause of most simple aneuploidy cases?

Meiotic non-disjunctions Signup and view all the answers

Which of the following is a well-known example of mental retardation related to chromosomal abnormalities?

Down syndrome Signup and view all the answers

What is the consequence of monosomy?

One member of a chromosome pair is missing Signup and view all the answers

In chorionic villus sampling, what tissue is primarily analyzed for a karyotype?

Tissue from the fetal side of the placenta Signup and view all the answers

What defines triploidy in terms of chromosome number?

Two additional chromosome sets (3n) Signup and view all the answers

What is a common risk associated with chorionic villus sampling performed at 10-12 weeks' gestation?

Miscarriage (1-2%) Signup and view all the answers

Which term describes conditions where genetic and phenotypic sex do not correspond?

Intersexes Signup and view all the answers

What does aneuploidy specifically refer to?

Less or more than the normal diploid number Signup and view all the answers

Which of the following congenital anomalies is associated with polydactylism?

Extra fingers and toes Signup and view all the answers

What is the consequence if a deletion includes the centromere of a chromosome?

The chromosome will be lost during cell division Signup and view all the answers

In Cri du Chat syndrome, which chromosome band is typically deleted?

Band p15.3 of chromosome 5 Signup and view all the answers

What best describes the result of a chromosome duplication?

Unbalanced gene dosage causing developmental abnormalities Signup and view all the answers

What type of chromosome alteration involves a segment of the chromosome being repeated?

Duplication Signup and view all the answers

Which of the following is a characteristic symptom of Cri du Chat syndrome?

Moon-shaped face Signup and view all the answers

What condition can occur when a haploinsufficient gene is present?

Recessive mutation becomes expressed Signup and view all the answers

Which of the following describes the effect of a deletion in terms of gene product imbalance?

It can create lethal homozygous conditions Signup and view all the answers

What is the result when nondisjunction occurs in a mother with the genotypes XBXb and XBY?

Klinefelter Syndrome and Super Female Signup and view all the answers

How does dosage compensation work in XX females?

They inactivate one of their X chromosomes. Signup and view all the answers

Why might a normal XX female with one X inactivated not be equivalent to a Turner female?

Turner syndrome results from the absence of one entire chromosome. Signup and view all the answers

Which of the following statements about Down Syndrome is correct?

It usually occurs due to nondisjunction of chromosome 21 during meiosis. Signup and view all the answers

What percentage of Down Syndrome cases are associated with defective eggs from mothers?

95% Signup and view all the answers

What is a common characteristic associated with Down Syndrome?

Flattened facial patterning Signup and view all the answers

How does the incidence of trisomy compare to monosomy in terms of viability?

Trisomy is generally more viable than monosomy. Signup and view all the answers

What could lead to inconsistencies between syndromes and X inactivation?

Random inactivation and incomplete inactivation for some genes Signup and view all the answers

What genetic abnormality is characterized by a deletion on the maternal chromosome 15?

Angelman syndrome Signup and view all the answers

Which of the following conditions is associated with the paternal deletion of a gene where the maternal copy is imprinted?

Prader-Willi syndrome Signup and view all the answers

In which chromosomal abnormality does a person have an extra chromosome 21?

47,XX,+21 Signup and view all the answers

What mechanism is primarily responsible for the maintenance of genomic imprinting?

DNA methlyation Signup and view all the answers

What feature is typical of Angelman syndrome?

Frequent laughter and smiling Signup and view all the answers

What kind of chromosomal abnormality is indicated by 46,XY,t(2;4)(p12;q12)?

Reciprocal arrangement of chromosomes Signup and view all the answers

Which of the following best describes heterochromatin?

Tightly packed and gene repressive Signup and view all the answers

Which gene is an example of one that is imprinted?

Insulin-like growth factor 2 (Igf2) Signup and view all the answers

Study Notes

Chromosomal Disorders

Chromosomal disorders arise from abnormalities in chromosome structure or number
Causes include ionizing radiation, viral infections, and chemical toxins, as well as meiotic or mitotic nondisjunctions.
Meiotic nondisjunctions are prominent in most simple aneuploidy cases (monosomy or trisomy).
Mitotic nondisjunctions are also possible.
Clinical presentations of chromosomal abnormalities often include infertility/sterility, intersex conditions (mismatch in genetic and phenotypic sex), multiple congenital malformations, and mental retardation (e.g., Down syndrome, fragile X syndrome)
Sterility is the inability to produce offspring, whereas infertility is the inability to conceive.

Types of Chromosomal Abnormalities

Numerical Abnormalities:
- Euploid: Normal chromosome set (2n)
- Polyploidy: Extra chromosome sets (3n, 4n, etc.) - triploidy, tetraploidy
- Aneuploidy: Fewer or more chromosomes than the normal diploid number
  - Monosomy: One member of a chromosome pair is missing (2n-1)
  - Trisomy: One chromosome set consists of three copies (2n+1)
Structural Abnormalities
- Translocations: Movement of a chromosome segment to a non-homologous chromosome
  - Robertsonian translocations: Fusion of acrocentric chromosomes.
  - Reciprocal translocations: Exchange of segments between two non-homologous chromosomes
- Deletions: Loss of a chromosome segment
- Duplications: Repetition of a chromosome segment
- Ring chromosomes: Formation of a ring-shaped chromosome due to breaks and rejoining
- Inversions: Reversal of a chromosome segment
Specific Syndromes Associated With Anomalies:
- Cri-du-chat syndrome: Deletion of a portion of chromosome 5
- Down syndrome: Trisomy 21
- Edwards syndrome: Trisomy 18
- Patau syndrome: Trisomy 13
- Klinefelter syndrome: 47,XXY
- Turner syndrome: 45, X
- Jacob's syndrome: 47, XYY

Karyotypes

A karyotype is a visual representation of an organism's chromosomes
It displays the total chromosome number and sex chromosome composition, with any detectable anomalies noted.

Genetic Testing

Amniocentesis: Prenatal test to analyze amniotic fluid for fetal chromosomal abnormalities (14-16 weeks gestation)
Chorionic villus sampling (CVS): Prenatal test using chorionic villi to assess fetal chromosomal abnormalities (10-12 weeks gestation)

Other Key Concepts

Mosaicism: A condition where different cells in an individual have different chromosomes.
Nondisjunction: The failure of chromosomes to separate properly during cell division
- May occur during meiosis I or II
Imprinting: The expression of a gene is influenced by the parent of origin of the gene.

Epigenetics & Genomic Imprinting

Some genes are expressed differently depending on the parent they inherit from.
Imprinting is a form of epigenetic modification in which genes are expressed preferentially from either the mother or the father.
This is usually maintained by DNA methylation (addition of methyl groups to DNA).
Examples include Angelman and Prader-Willi syndromes, both caused by defects in genomic imprinting.

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Description

This quiz explores key concepts related to Down Syndrome and Turner Syndrome, including incidence rates, maternal age factors, and mosaicism types. Test your knowledge on the genetic aspects and karyotypes related to these conditions.