Question 1

Increased sister chromatid exchange can be found in cells from people with which of the following disorders?&#10;A) Angelman syndrome&#10;B) Fragile X syndrome&#10;C) Bloom syndrome&#10;D) Chromosome 22q11 deletion syndrome&#10;E) Oculocutaneous albinism

Accepted Answer

Bloom syndrome is a rare autosomal recessive disorder characterized by short stature, sensitivity to sunlight, and increased sister chromatid exchange. This increased exchange is due to defects in DNA helicase, which is involved in the unwinding and separation of DNA strands during replication and repair. The other disorders listed do not have a known association with increased sister chromatid exchange.

Question 2

What molecular feature of the chromosome 22q11 region leads to its propensity for deletions?&#10;A) High GC content&#10;B) The presence of a fragile site&#10;C) It is flanked by low copy repeats&#10;D) It is imprinted&#10;E) Secondary structure of the DNA

Accepted Answer

The region of chromosome 22q11 is flanked by low copy repeats (LCRs), which are stretches of DNA that are nearly identical and prone to misalignment during DNA replication. This can lead to unequal crossover events between the LCRs, resulting in deletions or duplications of the intervening segment.

Question 3

Which of the following is the most common finding in chromosomal analysis of Down syndrome?&#10;A) Trisomy 21 with the extra copy being maternal in origin&#10;B) Trisomy 21 with the extra copy being paternal in origin&#10;C) Robertsonian translocation involving chromosome 21&#10;D) Mosaicism for trisomy 21&#10;E) A and B are equally likely

Accepted Answer

Trisomy 21 is the most common chromosomal abnormality seen in Down syndrome, accounting for approximately 95% of cases. The extra copy of chromosome 21 is most often maternal in origin, with only 5% due to paternal non-disjunction. Robertsonian translocations involving chromosome 21 occur in about 4% of cases, while mosaicism for trisomy 21 accounts for less than 1%. Therefore, A is the most common finding in chromosomal analysis of Down syndrome.

Question 4

Which of the following is true of Fragile X syndrome?&#10;A) Affected individuals have testicular atrophy beginning at puberty&#10;B) Accurate prenatal diagnosis of full-mutation females is available&#10;C) Premutation carriers show no phenotypic effects&#10;D) Expansion of premutation alleles occurs primarily during spermatogenesis&#10;E) It is almost always associated with a trinucleotide expansion in the 5' untranslated portion of FMR1

Accepted Answer

Fragile X syndrome is almost always caused by a trinucleotide expansion (CGG) in the 5' untranslated region of the FMR1 gene on the X chromosome. This expansion leads to methylation and decreased expression of the FMR1 gene, resulting in the clinical features of Fragile X syndrome.

Answer: A
 Testicular atrophy is a common feature of Fragile X syndrome in males, and typically begins during puberty. However, not all males with Fragile X syndrome will have testicular atrophy.

Answer: B
 Accurate prenatal diagnosis of full-mutation females with Fragile X syndrome is available through chorionic villus sampling or amniocentesis. However, accurate prenatal diagnosis for males with Fragile X syndrome is not currently available.

Answer: D
 Expansion of premutation CGG repeats in FMR1 occurs primarily during oogenesis in women. In males, expansion of CGG repeats can occur during both spermatogenesis and post-zygotic mitotic amplification. Premutation carriers may also show phenotypic effects, such as Fragile X-associated tremor/ataxia syndrome (FXTAS) in older individuals.

Question 5

Which of the following is a definite indication for chromosome analysis?&#10;A) Multiple congenital abnormalities&#10;B) Diaphragmatic hernia&#10;C) Single palmar crease&#10;D) Cleft lip&#10;E) All of the above

Accepted Answer

Multiple congenital abnormalities suggest a potential chromosomal abnormality and require chromosome analysis to confirm or rule out any such abnormalities. The other options, such as diaphragmatic hernia, single palmar crease, and cleft lip, may or may not be associated with chromosomal abnormalities and are not definite indications for chromosome analysis.

Question 6

What is the most common defect causing male pseudohermaphroditism?&#10;A) Lack of testosterone production&#10;B) Defective androgen receptors&#10;C) Mosaicism for 45,X/46,XY&#10;D) Inappropriate estrogen expression&#10;E) Loss of Y chromosome pseudoautosomal region

Accepted Answer

Male pseudohermaphroditism is a condition where an individual has external male genitalia but underdeveloped or absent internal male reproductive organs. The most common cause of male pseudohermaphroditism is defective androgen receptors, leading to an inability to respond to androgens such as testosterone, which are necessary for the development of male reproductive organs.

Question 7

Which of the following is a major risk factor for Down syndrome?&#10;A) Folic acid exposure&#10;B) Hyperthermia during the first trimester&#10;C) Toxoplasmosis infection during pregnancy&#10;D) Advanced maternal age&#10;E) Any of the above

Accepted Answer

Advanced maternal age, or being over the age of 35, is a major risk factor for Down syndrome. The chance of having a baby with Down syndrome increases as a woman gets older.

Question 8

Which of the following is the most common inherited cause of learning difficulties?&#10;A) Fragile X syndrome&#10;B) Down syndrome&#10;C) Turner syndrome&#10;D) Klinefelter syndrome&#10;E) Williams syndrome

Accepted Answer

Fragile X syndrome is the most common inherited cause of learning difficulties, affecting 1 in 4,000 males and 1 in 8,000 females. It is caused by a mutation in the FMR1 gene on the X chromosome and can lead to intellectual disability, behavioral and emotional problems, and physical characteristics such as a long face and large ears. Down syndrome, Turner syndrome, Klinefelter syndrome, and Williams syndrome are all genetic conditions that can cause learning difficulties, but they are not as common as Fragile X syndrome.

Question 9

Individuals with Klinefelter syndrome are usually infertile because...&#10;A) Their testes do not descend&#10;B) They lack sperm in their semen&#10;C) Their sperm are nonmotile&#10;D) They exhibit feminization of their genitalia&#10;E) Problems with non-disjunction

Accepted Answer

Individuals with Klinefelter syndrome have an extra X chromosome, resulting in XXY genotype. As a result, their testes are underdeveloped and do not produce sufficient amounts of testosterone required for normal sperm development. Thus, Klinefelter syndrome individuals lack sperm in their semen and are usually infertile.

Question 10

What determines whether a child with a chromosome 15q11-q13 deletion will have Angelman syndrome or Prader-Willi syndrome?&#10;A) The size of the deletion&#10;B) The location of the 3' border of the deletion&#10;C) The location of the 5' border of the deletion&#10;D) The parent of origin of the deletion&#10;E) Variation in a gene on a different chromosome

Accepted Answer

The parent of origin of the deletion determines whether the child will have Angelman syndrome or Prader-Willi syndrome. If the deletion is inherited from the mother, the child will have Angelman syndrome, while if it is inherited from the father, the child will have Prader-Willi syndrome. This is due to the fact that genes in this region are subject to genomic imprinting, where only one copy of the gene is expressed depending on whether it came from the mother or father.

Deck 18: Chromosome Disorders