Genetic Disorders MCQs Examination (Part 2)

Question 1

A genetic variant with at least two alleles occurring in at least 1% of the population is termed

Accepted Answer

Mutation — A polymorphism is defined as a genetic variant that has at least two alleles and occurs in at least 1% of the population, distinguishing it from rare mutations. ---

Question 2

In type 1 diabetes, approximately how many genes contribute most significantly to disease risk?

Accepted Answer

While 20-30 genes — While 20-30 genes are implicated in type 1 diabetes, only 6-7 are most important, with a few HLA alleles contributing more than 50% of the risk. ---

Question 3

Which environmental factor commonly unmasks the genetic trait in type 2 diabetes mellitus?

Accepted Answer

Obesity and weight gain — Type 2 diabetes has features of a complex multigenic disorder where obesity and weight gain often unmask the diabetic genetic trait, demonstrating gene-environment interaction. ---

Question 4

Approximately what percentage of first-trimester spontaneous abortions have chromosomal abnormalities?

Accepted Answer

50% — Approximately 50% of first-trimester spontaneous abortions have demonstrable chromosomal abnormalities, highlighting the importance of chromosomal integrity for fetal development. ---

Question 5

The G-banding technique uses which stain to visualize chromosomes?

Accepted Answer

Giemsa stain — The G-banding (Giemsa banding) technique is widely used to stain chromosomes, producing distinctive patterns of alternating light and dark bands that allow identification of each chromosome. ---

Question 6

A chromosome number of 69 (3n) would be classified as

Accepted Answer

Since it's a multiple other than 2n, it's also polyploid (specifically triploid). — 69 chromosomes (3n) is an exact multiple of the haploid number (n=23), making it euploid. Since it's a multiple other than 2n, it's also polyploid (specifically triploid). ---

Question 7

The chief cause of aneuploidy is

Accepted Answer

Nondisjunction during meiosis — Nondisjunction of homologous chromosomes during the first meiotic division or failure of sister chromatids to separate during the second meiotic division is the chief cause of aneuploidy. ---

Question 8

Monosomy involving an autosome is

Accepted Answer

Incompatible with life — Monosomy involving an autosome is incompatible with life, whereas trisomies of certain autosomes and monosomy of sex chromosomes can be compatible with life. ---

Question 9

Mosaicism is defined as

Accepted Answer

Mosaicism — Mosaicism is the presence of two or more populations of cells with different complements of chromosomes in the same individual, usually resulting from postzygotic mitotic nondisjunction. ---

Question 10

In cytogenetic notation, "p" denotes

Accepted Answer

The short arm of a chromosome — In cytogenetic shorthand, "p" (from French "petit") denotes the short arm of a chromosome, while "q" denotes the long arm. ---

Question 11

The notation 46,XX,t(2;5)(q31;p14) indicates

Accepted Answer

Reciprocal translocation between chromosomes 2 and 5 — This notation indicates a reciprocal translocation involving the long arm of chromosome 2 at region 3 band 1, and the short arm of chromosome 5 at region 1 band 4. ---

Question 12

Robertsonian translocation typically involves

Accepted Answer

Two acrocentric chromosomes — Robertsonian (centric) fusion is a reciprocal translocation between two acrocentric chromosomes, involving the short arm of one and long arm of the other, forming one large and one very small chromosome. ---

Question 13

The most common isochromosome in live births involves

Accepted Answer

Long arm of the X chromosome [i(Xq)] — The most common isochromosome in live births is i(Xq), involving the long arm of the X chromosome, resulting in monosomy for Xp genes and trisomy for Xq genes. ---

Question 14

An isochromosome results from

Accepted Answer

Horizontal division of the centromere — Isochromosomes result when the centromere divides horizontally (rather than vertically), leading to loss of one arm and duplication of the other, creating two identical arms. ---

Question 15

Which statement about chromosomal abnormalities is correct?

Accepted Answer

Loss of chromosomal material produces more severe defects than gain — In general, loss of chromosomal material produces more severe defects than does gain of chromosomal material, which is why monosomies are typically more severe than trisomies. ---

Question 16

Sex chromosome imbalances compared to autosomal imbalances are

Accepted Answer

Tolerated much better — Imbalances of sex chromosomes (excess or loss) are tolerated much better than similar imbalances of autosomes, often producing subtle abnormalities. ---

Question 17

Quantitative trait loci (QTLs) include all of the following EXCEPT

Accepted Answer

Hair color — QTLs are normal phenotypic characteristics governed by multigenic inheritance showing continuous variation, including hair color, eye color, height, and intelligence. Down syndrome is a chromosomal disorder. ---

Question 18

The most common chromosomal disorder is

Accepted Answer

Trisomy 21 (Down syndrom — Trisomy 21 (Down syndrome) is the most common chromosomal disorder, occurring with notable frequency compared to other chromosomal abnormalities. ---

Question 19

In about 95% of Down syndrome cases, the extra chromosome 21 is of

Accepted Answer

Maternal origin — In approximately 95% of trisomy 21 cases, the extra chromosome is of maternal origin, resulting from meiotic nondisjunction in the ovum. ---

Question 20

Maternal age has the strongest correlation with which type of Down syndrome?

Accepted Answer

Meiotic nondisjunction form — Maternal age has a strong influence on Down syndrome due to meiotic nondisjunction (95% of cases), with incidence increasing dramatically with advancing maternal age. ---

Question 21

The translocation form of Down syndrome accounts for approximately what percentage of cases?

Accepted Answer

25% — About 4% of trisomy 21 patients have the extra chromosomal material as a translocation of the long arm of chromosome 21 to chromosome 22 or 14, often familial. ---

Question 22

Mosaic Down syndrome accounts for approximately what percentage of cases?

Accepted Answer

10% — Approximately 1% of trisomy 21 patients are mosaics with a mixture of 46- and 47-chromosome cells, resulting from mitotic nondisjunction during early embryogenesis. ---

Question 23

Which cardiac defect is most common in Down syndrome?

Accepted Answer

Endocardial cushion defects — Approximately 40% of Down syndrome patients have congenital heart disease, most commonly endocardial cushion defects including atrial and ventricular septal defects. ---

Question 24

Children with trisomy 21 have what increased risk of developing acute leukemia?

Accepted Answer

10- to 20-fold — Children with trisomy 21 have a 10- to 20-fold increased risk of developing acute leukemia, including both acute lymphoblastic and acute myeloid leukemias. ---

Question 25

Most Down syndrome patients older than 40 develop neuropathologic changes characteristic of

Accepted Answer

Alzheimer disease — Virtually all patients with trisomy 21 older than age 40 develop neuropathologic changes characteristic of Alzheimer disease. ---

Question 26

The current median age at death for persons with Down syndrome is

Accepted Answer

25 years — Improved medical care has increased longevity in Down syndrome from a median of 25 years in 1983 to currently 47 years. ---

Question 27

Most Down syndrome patients have an IQ in which range?

Accepted Answer

25 to 50 — Down syndrome is a leading cause of severe mental retardation; approximately 80% have an IQ of 25 to 50. ---

Question 28

Trisomy 18 is also known as

Accepted Answer

Edwards syndrome — Trisomy 18 is also known as Edwards syndrome, occurring less commonly than trisomy 21 and associated with severe malformations and early death. ---

Question 29

Trisomy 13 is also known as

Accepted Answer

Patau syndrome — Trisomy 13 is also known as Patau syndrome, associated with increased maternal age, severe malformations, and death usually within the first year of life. ---

Question 30

The 22q11.2 deletion syndrome was previously thought to represent which two separate disorders?

Accepted Answer

DiGeorge syndrome and velocardiofacial syndrome — The 22q11.2 deletion syndrome encompasses what were previously believed to be two different disorders: DiGeorge syndrome and velocardiofacial syndrome. ---

Question 31

Which gene on chromosome 22 is suspected to be responsible for DiGeorge syndrome features?

Accepted Answer

TBX1 — The transcription factor gene TBX1 in the 22q11.2 region is suspected to be responsible for DiGeorge syndrome features, as its loss correlates with syndrome occurrence. ---

Question 32

Patients with 22q11.2 deletion are at particularly high risk for

Accepted Answer

Psychoses such as schizophrenia and bipolar disorder — In addition to malformations, patients with 22q11.2 deletion are at particularly high risk for developing psychoses such as schizophrenia and bipolar disorder. ---

Question 33

The diagnosis of 22q11.2 deletion syndrome is established by

Accepted Answer

Fluorescence in situ hybridization (FISH) — While diagnosis may be suspected clinically, it can only be established by detection of the deletion using fluorescence in situ hybridization (FISH). ---

Question 34

Clinical features of 22q11.2 deletion syndrome include all EXCEPT

Accepted Answer

Hepatic involvement is not characteristic. — Features include cardiac defects, palate abnormalities, facial dysmorphism, developmental delay, thymic hypoplasia, and parathyroid hypoplasia. Hepatic involvement is not characteristic. ---

Question 35

When T cell immunodeficiency and hypocalcemia dominate in 22q11.2 deletion, the patient is said to have

Accepted Answer

Velocardiofacial syndrome — When T cell immunodeficiency and hypocalcemia are dominant features, patients are said to have DiGeorge syndrome, while velocardiofacial syndrome has pronounced dysmorphology and cardiac defects. ---

Question 36

According to the common disease-common variant hypothesis, complex disorders occur when

Accepted Answer

Many polymorphisms with modest effects are co-inherited — Complex multigenic disorders occur when many polymorphisms, each with modest effect and low penetrance, are co-inherited, according to the common disease-common variant hypothesis. ---

Question 37

A ring chromosome forms when

Accepted Answer

A ring chromosome — A ring chromosome is a variant of deletion where, after loss of segments from each end of the chromosome, the remaining arms unite to form a ring structure. ---

Question 38

Deletion of which chromosome's short arm causes cri du chat syndrome?

Accepted Answer

Chromosome 5 — Cri du chat syndrome results from partial deletion of the short arm of chromosome 5 (5p-), named for the characteristic cat-like cry of affected infants. ---

Question 39

An inversion occurs when

Accepted Answer

Two interstitial breaks occur and segment reunites after turnaround — Inversions occur when there are two interstitial breaks in a chromosome and the segment between them reunites after rotating 180 degrees (complete turnaround). ---

Question 40

A balanced reciprocal translocation involves

Accepted Answer

Loss of genetic material — Balanced reciprocal translocations involve exchange of chromosomal material between two chromosomes with no net gain or loss of genetic material, potentially causing problems during meiosis. ---

Question 41

Which best describes complex multigenic disorders?

Accepted Answer

disorders are caused by interactions between variant forms of genes and environmental factors. — Complex multigenic (or multifactorial/polygenic) disorders are caused by interactions between variant forms of genes and environmental factors. ---

Question 42

Anaphase lag can lead to aneuploidy through

Accepted Answer

Failure of pairing followed by random assortment — Failure of pairing of homologous chromosomes followed by random assortment (anaphase lag) can lead to aneuploidy, in addition to nondisjunction. ---

Question 43

Mosaicism affecting which chromosomes is most common?

Accepted Answer

Sex chromosomes — Mosaicism affecting sex chromosomes is common, whereas autosomal mosaicism is relatively uncommon. ---

Question 44

In most chromosomal disorders, the parents are

Accepted Answer

Normal with de novo changes in offspring — In most cases, chromosomal disorders result from de novo changes (parents are normal and risk of recurrence in siblings is low), with translocation Down syndrome being an important exception. ---

Question 45

Fertilization of a gamete with an extra chromosome (n+1) by a normal gamete produces

Accepted Answer

Trisomic zygote (2n+1) — Fertilization of a gamete containing an extra chromosome (n+1) by a normal gamete (n) results in a trisomic zygote with 2n+1 chromosomes. ---

Question 46

Polyploidy (3n, 4n) generally results in

Accepted Answer

Spontaneous abortion — Polyploidy, where chromosome numbers are 3n or 4n (exact multiples of the haploid number other than 2n), generally results in spontaneous abortion. ---

Question 47

Which THREE autosomal trisomies were among the first chromosomal abnormalities identified?

Accepted Answer

13, 18, and 21 — Trisomies 21, 18, and 13 were among the first chromosomal abnormalities identified, along with cri du chat syndrome (5p deletion). ---

Question 48

The incidence of Down syndrome in women younger than 20 years is approximately

Accepted Answer

1 in 1550 live births — Down syndrome occurs in approximately 1 in 1550 live births in women younger than 20 years, increasing dramatically to 1 in 25 in women older than 45 years. ---

Question 49

The immune dysfunction in Down syndrome primarily affects

Accepted Answer

Mainly T cell function — Patients with Down syndrome demonstrate abnormal immune responses with abnormalities affecting mainly T cell functions, predisposing to infections and thyroid autoimmunity. ---

Question 50

In the translocation form of Down syndrome, the translocated chromosome is most commonly inherited from

Accepted Answer

The translocation form — The translocation form is frequently (but not always) familial, with the translocated chromosome inherited from a parent who is typically a carrier of a robertsonian translocation. ---

Genetic Disorders MCQs Examination (Part 2)

Questions, Answers & Explanations