Weekly Year 2: Molecular Genetics and Cytogenetics Exam -...

Question 1

What is the second most common genetic cause of mental retardation after Down syndrome?

Accepted Answer

Fragile X syndrome — Fragile X syndrome is the second most common genetic cause of mental retardation after Down syndrome, with a frequency of 1 in 1550 for affected males and 1 in 8000 for affected females.

Question 2

What is the normal number of CGG repeats in the FMR1 gene?

Accepted Answer

5-44 — The normal number of CGG repeats in the FMR1 gene is typically between 5 and 44.

Question 3

What defines a premutation in fragile X syndrome?

Accepted Answer

45-200 CGG repeats — A premutation in fragile X syndrome is characterized by the presence of 45-200 CGG repeats in the FMR1 gene.

Question 4

During which process can premutations be converted to full mutations in fragile X syndrome?

Accepted Answer

Meiosis — Premutations can be converted to full mutations during meiosis, particularly in the female germline, due to the expansion of CGG repeats.

Question 5

What is the only distinctive physical abnormality detected in at least 90% of postpubertal males with fragile X syndrome?

Accepted Answer

Macroorchidism — Macroorchidism (enlarged testes) is the only distinctive physical abnormality detected in at least 90% of postpubertal males with fragile X syndrome.

Question 6

What phenomenon describes the worsening of clinical features with each successive generation in fragile X syndrome?

Accepted Answer

Anticipation — Anticipation is the phenomenon where clinical features of a genetic disorder worsen or appear earlier in successive generations, often due to expansion of repeat sequences like in Fragile X syndrome.

Question 7

What percentage of carrier females with fragile X are affected (mentally retarded)?

Accepted Answer

30-40% — Approximately 30-40% of carrier females with fragile X syndrome are affected with mental retardation, though severity can vary.

Question 8

What is the molecular basis for fragile X syndrome?

Accepted Answer

Trinucleotide repeat expansion in the FMR1 gene — Fragile X syndrome is caused by an expansion of CGG trinucleotide repeats in the FMR1 gene, leading to its silencing and lack of functional protein.

Question 9

What is the function of FMRP (Familial Mental Retardation Protein)?

Accepted Answer

RNA binding and transport — FMRP is an RNA-binding protein that plays a crucial role in RNA transport and local protein synthesis, particularly in neuronal development.

Question 10

What condition affects approximately 30% of females carrying the fragile X premutation?

Accepted Answer

Fragile X-associated primary ovarian insufficiency (FXPOI) — Approximately 30% of females carrying the fragile X premutation develop Fragile X-associated primary ovarian insufficiency (FXPOI).

Question 11

What syndrome develops in approximately one-third of premutation-carrying males in their sixth decade?

Accepted Answer

Alzheimer disease — Fragile X-associated tremor/ataxia syndrome (FXTAS) develops in approximately one-third of premutation-carrying males in their sixth decade.

Question 12

In Huntington disease, when does conversion from premutation to full mutation occur?

Accepted Answer

During spermatogenesis — In Huntington disease, the conversion from a premutation to a full mutation typically occurs during spermatogenesis (male gamete formation).

Question 13

Two pure breeding parents are crossed similar to Mendel's Parental generation. A tall plant is crossed with a short plant. What is the expected outcome for the F1 generation?

Accepted Answer

All tall plants — If a pure breeding tall plant (homozygous dominant) is crossed with a pure breeding short plant (homozygous recessive), all offspring in the F1 generation will be heterozygous and express the dominant tall phenotype.

Question 14

If the cross from #1 is continued, what would be the expected outcome in the F2 generation?

Accepted Answer

75% tall and 25% short plants — When F1 heterozygous plants are crossed, the F2 generation will exhibit a 3:1 phenotypic ratio (75% tall, 25% short) and a 1:2:1 genotypic ratio (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive).

Question 15

Which of the following is the BEST definition of inbreeding?

Accepted Answer

Crossing of closely related individuals — Inbreeding is defined as the mating of closely related individuals, which increases homozygosity and can expose recessive alleles.

Question 16

Referring to the Punnett square for a cross between BB x bb (B = black rat, b = white rat) and complete dominance, which of the following accurately represents the phenotypic and genotypic ratios of the F1 generation?

Accepted Answer

Genotype: 100% Bb; Phenotype: 100% black — A cross between BB (homozygous dominant) and bb (homozygous recessive) will produce F1 offspring that are all Bb (heterozygous). With complete dominance of B over b, all Bb individuals will exhibit the black phenotype.

Question 17

Using the information in the Punnett square above (BB x bb), how would we refer to the parents and the offspring?

Accepted Answer

Parents: P generation; Offspring: F1 generation — The initial parents in a cross are referred to as the P (parental) generation, and their immediate offspring are the F1 (first filial) generation.

Question 18

If we were to cross the offspring in the Punnett square above (known as the F1 generation), what will be the genotypic and phenotypic ratios of the F2 generation?

Accepted Answer

Genotype: 1 BB : 2 Bb : 1 bb; Phenotype: 3 black : 1 white — Crossing F1 heterozygotes (Bb x Bb) results in an F2 generation with a genotypic ratio of 1 BB : 2 Bb : 1 bb and a phenotypic ratio of 3 black : 1 white due to complete dominance.

Question 19

Recombination frequency of < 0.5 suggests

Accepted Answer

Genes are linked and close together — A recombination frequency of less than 0.5 (or 50%) indicates that genes are linked on the same chromosome and are located relatively close to each other.

Question 20

Use the information and the Punnett square below to respond to the next three questions: Hemophilia is an X linked recessive disorder. What are the chances that this couple will have a child with hemophilia?

Accepted Answer

50% — Let's assume the father is unaffected (X^H Y) and the mother is a carrier (X^H X^h). The Punnett square would show a 25% chance of an affected son (X^h Y), a 25% chance of an unaffected son (X^H Y), a 25% chance of a carrier daughter (X^H X^h), and a 25% chance of an unaffected daughter (X^H X^H).

Question 21

The mother undergoes amniocentesis during her pregnancy, and is told that the child she is carrying is a boy. What are the chances that her son will have hemophilia?

Accepted Answer

100% — Given the mother is a carrier (X^H X^h) and the child is a boy (XY), there is a 50% chance he will inherit the X^h chromosome from his mother, resulting in hemophilia (X^h Y).

Question 22

The mother has a second pregnancy four years later. She is told that her baby will be a girl. What are the chances that her daughter will have hemophilia?

Accepted Answer

25% — Hemophilia is X-linked recessive. A female offspring would need to inherit two copies of the recessive allele (X^h X^h) to be affected. Since the father is assumed to be unaffected (X^H Y), he can only pass on an X^H allele. Therefore, a daughter will at best be a carrier (X^H X^h), not affected with hemophilia.

Question 23

The principle that states that alleles separate during gamete formation is the

Accepted Answer

Law of Segregation — The Law of Segregation states that during gamete formation, the alleles for each gene separate from each other so that each gamete carries only one allele for each gene.

Question 24

If you crossed two heterozygous plants, how many of the offspring will also be heterozygous?

Accepted Answer

1/2 — When crossing two heterozygous individuals (e.g., Aa x Aa), the genotypic ratio of the offspring is 1 AA : 2 Aa : 1 aa. Therefore, 1/2 of the offspring will be heterozygous (Aa).

Question 25

What is the minimum population frequency required for a genetic variant to be classified as a polymorphism?

Accepted Answer

10% — 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 26

In complex multigenic disorders, what percentage of type 1 diabetes risk is contributed by a few HLA alleles?

Accepted Answer

30-50% — In type 1 diabetes, a few HLA alleles contribute significantly to the genetic risk, estimated to be around 30-50% of the overall genetic predisposition.

Question 27

Which environmental factor is most commonly associated with unmasking the genetic trait in type 2 diabetes mellitus?

Accepted Answer

Diet and lifestyle (obesity, inactivity) — Diet and lifestyle factors, including obesity and physical inactivity, are the most significant environmental contributors that unmask the genetic predisposition to type 2 diabetes mellitus.

Question 28

What proportion of newborn infants has some form of chromosomal abnormality?

Accepted Answer

1 in 200 — Approximately 1 in 200 newborn infants has some form of chromosomal abnormality, with about half of these being clinically significant.

Question 29

What percentage of first-trimester spontaneous abortions are associated with chromosomal abnormalities?

Accepted Answer

40-50% — Approximately 40-50% of first-trimester spontaneous abortions are associated with chromosomal abnormalities, making them a significant cause of early pregnancy loss.

Question 30

Which staining technique is widely used to produce distinctive banding patterns on chromosomes?

Accepted Answer

Giemsa staining (G-banding) — Giemsa staining, specifically G-banding (Giemsa banding), is the most common and widely used technique for producing distinctive banding patterns on chromosomes, allowing for their identification and analysis.

Question 31

A chromosome number of 3n is classified as

Accepted Answer

Triploidy — A chromosome number of 3n (three complete sets of chromosomes) is classified as triploidy, a type of polyploidy.

Question 32

What is the primary cause of aneuploidy?

Accepted Answer

Nondisjunction during meiosis or mitosis — The primary cause of aneuploidy (an abnormal number of chromosomes) is nondisjunction, which is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis.

Question 33

What is the chromosome count in a trisomic zygote?

Accepted Answer

2n - 1 — A trisomic zygote has an extra copy of one chromosome, resulting in a total of 2n + 1 chromosomes.

Question 34

Which type of chromosomal abnormality is better tolerated: autosomal or sex chromosomal?

Accepted Answer

Both are equally tolerated — Sex chromosomal abnormalities are generally better tolerated than autosomal abnormalities, partly due to X-inactivation in females and the smaller number of genes on the Y chromosome.

Question 35

What does mosaicism refer to in cytogenetics?

Accepted Answer

Having a mixture of cells with different chromosome numbers or structures within the same individual — Mosaicism refers to the presence of two or more distinct cell populations with different genotypes or chromosome complements within a single individual, originating from a single zygote.

Question 36

In cytogenetic notation, what does "p" represent?

Accepted Answer

The short arm of a chromosome — In cytogenetic notation, 'p' refers to the short arm of a chromosome (from the Latin 'petit', meaning small).

Question 37

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

Accepted Answer

Polymorphism — A genetic variant with at least two alleles occurring in at least 1% of the population is defined as a polymorphism.

Question 38

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

Accepted Answer

10-15 — While many genes contribute to type 1 diabetes risk, a specific set of about 10-15 genes, particularly within the HLA complex, contribute most significantly to the disease susceptibility.

Question 39

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

Accepted Answer

Obesity and sedentary lifestyle — Obesity and a sedentary lifestyle are the most significant environmental factors that unmask the genetic predisposition to type 2 diabetes mellitus.

Question 40

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

Accepted Answer

40-50% — Chromosomal abnormalities are a major cause of miscarriage, accounting for approximately 40-50% of first-trimester spontaneous abortions.

Question 41

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

Accepted Answer

Giemsa stain — The G-banding technique uses Giemsa stain, after treatment with trypsin, to visualize the characteristic banding patterns on chromosomes.

Question 42

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

Accepted Answer

Triploidy — A chromosome number of 69, which represents three complete sets of chromosomes (3n), is classified as triploidy.

Question 43

The chief cause of aneuploidy is

Accepted Answer

Nondisjunction — Nondisjunction, the failure of homologous chromosomes or sister chromatids to separate properly during cell division, is the chief cause of aneuploidy.

Question 44

Monosomy involving an autosome is

Accepted Answer

Rarely viable — Monosomy involving an autosome (lack of one chromosome from a pair) is typically not viable and results in miscarriage. The only common viable autosomal monosomy is not seen.

Question 45

Mosaicism is defined as

Accepted Answer

Having a mixture of cells with different chromosome complements within one individual — Mosaicism is the presence of two or more cell lines with different chromosome numbers or structures within a single individual that originated from a single zygote.

Question 46

In cytogenetic notation, "p" denotes

Accepted Answer

The short arm of a chromosome — In cytogenetic notation, 'p' signifies the short arm of a chromosome (from the French 'petit' for small).

Question 47

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

Accepted Answer

A female with a translocation between the long arm of chromosome 2 and the short arm of chromosome 5 — 46,XX indicates a female with 46 chromosomes. t(2;5)(q31;p14) indicates a reciprocal translocation involving chromosome 2 (at band q31) and chromosome 5 (at band p14). The q arm is the long arm and the p arm is the short arm.

Question 48

Robertsonian translocation typically involves

Accepted Answer

The fusion of two acrocentric chromosomes at their centromeres — Robertsonian translocations occur between two acrocentric chromosomes (chromosomes with centromeres near the ends) where their long arms fuse at the centromeric region, with the loss of the short arms.

Question 49

What is the lifetime frequency of genetic disease per 1000 individuals?

Accepted Answer

670 — The lifetime frequency of genetic disease is estimated to be 670 per 1000 individuals, meaning a significant portion of the population will be affected by some form of genetic condition.

Question 50

Approximately how many genes do human beings have?

Accepted Answer

20,000 — The current estimate for the number of protein-coding genes in the human genome is around 20,000 to 25,000.

Weekly Year 2: Molecular Genetics and Cytogenetics Exam -...

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