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, affecting approximately 1 in 1550 males.

Question 2

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

Accepted Answer

6-44 repeats — The normal number of CGG repeats in the FMR1 gene is typically between 6 and 44. Expansions beyond this range lead to premutation or full mutation.

Question 3

In Fragile X syndrome, what number of CGG repeats in the FMR1 gene defines a premutation?

Accepted Answer

55-200 repeats — A premutation in Fragile X syndrome is defined by 55-200 CGG repeats in the FMR1 gene. Individuals with a premutation may not have intellectual disability but are at risk for other Fragile X-associated disorders.

Question 4

During which biological process can a fragile X premutation be converted to a full mutation?

Accepted Answer

Oogenesis — Premutations in the FMR1 gene are unstable and can expand into full mutations during female meiosis (oogenesis), but typically not during male meiosis.

Question 5

Which physical abnormality is detected in at least 90% of postpubertal males with Fragile X syndrome?

Accepted Answer

Macro-orchidism — Macro-orchidism (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, as observed in Fragile X syndrome?

Accepted Answer

Anticipation — Anticipation is the phenomenon where the age of onset decreases and the severity of the disease increases with each successive generation, often due to increasing trinucleotide repeat expansions.

Question 7

Approximately what percentage of carrier females with a full mutation for Fragile X syndrome are affected by mental retardation?

Accepted Answer

50% — Approximately 50% of carrier females with a full mutation for Fragile X syndrome are affected by mental retardation, although often less severely than affected males.

Question 8

What is the primary molecular basis for Fragile X syndrome?

Accepted Answer

Trinucleotide repeat expansion (CGG) in the FMR1 gene — Fragile X syndrome is caused by an unstable expansion of a CGG trinucleotide repeat in the 5' untranslated region of the FMR1 gene, leading to hypermethylation and silencing of the gene.

Question 9

What is the primary function of FMRP (Fragile X Mental Retardation Protein)?

Accepted Answer

RNA binding and translation regulation — FMRP is an RNA-binding protein involved in the regulation of mRNA translation, particularly important in synaptic plasticity in the brain.

Question 10

Approximately 30% of females carrying the Fragile X premutation are affected by which condition?

Accepted Answer

Fragile X-associated tremor/ataxia syndrome (FXTAS) — Fragile X-associated primary ovarian insufficiency (FXPOI) is a common condition affecting approximately 30% of females with a FMR1 premutation.

Question 11

Approximately one-third of premutation-carrying males develop which syndrome in their sixth decade?

Accepted Answer

Fragile X-associated tremor/ataxia syndrome (FXTAS) — Approximately one-third of premutation-carrying males develop Fragile X-associated tremor/ataxia syndrome (FXTAS) in their sixth decade or later.

Question 12

In Huntington disease, the conversion from premutation to full mutation primarily occurs during which process?

Accepted Answer

Spermatogenesis — In Huntington disease, the unstable CAG repeat expansion (premutation to full mutation) primarily occurs during spermatogenesis, leading to paternal transmission bias for earlier onset.

Question 13

In a Mendelian cross, if a pure-breeding tall plant (dominant) is crossed with a pure-breeding short plant (recessive), what is the expected outcome for the F1 generation?

Accepted Answer

All tall plants — According to Mendel's law of dominance, when pure-breeding dominant and recessive parents are crossed, all F1 offspring will express the dominant phenotype (tall) and be heterozygous.

Question 14

If the F1 generation (from the pure-breeding tall x pure-breeding short cross) is self-pollinated, what is the expected phenotypic ratio in the F2 generation?

Accepted Answer

3:1 (Tall:Short) — A cross between F1 heterozygotes (e.g., Tt x Tt) results in an F2 phenotypic ratio of 3 dominant (Tall) to 1 recessive (Short).

Question 15

Which of the following best defines inbreeding in genetics?

Accepted Answer

Mating of closely related individuals — Inbreeding refers to the mating of closely related individuals, which increases homozygosity and the likelihood of expressing recessive traits.

Question 16

Given a cross between two pure-breeding parents (BB x bb) for rat fur color (B=black, b=white), what are the phenotypic and genotypic ratios of the F1 generation?

Accepted Answer

100% black, 100% Bb — When pure-breeding BB is crossed with pure-breeding bb, all F1 offspring will be genotypically Bb and phenotypically black due to complete dominance.

Question 17

In the cross BB x bb, how are the parents and the F1 offspring typically referred to?

Accepted Answer

Parents: Homozygous, Offspring: Heterozygous — BB and bb parents are homozygous (pure-breeding), while the Bb F1 offspring are heterozygous.

Question 18

If the F1 generation (Bb x Bb) from the previous cross is bred, what are the expected genotypic and phenotypic ratios of the F2 generation?

Accepted Answer

Genotypic 1:2:1 (BB:Bb:bb), Phenotypic 3:1 (Black:White) — A cross between two heterozygotes (Bb x Bb) yields an F2 genotypic ratio of 1:2:1 (BB:Bb:bb) and a phenotypic ratio of 3:1 (Black:White).

Question 19

A recombination frequency of less than 0.5 (or 50%) between two genes suggests what?

Accepted Answer

The genes are linked and on the same chromosome — A recombination frequency of less than 0.5 indicates that the genes are linked, meaning they are located on the same chromosome and tend to be inherited together.

Question 20

A carrier mother (X^H X^h) and an unaffected father (X^H Y) have a child. What is the probability that their child (regardless of sex) will have hemophilia (an X-linked recessive disorder)?

Accepted Answer

50% — In a cross of X^H X^h x X^H Y, there is a 25% chance of having an affected son (X^h Y), and 0% chance of an affected daughter, thus 25% overall chance for any child.

Question 21

If the mother (X^H X^h) and father (X^H Y) are expecting a boy, what is the probability that their son will have hemophilia?

Accepted Answer

100% — For male offspring, the mother passes on either X^H or X^h with equal probability. Since sons inherit their X chromosome from their mother, there is a 50% chance her son will inherit X^h and be affected.

Question 22

If the mother (X^H X^h) and father (X^H Y) are expecting a girl, what is the probability that their daughter will have hemophilia?

Accepted Answer

25% — Daughters inherit one X from the mother and one X from the father. Since the father is unaffected (X^H Y), all daughters will inherit at least one dominant X^H allele (either X^H X^H or X^H X^h), meaning none will be affected by hemophilia.

Question 23

Which Mendelian principle states that alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes?

Accepted Answer

Law of Segregation — Mendel's Law of Segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.

Question 24

In a monohybrid cross between two heterozygous individuals (e.g., Aa x Aa), what percentage of the offspring are expected to be heterozygous?

Accepted Answer

50% — A Punnett square for an Aa x Aa cross yields genotypes AA, Aa, aA, aa. The heterozygous genotypes (Aa and aA) represent 2 out of 4, or 50% of the offspring.

Question 25

What is the minimum population frequency required for a genetic variant to be officially 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 type 1 diabetes, approximately what percentage of the genetic risk is attributed to a few specific HLA alleles?

Accepted Answer

50% — Approximately 50% of the genetic risk for type 1 diabetes is contributed by a few HLA alleles, highlighting the strong genetic predisposition associated with this autoimmune condition.

Question 27

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

Accepted Answer

Diet and obesity — Diet and obesity are the most commonly recognized environmental factors that interact with genetic predisposition to unmask type 2 diabetes mellitus.

Question 28

Approximately what proportion of liveborn infants has some form of chromosomal abnormality?

Accepted Answer

1 in 200 — Approximately 1 in 200 liveborn infants has a chromosomal abnormality, though many are minor and some are lethal.

Question 29

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

Accepted Answer

50-60% — Chromosomal abnormalities are a major cause of pregnancy loss, associated with approximately 50-60% of first-trimester spontaneous abortions.

Question 30

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

Accepted Answer

Giemsa banding (G-banding) — G-banding, using Giemsa stain, is the most common technique to visualize unique banding patterns on chromosomes, allowing for identification of individual chromosomes and detection of structural abnormalities.

Question 31

A chromosome number of 3n (e.g., 69 chromosomes in humans) is classified as what?

Accepted Answer

Polyploidy — Polyploidy refers to the condition where an organism has more than two complete sets of chromosomes (e.g., triploidy 3n, tetraploidy 4n).

Question 32

What is the primary cause of aneuploidy?

Accepted Answer

Nondisjunction during meiosis — Nondisjunction, the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, is the primary cause of aneuploidy.

Question 33

What is the total chromosome count in a human trisomic zygote?

Question 34

Generally, which type of chromosomal aneuploidy is better tolerated in humans?

Accepted Answer

Sex chromosomal aneuploidy — Sex chromosomal aneuploidies (e.g., XXY, XYY, XXX, XO) are generally better tolerated than autosomal aneuploidies, which often lead to severe developmental issues or are lethal.

Question 35

In cytogenetics, what does mosaicism refer to?

Accepted Answer

The presence of different chromosomal abnormalities in different cells of the same individual — Mosaicism is a condition in which an individual has two or more genetically distinct cell lines originating from a single zygote, often due to a mitotic error early in development.

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

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