Q: According to the Lyon Hypothesis, what is the formula to calculate the number of Barr bodies in an individual with abnormal sex chromosome complements?

$$\text{Number of Barr bodies} = \text{Number of X chromosomes} - 1$$

Question 1

Which pharmacological agent is commonly used to arrest cells in metaphase for human karyotype preparation, and what is a key visual feature of G-banding?

Accepted Answer

Colchicine; alternating light and dark bands corresponding to regions of DNA density

Question 2

A cytogeneticist is analyzing chromosomes and notes that chromosome 13 is involved in a Robertsonian translocation. Which structural feature is characteristic of chromosome 13, and what other chromosomes share this feature?

Accepted Answer

Acrocentric; chromosomes 14, 15, 21, and 22 are also acrocentric

Question 3

Fluorescence In Situ Hybridization (FISH) is used to detect specific DNA sequences on chromosomes. Compared to traditional G-banded karyotyping, what is a key advantage of FISH?

Accepted Answer

FISH has higher resolution, allowing detection of submicroscopic chromosomal changes

Question 4

In a pedigree, which feature most strongly suggests autosomal recessive inheritance?

Accepted Answer

Affected individuals often have unaffected parents and the trait can skip generations

Question 5

A pedigree shows an affected son with unaffected parents. Which rule allows you to rule out X-linked recessive inheritance?

Accepted Answer

An affected son must have an affected mother if the trait is X-linked recessive

Question 6

Which statement best describes mitochondrial (maternal) inheritance compared to autosomal and X-linked inheritance?

Accepted Answer

Only mothers transmit mitochondrial DNA to all their children, regardless of sex

Question 7

A family shows a genetic disorder where some individuals with the mutant allele do not express the phenotype, while others show varying degrees of severity. Which two genetic concepts explain this observation?

Accepted Answer

Incomplete penetrance and variable expressivity

Question 8

According to the Lyon Hypothesis, what is the formula to calculate the number of Barr bodies in an individual with abnormal sex chromosome complements?

Accepted Answer

$$	ext{Number of Barr bodies} = 	ext{Number of X chromosomes} - 1$$

Question 9

Why does random X-inactivation in female embryonic development result in cellular mosaicism, as seen in the coat coloration of Calico cats?

Accepted Answer

Different cells inactivate different X chromosomes, leading to patches expressing different alleles

Question 10

During meiosis, nondisjunction can occur in either Meiosis I or Meiosis II. Which gametic outcomes are expected if nondisjunction occurs in Meiosis II?

Accepted Answer

Two normal ($$n$$), one with $$n+1$$, and one with $$n-1$$

Question 11

Trisomies 21, 18, and 13 are the only common viable autosomal trisomies in live births. What is the primary molecular mechanism explaining the increased risk of Down syndrome with maternal age?

Accepted Answer

Increased frequency of meiotic nondisjunction in oocytes as maternal age increases

Question 12

A pregnant woman is considering prenatal testing. Which method is non-invasive, can be performed earliest in pregnancy, and carries no risk to the fetus?

Accepted Answer

Non-Invasive Prenatal Testing (NIPT); uses maternal blood to analyze cell-free fetal DNA

Question 13

A woman with a 47,XXY karyotype is analyzed for Barr bodies. According to the Lyon Hypothesis, how many Barr bodies would be present in her somatic cells?

Accepted Answer

$$2 - 1 = 1$$ Barr body

Question 14

A couple has a child with a mitochondrial disorder. The mother is affected, but the father is not. What is the probability that their next child will inherit the disorder?

Accepted Answer

100%, because all children inherit mitochondria from their mother

Question 15

A pedigree shows a trait that appears in every generation and is transmitted from both males and females to their offspring. Which inheritance pattern is most likely?

Accepted Answer

Autosomal dominant

Question 16

A genetic counselor is analyzing a family with a late-onset disorder. Why might this complicate pedigree analysis?

Accepted Answer

Affected individuals may not show symptoms until after reproductive age, making it difficult to identify carriers

Question 17

A karyotype analysis reveals a Robertsonian translocation involving chromosomes 14 and 21. What is a potential clinical consequence of this rearrangement in offspring?

Accepted Answer

Increased risk of Down syndrome due to unbalanced gametes

Question 18

A couple has a child with trisomy 18. Which prenatal diagnostic test would have provided the earliest definitive diagnosis with the lowest risk to the fetus?

Accepted Answer

Non-Invasive Prenatal Testing (NIPT)

Question 19

A cell biologist is studying the outcomes of nondisjunction in Meiosis I. What is the expected distribution of chromosomes in the resulting gametes?

Accepted Answer

Two gametes with $$n+1$$ and two with $$n-1$$

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