Question 1

Which of the following best describes a Quantitative Trait Locus (QTL)?

Accepted Answer

A chromosomal region associated with variation in a quantitative trait.

Question 2

In a QTL mapping experiment with tomatoes, why are purebred parents with extreme phenotypes used as the starting point?

Accepted Answer

To maximize genetic variation in the F1 and subsequent generations for mapping trait-associated loci.

Question 3

What is the primary role of a Single Nucleotide Polymorphism (SNP) in QTL mapping?

Accepted Answer

To serve as a unique genomic marker linked to a region associated with the trait.

Question 4

Which of the following statements correctly compares QTL mapping and Genome-Wide Association Studies (GWAS)?

Accepted Answer

QTL mapping uses controlled crosses to identify trait-associated regions, while GWAS uses natural populations to correlate SNPs with phenotypes.

Question 5

A GWAS identifies a SNP that is statistically associated with a disease. What is the most accurate interpretation of this result?

Accepted Answer

The SNP is a marker that is likely linked to the actual causal variant.

Question 6

In the context of QTL mapping, what is the purpose of creating introgression lines (ILs)?

Accepted Answer

To isolate and study the effect of specific chromosomal regions from one parent in the genetic background of the other.

Question 7

Which of the following best explains why SNPs used in GWAS are often not the causal variants themselves?

Accepted Answer

SNPs are used as markers because they are physically close to the causal variant and inherited together due to linkage disequilibrium.

Question 8

A tomato breeder wants to increase fruit size using QTL mapping. After identifying a QTL associated with larger fruit, what is the next logical step?

Accepted Answer

Select plants with the SNP marker linked to the QTL and breed them to fix the large-fruit allele.

Question 9

Which equation best represents the total phenotypic variance ($$V_P) in a $$quantitative trait?

Accepted Answer

$$V_P = V_G + V_E$$

Question 10

In a GWAS for Crohn’s disease, a significant peak is observed on chromosome 6. What does this indicate?

Accepted Answer

A SNP on chromosome 6 is statistically associated with Crohn’s disease, suggesting a nearby causal variant may influence disease risk.

Question 11

Which of the following scenarios best illustrates additive genetic effects in a quantitative trait?

Accepted Answer

Each gene contributing to the trait increases the phenotype by a fixed amount, and the total effect is the sum of all gene effects.

Question 12

Why is it important to distinguish between a GWAS SNP and a causal variant?

Accepted Answer

Because the GWAS SNP is a marker, while the causal variant is the actual DNA change responsible for the trait.

Question 13

A researcher finds that a specific SNP is present in 80% of individuals with a disease and 20% of healthy controls. What does this suggest?

Accepted Answer

The SNP is statistically associated with the disease and may be linked to a causal variant.

Question 14

Which of the following best describes the process of QTL mapping in plants?

Accepted Answer

Crossing two parents with contrasting phenotypes, generating recombinant offspring, and using genetic markers to identify chromosome regions associated with the trait.

Question 15

Which of the following is NOT a feature of quantitative traits?

Accepted Answer

They are controlled by a single gene with two alleles.

Question 16

If a QTL explains 30% of the variation in tomato fruit weight, what does this imply about the genetic architecture of the trait?

Accepted Answer

Other genes and possibly environmental factors account for the remaining 70% of the variation.

Question 17

Which of the following best describes linkage disequilibrium in the context of GWAS?

Accepted Answer

It is the non-random association of alleles at different loci, allowing SNPs to serve as markers for nearby causal variants.

Question 18

A bell-shaped curve is often observed for quantitative traits in a population. What does this indicate about the genetic basis of the trait?

Accepted Answer

The trait is influenced by multiple genes, each contributing additively to the phenotype.

Quantitative Genetics: Mapping Additive Alleles with QTLs and GWAS

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