BLock 7 Linkage Analysis MCQ's

2.

What does a LOD score of 3.5 mean?

The disease and the marker are not linked
The result is uninformative
The DNA typing has had problems
The result is promising, but more members of the pedigree must be typed to get a meaningful result
The odds are better than 1000:1 that the disease is linked to this marker

Correct Answer

A. The odds are better than 1000:1 that the disease is linked to this marker

Explanation

A LOD score of 3.5 indicates that the odds are better than 1000:1 that the disease is linked to the marker. This means that there is a strong likelihood of a genetic association between the disease and the marker based on the statistical analysis. It suggests that further investigation and testing should be done to confirm this finding and gather more evidence.

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3.

If two loci show genetic linkage then

They are not always on the same chromosome
Mutations at both loci cause the same disease
Alleles at the same loci interact to cause the same disease
If recombination fraction between loci is 0.05 then this means that on average alleles in coupling at these loci will cosegregate in 95% of meioses
Alleles at these loci must show linkage disequilibrium

Correct Answer

A. If recombination fraction between loci is 0.05 then this means that on average alleles in coupling at these loci will cosegregate in 95% of meioses

Explanation

If the recombination fraction between loci is 0.05, it means that there is a 5% chance of recombination occurring between the alleles at these loci during meiosis. This implies that 95% of the time, the alleles will remain together and cosegregate. This supports the idea of genetic linkage, as alleles that are physically close to each other on the same chromosome are more likely to be inherited together. Therefore, the given statement is a valid explanation for the correct answer.

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4.

Osteogenesis imperfecta is a rare autosomal dominant. 1 and 2 represent two haplotypes of an RFLP marker that is genetically linked to the gene causing Osteogenesis imperfecta. Which individual is most likely to be an example of recombination?

Individual A
Individual B
Individual C
Individual D
Individual E

Correct Answer

A. Individual D

Explanation

Individual D is most likely to be an example of recombination because it carries one haplotype from one parent (1) and the other haplotype from the other parent (2). This suggests that a recombination event has occurred, resulting in the mixing of genetic material from both parents. The presence of both haplotypes in Individual D indicates that genetic recombination has taken place during meiosis.

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5.

Consider the table of LOD scores shown below for an autosomal dominant disease. The first line gives recombination frequencies, and each subsequent line represents the LOD score pattern found in different families typed with the same marker. Which statement best describes this pattern?

Tight linkage
Allelic heterogeneity
Loose linkage
Linkage disequilibrium
Locus heterogeneity

Correct Answer

A. Locus heterogeneity

Explanation

The given LOD score pattern found in different families typed with the same marker indicates locus heterogeneity. This means that the disease is caused by mutations in different genes or loci. The LOD scores vary significantly between families, suggesting that the disease is not tightly linked to a specific gene or locus.

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6.

Researchers are trying to find a microsatellite marker that is linked to the gene for acute intermittent porphyria, an autosomal dominant condition known to map to the 11 chromosome. In a study of four large families with this disease, an array of microsatellite markers on chromosome 11 were tested for linkage to the gene, and the LOD scores are displayed in the table below. Using conventional cutoff points for LOD scores, which of the markers show(s) significant linkage with the gene involved in acute intermittent porphyria?

S1
S2
S4
S1 and S4
S1, S2 and S4

Correct Answer

A. S1 and S4

Explanation

Based on the information provided, the researchers tested an array of microsatellite markers on chromosome 11 for linkage to the gene involved in acute intermittent porphyria. The LOD scores for each marker were displayed in a table. LOD scores are used to determine the likelihood of linkage between a marker and a gene, with higher scores indicating stronger evidence for linkage. The question asks which markers show significant linkage. Since the question does not provide specific cutoff points for LOD scores, it can be assumed that conventional cutoff points are used. Therefore, the markers with LOD scores above the conventional cutoff point are considered to show significant linkage. In this case, the LOD scores for S1 and S4 are above the cutoff point, indicating significant linkage with the gene involved in acute intermittent porphyria.

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1

7.

In a full genome linkage study in a large family from the Middle East with an autosomal dominant form of cardiomyopathy (disease of the heart), a marker on chromosome 3 was found to have the following LOD scores (Z) for the following values of 0: Based on this study, choose the BEST answer for the linkage of cariomyopathy in this family:

Cardiomyopathy must be on a gene exactly 5 cM from the chromosome 3 marker
Cardiomyopathy is closely linked to this marker in this family.
This must be a critical gene for cardiomyopathy since the LOD score is so high
Cardiomyopathy could be lined to this marker, but you will need to confirm linkage in another family.
Cardiomyopathy is linked somewhere on chromosome 3.

Correct Answer

A. Cardiomyopathy is closely linked to this marker in this family.

Explanation

The LOD scores indicate the strength of linkage between a marker and a disease gene. In this case, the LOD scores for the marker on chromosome 3 are high, suggesting a strong linkage between the marker and the gene responsible for cardiomyopathy in this family. Therefore, the best answer is that cardiomyopathy is closely linked to this marker in this family.

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8.

Another large family is discovered in China with an autosomal dominant form of Cardiomyopathy (disease of the heart) . The same marker on chromosome 3 was Tested for linkage for this disease and the following LODs cores (Z) for the following

Is not linked to the same chromosome three locus as the Middle Eastern family.
May be l inked to chromosome 3 , but further away from this marker
Shows that the linkage results from Middle Eastern family were misleading
Is also linked in this family, since the combined LOD (Z) scores obtained by Adding these two families are still greater than 3
Is due to genetic drift

Correct Answer

A. Is not linked to the same chromosome three locus as the Middle Eastern family.

Explanation

The given answer suggests that the new family discovered in China does not have the same linkage to the chromosome 3 locus as the Middle Eastern family. This means that the genetic cause of the cardiomyopathy in the Chinese family is not related to the same specific gene or region on chromosome 3 as the Middle Eastern family.

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