A mutation where insertion or deletion of a base pair results in mistranslation of the genetic code beyond that point is called a:
A. This refers to a frame-shift mutation. When a mutation results in misreading of a single triplet while other consecutive triplets are read correctly, it is called an in-frame mutation. If a mutation (usually a deletion or insertion) results misreading of all subsequent codons, then it is termed a frame-shift mutation.
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Which of the following genetic analyses studies the departure from independent segregation?
B. According to the law of segregation, inheritance of one trait occurs independently of another trait, but this is not always the case. This is due to random crossing over during meiosis, which allows the exchange of genetic segments that are significantly longer than a single gene. Such a crossing over results in the recombination of distant genetic loci. When two loci are close together, recombination is very unlikely and they are inherited as a single genetic element at a frequency significantly more than chance. This is called cosegregation, and the loci are said to be linked. Note that an essential condition for linkage is that the two loci must be on the same chromosome, called as syntenic loci; but not all syntenic loci are linked. Study of such linked loci is called linkage analysis. Restriction fragment length polymorphism is a method of genotyping single nucleotide polymorphisms using restriction endonucleases. Fluorescent in situ hybridization (FISH) involves denaturation of DNA on microscope slides and binding of sequence-specific DNA probes to the regions of interest on the exposed DNA strands. It is often used to study areas of deletion in chromosomes.
In genetic linkage studies, the LOD score above which linkage is conventionally thought to be significant is:
D. The distance between two loci can be measured in terms of the frequency with which they undergo recombination. For linked loci the frequency of recombination is less than 50%. Genetic distances are often expressed in centiMorgans (cM). One cM is equal to 1% recombination frequency between two loci, which can occur if nearly one million base pairs separate the two loci. LOD scores (log of odds) are used to estimate the likelihood that an observed recombination frequency is truly due to the loci being linked. It is given by the log of the ratio between the probabilities of the recombination frequency being the observed value (θ) to the expected value of 50% if they are not linked. A LOD of more than 3 indicates a linkage; less than −2 indicates no linkage. The value of θ at which LOD scores are greatest is the most likely estimate of recombination frequency.
In linkage analysis, LOD scores are used. Which of the following best describes the LOD score?
C. LOD scores were first used by Morton in 1955. It is a statistical method to establish linkage disequilibrium. A LOD (or log of odds) score is the common log of the likelihood that the recombination fraction has a certain value, θ, divided by the likelihood that it is 1/2. Conventionally, a LOD of 3, representing an odds of linkage of 1000: 1, is the accepted level for concluding linkage.
Two possible alleles of a gene are A and B. The genotype is distributed as AA, AB, and BB in the population. In a sample of 100 people, if the genotype frequency of AA is 40, AB is 54, and BB is 6, then the frequency of allele B is:
C. Genotype frequency measures the proportion of each genotype, AA or AB or BB, in a population. Gene frequency measures the frequency of each allele at a particular locus in the population. Here, the frequency of allele A is (40 × 2) + 54 = 134% or 1.34. The frequency of B is (6 × 2) + 54 = 66% or 0.66.
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