IMAT - International Medical Admissions Molecular Genetics and Inheritance Questions and Answers — Questions and Answers
Question 1: A scientist analyzing a sample of DNA finds that it contains 28% adenine. Based on Chargaff's rules, what percentage of guanine should be present in this sample?
- 28%
- 44%
- 22% (Correct answer)
- 56%
Correct answer: 22%
According to Chargaff's rules, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of guanine (G) is equal to the amount of cytosine (C). If adenine is 28%, then thymine is also 28%, making the total for A+T = 56%. The remaining percentage for G+C is 100% - 56% = 44%. Since G equals C, the percentage of guanine is 44% / 2 = 22%.
Question 2: In a dihybrid cross between two heterozygous individuals (AaBb), what is the expected phenotypic ratio of the offspring if the genes assort independently?
- 1:2:1
- 3:1
- 1:1:1:1
- 9:3:3:1 (Correct answer)
Correct answer: 9:3:3:1
This is a classic Mendelian dihybrid cross. When two individuals heterozygous for two independently assorting traits are crossed, the offspring exhibit a phenotypic ratio of 9:3:3:1. This ratio represents the four possible combinations of the two traits: 9/16 will show both dominant phenotypes, 3/16 will show the first dominant and second recessive phenotype, 3/16 will show the first recessive and second dominant phenotype, and 1/16 will show both recessive phenotypes.
Question 3: A frameshift mutation is caused by which of the following events?
- A substitution of one nucleotide for another.
- The insertion or deletion of one or more nucleotides. (Correct answer)
- A mutation that changes an amino acid codon to a stop codon.
- A mutation in a non-coding region of the DNA.
Correct answer: The insertion or deletion of one or more nucleotides.
Frameshift mutations occur when nucleotides are inserted or deleted from the DNA sequence in a number that is not a multiple of three. This alters the reading frame of the codons during translation, leading to a completely different and usually non-functional protein from the point of the mutation onwards.
Question 4: In the lac operon of E. coli, what is the role of allolactose?
- It acts as a co-repressor, activating the repressor protein.
- It binds to the operator, preventing transcription.
- It functions as an inducer, inactivating the repressor protein. (Correct answer)
- It binds to RNA polymerase, enhancing transcription.
Correct answer: It functions as an inducer, inactivating the repressor protein.
The lac operon is an inducible operon. In the presence of lactose, some of it is converted to allolactose. Allolactose acts as an inducer by binding to the repressor protein, causing a conformational change that prevents the repressor from binding to the operator. This allows RNA polymerase to transcribe the structural genes needed for lactose metabolism.
Question 5: A man with type A blood and a woman with type B blood have a child with type O blood. What is the probability that their next child will have type AB blood?
- 0%
- 25% (Correct answer)
- 50%
- 75%
Correct answer: 25%
For a child to have type O blood (genotype ii), they must inherit a recessive 'i' allele from each parent. This means the man with type A blood must have the genotype I^A i, and the woman with type B blood must have the genotype I^B i. A Punnett square of this cross (I^A i x I^B i) shows the possible offspring genotypes are I^A I^B (Type AB), I^A i (Type A), I^B i (Type B), and ii (Type O), each with a 25% probability.
Question 6: Which of the following processes occurs during transcription but NOT during DNA replication?
- Formation of a phosphodiester bond
- Unwinding of the DNA double helix
- Use of a polymerase enzyme
- Incorporation of Uracil (Correct answer)
Correct answer: Incorporation of Uracil
Transcription is the synthesis of RNA from a DNA template. RNA contains the nitrogenous base uracil (U) instead of thymine (T), which is found in DNA. During transcription, uracil is incorporated into the growing mRNA strand opposite to adenine in the DNA template. DNA replication involves synthesizing a new DNA strand, where thymine would be incorporated.
IMAT - International Medical Admissions Molecular Genetics and Inheritance Questions and Answers
A scientist analyzing a sample of DNA finds that it contains 28% adenine.
Based on Chargaff's rules, what percentage of guanine should be present in this sample?