The allele responsible for Duchenne\'s muscular dystrophy is found on the X-chro
ID: 211458 • Letter: T
Question
The allele responsible for Duchenne's muscular dystrophy is found on the X-chromosome and is dominated by the allele for the normal condition. Why is this disorder far more common among males? Select one: a. Men who possess the allele usually pass it on to a son. b. Duchenne's muscular dystrophy is an X-inked dominant disorder c. Men have only one X chromosome which is always expressed. d. Duchenne's muscular dystrophy is a Y-linked disorder. O Which effect may result from a nonsense mutation? Select one: O a. The genotype is altered, but the phenotype is unchanged O b. The first halr of a protein is synthesized, but the second hair is not c. The phenotype is altered, but the genotype is unchanged O d. The first half of a protein is correct, but the second haif is full of errors.Explanation / Answer
Answer 1.
Duchenne's muscular dystrophy is an X-linked recessive genetic disease. Even if we weren't aware of this information, the line, "disorder is far more common among males", is a giveaway that options a, b, and d, are all wrong since men pass on the Y chromosome to their sons and only males have Y. Apart from this, X-linked dominant diseases would show equal frequency in both genders.
Hence, c. Men have only one X-chromosome which is always expressed - because having only one of the X makes the recessive nature a dominant one as their is no counterpart.
Answer 2.
A nonsense mutation is one in which a codon has been altered to now express a stop codon. Hence the genetic segment under any circumstance will be altered. We cannot say for sure whether this will produce a genotypic or phenotypic change for sure because of multi-gene interactions to produce the overall effect or protein and also because of synergistic pathways that give rise to the same product even if one particular genetic segment or its protein is inactivated.
Hence, b. The first half of a protein is synthesized, but the second half is not - this option is correct while making the assumption that the nonsense mutation has occurred in the middle (the assumption validates the options b. and d. otherwise the entire question becomes baseless)
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