Genetics
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What this topic is really about
Cystic fibrosis is caused by recessive mutations in the CFTR gene on chromosome 7, so disease manifests only when an individual inherits two defective alleles, one from each parent. It is not X‑linked dominant (which would show sex‑biased patterns), nor autosomal dominant (one mutant allele suffices), and mitochondrial inheritance would be transmitted exclusively through mothers, so those distractors are inconsistent with CF genetics.
Mitochondria are inherited maternally because the egg cell contributes the vast majority of cytoplasm and organelles to the zygote, while sperm mitochondria are typically degraded after fertilization. This makes paternal inheritance (Option C) extremely rare in humans, rather than an equal contribution.
See the mechanism
A silent mutation changes a single nucleotide codon but, due to the redundancy of the genetic code, still codes for the same amino acid. A diagram for this topic isn't available yet — the worked example below walks the same reasoning step by step.
An exam-style question, fully explained
A point mutation that does not change the encoded amino acid is termed:
- Identify what the question tests: A point mutation that does not change the encoded amino acid is termed:.
- A silent mutation changes a single nucleotide codon but, due to the redundancy of the genetic code, still codes for the same amino acid.
- This differs from a missense mutation, which alters the codon to specify a different amino acid and can change the protein's function.
Traps the examiner sets
- The option of 1/2 is incorrect because that represents the probability of obtaining heterozygous offspring.
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