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Cystic fibrosis, which is usually lethal before the age of reproduction, is a homozygous recessive trait. Why do cases continue to arise, even though people with the disease rarely live to reproduce?
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13. Mendelian Genetics
Autosomal Inheritance
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When two average-height parents give birth to a child exhibiting achondroplasia, it is most likely due to a new mutation. This is because __________.
A
the frequency of achondroplasia is unknown
B
achondroplasia is a relatively rare disorder
C
such mutations are statistically predictable
D
None of the listed responses is correct.
E
achondroplasia is caused by an allele that is always expressed; therefore, the parents must not have the allele
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Understand that achondroplasia is a genetic disorder caused by a mutation in the FGFR3 gene, which affects bone growth.
Recognize that achondroplasia is an autosomal dominant disorder, meaning only one copy of the mutated gene is needed for the disorder to be expressed.
Since both parents are of average height, it is unlikely they carry the dominant allele for achondroplasia, as it would be expressed if they did.
Consider that the occurrence of achondroplasia in a child with average-height parents is likely due to a new mutation in the FGFR3 gene during gamete formation or early embryonic development.
Conclude that the rarity of achondroplasia and the fact that it is caused by a dominant allele that is always expressed supports the explanation that a new mutation is responsible for the child's condition.
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