Achondroplasia (dwarfism) is caused by an autosomal dominant mutation within the

Question

Achondroplasia (dwarfism) is caused by an autosomal dominant mutation within the fibroblast growth factor receptor-3 (FGFR3) gene on the short arm of chromosome 4. Pooled records from several large hospitals in the United States revealed that among 1,422,000 live births over a 20 year period, the number of babies born with achondroplasia was 31. Among those 31 babies, 18 of them had at least one parent with achondroplasia, and the remaining babies were born to two unaffected parents. What is the spontaneous mutation rate for achondroplasia?

Explanation / Answer

Spontaneous mutation and the associated changes with them refer to those heritable or non-heritable changes which occur due to un-induced mutations arising in an individual on their own. Such mutations are difficult to analyse and predict. Hence, many clinical syndromes arise due to these spontaneous mutations.

According to the information, there are total  1,422,000 samples analyzed in the study. It must be carefully noted that out of total 31 subjects, 18 subjects had parents with previous history of this disease. Since the disease is inherited in autosomal dominant manner, it suggests that the remaining cases i.e. 31-18 = 13 must have been occured only due to spontaneous mutations. Thus, the rate of spontaneous mutations in this population will be:

Rate = Total cases of spontaneous mutations/total population * time in years

Rate = 13/ 1,422,000 * 20

Rate = 0.0000045-1 year

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