CGC Medical Genetics & Genomic Knowledge 2

Question 1

A pedigree shows an affected father who has no affected sons but all affected daughters. Which inheritance pattern is most likely?

Accepted Answer

X-linked dominant

Answer

Fathers pass their only X to all daughters and none to sons, so absent male-to-male transmission with all daughters affected indicates X-linked dominant inheritance.

Question 2

Which concept best explains the variable severity of mitochondrial disease among maternally related family members?

Accepted Answer

Heteroplasmy

Answer

Heteroplasmy is the mix of mutant and normal mtDNA, and the proportion varies between cells and individuals, producing variable severity.

Question 3

Approximately what percentage of individuals with Down syndrome have free trisomy 21 due to meiotic nondisjunction?

Accepted Answer

95%

Answer

About 95% of Down syndrome cases are free trisomy 21, with the remainder due to translocations and mosaicism.

Question 4

What is the most common molecular mechanism underlying Prader-Willi syndrome?

Accepted Answer

Paternal 15q11-q13 deletion

Answer

About 70% of Prader-Willi cases result from a de novo deletion of the paternally inherited 15q11-q13 region.

Question 5

What CGG repeat size in FMR1 defines a full mutation associated with fragile X syndrome?

Accepted Answer

More than 200 repeats

Answer

More than 200 CGG repeats typically leads to FMR1 hypermethylation and gene silencing, causing fragile X syndrome.

CGC - Certified Genetic Counselor Practice Test

CGC - Certified Genetic Counselor Practice Test

CGC Medical Genetics & Genomic Knowledge 2