Genetics

Question 1

Melanie has 2 brothers affected with DMD. Melanie’s daughter is pregnant. What is the risk to melanie’s grandchild of developing DMD?
A. 1 in 2
B. 1 in 4
C. 1 in 16
D. 1 in 32
E. 1 in 64

Answer 1

C
Assume melanie’s mum is carrier (1), melanie is 1/2, melanic daughter is 1/4
1/2 (Probability of being a son) X 1/8 (probability if inheriting the allele) = 1/16

Question 2

The carrier frequency for CF in the Caucasian population in Aus is ~4%. The F508 mutation accounts for 75% of all mutations among carriers in this population. A girl is diagnosed with CF. DNA studies identify 1 F508 mutation but fail to identify a mutation in the other allele. DNA studies of her unaffected brother are normal. What is the risk of the brother being a carrier
<1%, 4%, 25%, 50%, 66%

Answer 2

50%
xY, xX (dad and mum)
Four probabilities: YX, Yx, Xx, xx (girl)
Probability of brother being a carrier: YX, Yx (50%

Question 3

In the pedigree shown above, the affected male has a rare autosomal recessive disorder. His niece and nephew have a newborn son (indicated by the arrow). What is the chance that the baby will have the same disorder?
1 in 18, 1 in 32, 1 in 36, 1 in 64, 1 in 128

Answer 3

the affected male’s brother and sister is 2/3 carrier
Niece and nephew: 2/3 X 1/2 = 1/3
Son: 1/3 X 1/3 X 1/4 (square) = 1/36

Question 4

What’s the underlying genetic lesion of Marfan syndrome

Answer 4

Point mutation in a single gene (FBN1 gene)