Several members of a family exhibit the symptoms of myotonic dystrophy, a condit

Question

Several members of a family exhibit the

symptoms of myotonic dystrophy, a condition

typified by wasting away of muscles and a

variety of other problems. The pedigree of the

family is shown to the right. The gene

responsible for the disease is on chromosome

19 and is preceded by a long untranslated

stretch of repeated GCT nucleotides that are 5’

to the gene. DNA is isolated from the living

members of the pedigree shown and the length

of the 5' region is determined. The numbers of

repeats for each individual are shown on the

pedigree.

a. Why are two numbers shown for each

individual? (1 point)

b. What do you think is the relationship

between the number of repeats and whether

the disease is present at birth (congenital

onset, shown in red)? (1 point)

Famlly with Myotanic Dystrophy 5/13 B/N15/13 6/7514/16 | 13/30 14/14 13/112 13/205 5/160 13/14 16/200 14/2100 13/780

Explanation / Answer

Ans

(a) Pedigree from myotonic dystrophy(ch=16,GCT repeat sequence) family with the proband are indicated by an arrow. The repeated numbers in the pedigree symbols represents repeat length in 4 individuals (male and female) who are genetically tested.

(b) Myotonic dystrophy is inherited autosomal dominant disease which shows that each son or daughter of a person affecting from disease has 50% chance of inheriting the disease. This disease affects males and females equally. The genetic change that causes disease by the GCT repeat expansion and it represents specific pattern of DNA. When the GCT pattern is repeated too many times in one copy of the DM gene, it disrupts the normal function of the protein made by the gene. In this pedigree analysis, large the repeat size, the younger the age at which a person will develop symptoms of DM and the more severe the symptoms will be, but the repeat size cannot predict the age when a person will develop symptoms so the largest repeat expansions are seen in congenital DM.

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