Question

The muscular dystrophy group contains thirty different genetic disorders, please choose TWO types and write briefly:

1- Description of the disease

2- Causes of the disease

3- Inheritance pattern of the disease

Answer 1

We choose Duchenne muscular dystrophy and Myotonic dystrophy

Duchenne muscular dystrophy

1- Description of the disease

• It is a severe form of muscular dystrophy
• It makes the muscles feeble
• The onset of this disease is around the age of four
• Lose the ability to walk by 12 years of age.
• Loss of muscle mass occurs in the thighs, pelvis and arms in the same order.
• Patients suffering from it face difficulty in standing

2- Causes of the disease

• It is caused by a mutation at the Xp21 locus in the dystrophin gene which is located at the X chromosome.
• The protein coded by the gene, Dystrophin, helps to attach the cytoskeleton of muscle fibers to the basal lamina
• Mutation renders the dystrophin nonfunctional which causes excess calcium to enter the cell membrane of muscle sells
• This results in the bursting of mitochondria
• Around 2/3 cases are inherited from the mother while 1/3 cases are due to a fresh mutation

3- Inheritance pattern of the disease

• The pattern of inheritance follows the X-linked recessive gene's inheritance pattern.
• It is very rare in females (1 in 50,000,000).
• An affected father and a carrier mother can pass the disease to their daughter (50% chance)

Myotonic dystrophy

• It is a genetic disorder that affects muscles and their function
• It causes muscle loss and feebleness
• Muscles often get contracted and are unable to go into relaxed state
• Can also result in cataracts, compromised intellectuality and heart conduction issues
• Early balding and sterility in males
• It affects around 1 in 8,000 individuals

2- Causes of the disease

• Type 1 myotonic dystrophy is caused by mutations in the DMPK gene
• Type 2 myotonic dystrophy is caused by mutations in the CNBP gene
• Type 2 myotonic dystrophy is a milder form while the symptoms of Type 1 may be visible at birth

3- Inheritance pattern of the disease

• It is an autosomal dominant disorder
• If at least one copy of dominant allele for myotonic dystrophy is present in an individual, then s/he will be diseased
• If one parent is heterozygous affected and the other is unaffected, then offspring has a 50% chance of being affected
• If either parent is homozygous for myotonic dystrophy, then all offsprings shall be diseased with the same
• It both parents are heterozygous, then the offspring has a 25% chance of being born normal/unaffected

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