Question

Learning Task 18-02: Genetic Disorders

1. Research and write a brief description of each genetic disorder. 1- Huntington Disease 2- Tay-Sachs Disease 3-Marfan Syndrome 4- Cystic Fibrosis 5-Duchenne Muscular Dystrophy
2. For each disease, state the chances that a child would be born with the disease:
   • if one parent carries the gene; and
   • if both parents carry the gene.

3. Explain how you arrived at your answers. Use Punnett squares to help justify your response for each disease.

Answer 1

Answer:

1. Huntington's Disease (HD): is a fatal inherited disease. It is a neural disorder due to the degeneration of neurons secreting GABA in corpus striatum and substantia niagra. It usually manifests in age group of 30 - 40. It is characterized by Chorea, hypotonia and dementia. In severe cases by lateral wasting of muscles occur. It has no cure but some symptoms can be managed by Occupational- and Physio- therapies.

HD is inherited in an autosomal dominant pattern, meaning that one copy of the altered gene in each cell is enough to cause the disorder. Child inheriting HD from both parents will not survive to birth.

H represents the affected allele for Huntington's disease and h is the normal allele. Below is the Punnet square showing inheritance pattern of HD and percentages of affected individuals in each case.

2. Tay Sach's Disease - is a genetic disorder common among people with certain ancestry like Eastern European Jews. The disease leads to destruction of nerve cells in brain and spinal cord by a fatty substance. The disease becomes apparent by 3 - 5 months when child loses its ability to crawl or make sounds etc. It usually leads to death by 4 years of age.

This disease in inherited by autosomal recessive pattern. This means those who carry only one copy of the allele, they carry the disease but do not have the associated symptoms. Only when both parents, who are carrying the affected allele pass it to the child, then the disease is fully manifested.

t represents the affected allele for Tay Sach's disease and T is the normal allele. Below is the Punnet square showing inheritance pattern of this disease and percentages of affected individuals in each case.

3. Marfan Syndrome - or MFS affects the connective tissue. Since connective tissue is present in many organs and hence MFS is a multi-organ disease. It is an inherited disorder. The most commonly affected parts are eyes and large heart vessels (aorta). With MFS people may also get some additional heart problems. MFS affected persons show some typical features like - larger than body arm span, thin and long face, long fingers and toes. They also show scoliosis of spine.

MFS is inherited as autosomal dominant patters, suggesting that one copy of the MFS gene is sufficient to cause the disease. At least 25% of the cases happen due to new mutation and such cases don't have any family history of MFS.

M represents the affected allele for Marfan Syndrome and m is the normal allele. Below is the Punnet square showing inheritance pattern of MFS and percentages of affected individuals in each case.

4. Cystic Fibrosis (CF) - is a hereditory disease that affects lungs and digestive system. CF affects the cells that produce mucus, sweat and digestive juices. It causes thickening of their secretions. Due to this CF could be fatal as it clogs up passageways and create blocks. Person's affected with CF will have a shorter than normal life span.

It is an autosomal recessive disorder. Two parents carrying this gene should pass it on to the progeny to have CF. When only one parent passes the gene the child becomes a carrier and is not affected with symptoms.

c represents the affected allele for Cystic Fibrosis and C is the normal allele. Below is the Punnet square showing inheritance pattern of CF and percentages of affected individuals in each case.

5. Duchenne Muscular Dystrophy (DMD) - is progressive form of muscular dystrophy and is mostly seen in males. It causes atrophy of skeletal and heart muscles. This muscle weakness is noticeable by the age of 4. It is usually caused by mutation for DMD gene which codes for dystrophin. Children with DMD show tight or rigid joints, Scoliosis and weakened respiratory and heart muscles leading to breathlessness and cardiomyopathy.

It is an X- linked disorder. Usually the male are affected because they carry only copy of X chromosome. Females have 2 copies and hence they become carriers. There is very rare chance of females contracting DMD.

XD represents the affected allele in both males and females. X and Y are normal alleles.  Below is the Punnet square showing inheritance pattern of DMD and percentages of affected individuals in each case.