Option 1 - Down syndrome: Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. This additional genetic material affects the physical and intellectual development of individuals with Down syndrome. It is a genetic disease because it is caused by an abnormality in the person`s genes.

Option 2 - Elephantiasis: Elephantiasis, also known as lymphatic filariasis, is not a genetic disease. It is caused by a parasitic infection transmitted through mosquito bites. The parasite blocks the lymphatic system, leading to severe swelling and thickening of the skin.

Option 3 - Asthma: Asthma is a chronic respiratory condition, but it is not a genetic disease. It is primarily caused by environmental factors such as allergies and exposure to irritants.

Option 4 - Night blindness: Night blindness refers to the inability to see clearly in dim lighting conditions. While it can be hereditary in some cases, it is not considered a genetic disease on its own.

Therefore, the correct answer is option 1, Down syndrome, as it is a genetic disease caused by an abnormality in the person`s genes.