Down syndrome, hemophilia,and sickle-cell anaemia are all serious conditions that have a genetic cause. Down Syndrome is caused by having an extra chromosome 21; this results from an error during formation of egg or sperm cells. One of the gametes ends up with both copies of chromosome 21, instead of just one.
The gene for hemophilia is recessive, and carried on the X chromosome. If the person with a copy of the gene is female, she will not show the disease as she will have a "good" copy of the gene on her other X chromosome to block it. Males have a Y chromosome, and therefore no "good" copy to block it.
Sickle-cell anemia is also caused by a recessive gene, but it is not carried on a sex chromosome. If the person recieves one copy of the gene, they are a carrier; two copies, and they have sickle-cell anemia.
All three diseases are inherited, and each are the result of a different mechanism:
Down syndrome is caused by inheriting 3 copies of chromosome 21 (a trisomy).
Hemophilia caused by the loss of the gene for a clotting factor (Factor VIII if hemophilia A) and is a sex-linked disease expressed almost exclusively in males.
Sickle cell anemia is caused by a mutation in a gene for hemoglobin that results in an altered hemoglobin molecule.