These reproductive cells each have only 23 unpaired chromosomes. The human being that results is genetically unique, with a blueprint half from each parent. Each child gets half of its genes from each parent. Genes in each pair carry instructions for either dominant or recessive traits. If a parent passes on the gene for a dominant condition, there is a 50 percent chance that each child will have the same condition.
Dominant disorders are quite variable, with symptoms ranging from none to severe.
Marfan syndrome, which affects connective tissue, also is a dominant trait. If both parents are carriers of recessive genes, it can cause a birth defect - even if they are unaware of it. If only one parent passes on the gene for the disorder, the normal gene received from the other parent will prevent inheritance of the condition. Recessively inherited diseases are often severe and may lead to early death. The X and Y chromosomes are the sex-determining chromosomes.