Some disorders are inherited. They are passed from parents to children because the child inherits certain alleles of a gene. Two examples are polydactyly and cystic fibrosis.
Polydactyly means having extra fingers or toes. It is caused by a dominant allele. Because the allele is dominant, a child only needs to inherit one copy of it from either parent to have the disorder.
Cystic fibrosis is a disorder of cell membranes. It is caused by a recessive allele. A child must inherit two copies of the recessive allele, one from each parent, to have the disorder. A person with only one copy does not have cystic fibrosis, but is a carrier who can pass the allele on to their children.
Embryo screening checks the genes of embryos made by IVF for harmful alleles, so that an embryo without the disorder can be chosen. Screening may reduce suffering and could save the cost of long-term treatment. However, there are social and ethical issues. Some people think that destroying embryos is wrong, that screening could lead to parents choosing embryos for other characteristics, and that it may make people with the disorder feel less valued. Gene therapy, which aims to alter faulty alleles, raises similar ethical questions.