Some disorders are inherited. They are caused by the inheritance of certain alleles. Polydactyly means having extra fingers or toes, and it is caused by a dominant allele. A child only needs one copy of this allele, from either parent, to be born with extra digits.
Cystic fibrosis is a disorder of cell membranes. It is caused by a recessive allele, so a child only has the disorder if they inherit two copies. A person with one cystic fibrosis allele and one normal allele does not have the disorder. They are a carrier and can pass the allele on. Two parents who do not have cystic fibrosis can therefore have a child who does, if both are carriers.
Embryo screening tests an embryo for harmful alleles before it is placed in the womb. It may reduce suffering by helping families avoid serious disorders. There are also issues. Some people think it is wrong to destroy embryos. It costs money, and people worry it could be used to choose features of a baby. You may be asked to weigh up the economic, social and ethical arguments.
