 A genetic disorder is a genetic problem caused by one or more abnormalities in the genome. Most genetic disorders are quite rare and affect one person in every several thousands or millions. Genetic disorders may be hereditary, meaning that they are passed down from the parent's genes. In other genetic disorders, defects may be caused by new mutations or changes to the DNA. In such cases, the defect will only be passed down if it occurs in the germline. Some types of recessive gene disorders confirm an advantage in certain environments when only one copy of the gene is present. A single-gene or monogenet disorder is the result of a single mutated gene. Over 6,000 human diseases are caused by single-gene defects. Single-gene disorders can be passed on to subsequent generations in several ways. Genomic imprinting and unit parental disamy, however, may affect inheritance patterns. The divisions between recessive and dominant types are not hard and fast although the divisions between autosomal and x-linked types are since the latter types are distinguished purely based on the chromosomal location of the gene. For example, achondroplasia is typically considered a dominant disorder, but children with two genes or achondroplasia have a severe skeletal disorder of which achondroplastics could be viewed as carriers. Sickle cell anemia is also considered a recessive condition, but heterozygous carriers have increased resistance to malaria in early childhood, which could be described as a related dominant condition. But a couple where one partner both are sufferers or carriers of a single-gene disorder wish to have a child, hey can do so through in vitro fertilization, which enables pre-implantation genetic diagnosis to occur to check whether the embryo has the genetic disorder. Most congenital metabolic disorders known as inborn errors of metabolism result from single-gene defects.