Genetic disorder is a problem caused by one or more abnormalities in the genome, that is present from birth (congenital). The most common genetic disorders, ecxluding chromosomal abnormalities (a sub-type), includes:

• 22q11.2 deletion syndrome
• Canavan disease
• Charcot-marie-Tooth disease
• Color blindness
• Cri du chat
• Cystic fibrosis
• Duchenne muscular dystrophy
• Hemochromatosis
• Hemophilia
• Neurofibromatosis
• Phenylketonuria
• Polycystic kidney disease
• Sickle-cell disease
• Spinal muscular atrophy
• Tay-Sachs disease

• Heritable (i.e. passed down from the parents' genes), including:
  • Single-gene disorders, of which probabilities can be calculated with a Punnett square as they follow Mendelian inheritance, including:
    • Autosomal dominant, where only 1 mutated copy of the gene is required (i.e. only 1 affected parent required). The chance a child will inherit the mutated gene is 50%. Examples include familial hypercholesterolemia, polycystic kidney disease, neurofibromatosis type 1, hereditary spherocytosis, Marfan syndrome, Huntington's disease
    • Autosomal recessive, where 2 mutated copies of the gene is quired (i.e. both parents must be affected). If parents are both carriers, children have a 25% risk. Examples include sickle cell anemia, cystic fibrosis, Tay-Sachs disease, phenylketonuria, mucopolysaccharidoses, lysosomal acid lipase deficiency, glycogen storage diseases, galactosemia
    • X-linked dominant, which are caused by mutations in the X chromosome. Both males and females are affected. Males are more SEVERELY affected. All of an affected man's daughters will be AFFECTED, but his sons will not be (since they receive the father's Y chromosome). A woman's children have 50% chance of being affected
    • X-linked recessive, which are caused by mutations in the X chromosome. Males are more FREQUENTLY affected than females. All of an affected man's daughters will be CARRIERS, and his sons will not be affected. A woman's children have 50% chance of being carriers. Examples include Duchenne muscular dystrophy, hemophilia
    • Y-linked, which are caused by mutations on the Y chromosome. They are only transmitted from fathers to their sons. Females can never be affected since they do not have a Y chromosome. Examples include infertility
    • Mitochondrial (aka maternal inheritance), are mutations in mitochondrial DNA. Because only egg cells contribute to mitochondria, only mothers can pass on mitochondrial conditions. Examples include Leber's hereditary optic neuropathy
  • Multifactorial and polygenic disorders
• Non-heritable (sporadic mutation), where defects may be caused by new mutations or changes to the DNA. Where this occurs, the defect will only be heritable if it occurs in the germ line
• The same disease (e.g. some forms of cancers), can be inherited in some condition, a new mutation in others, and caused by environmental causes in still others

• Almost always affected by the environmental factors and events in a person's development

Screening:

• Alpha-fetoprotein (see page)
• Nuchal translucency (see page)

Confirmation:

• Amniocentesis (see page)
• Chorionic villus sampling (see page)
• Peripheral umbilical blood sampling (PUBS, aka cordocentesis), is a prenatal test, to Dx genetic and other fetal problems (e.g. fetus hemolytic disease). Fetal and maternal blood is typically connected in utero with 1 vein and 2 arteries. The umbilical vein is responsible for delivery oxygenated blood to the fetus from the other. It is usually done in trimester 2-3, when the umbilical cord vessels can be punctured with a needle. Alternatively, it can provide rapid chromosome analysis, when information can't be obtained through amniocentesis, CVS, or U/S, or the results of these tests were inconclusive. The test carries a significant risk of bleeding of the puncture site, and has a higher risk of miscarriage [than amniocentesis or CVS] at 3%

Patient information

It seems like PUBS causes a higher risk, of 3%, and is also better done later . So unless if there's good reason, the choice is usually between amniocentesis and CVS. What's the difference between them ?
It's a weigh of choices. Amniocentesis has a lower risk of miscarriage of 0.5%, but you have to wait until 15 weeks gestation. CVS on the other hand, has a higher risk of miscarriage, but is the preferred method before 15 weeks gestation. So it's weighing "knowing earlier" with "risk of miscarriage". That all depends on the risk, and thoughts and feelings of the parents .

• Depends on the genetic defect or abnormality

• Most genetic disorders are quite rare, affecting 1 person in every several thousand or millions
• Some recessive gene disorders confer an evolutionary advantage in certain environments, when only 1 copy of the gene is present

• Chromosomal abnormality (type of genetic disorder)
• Down syndrome (contains more screens, specific for Down's)
• Alpha-fetoprotein