health

Hereditary birth defects are caused when a mistake is made as DNA
divides and forms a template to transcribe copies. Then a process called
translation enters in to create the final product. Many disorders of genetic
origin occur in translation; the final product will be a mutation or change
from the normal final product. One might relate the process to a group of
workers on an assembly line making peanut butter and jelly sandwiches.
Each person has a job to do: Unwrapping the loaf of bread, spreading
the peanut butter, spreading the jelly, putting the top and bottom slices
together. The process moves like clockwork, but if the jelly spreader drops
his knife and misses out on his job, no jelly will be on the next sandwich.
This simple illustration shows how one small slip can change the outcome
of a product. Likewise, even small changes in the primary structure of
a protein may affect the protein’s properties. For example, in sickle-cell
disease*, a single molecule of valine replaces the glutamic acid molecule
that makes hemoglobin, the protein that carries oxygen in the red blood
cells. The tiny error, one incorrect amino acid in a series of 287 amino
acids, causes a change in the chemistry of the hemoglobin molecule that
results in a deformity in the red blood cell and causes all of the problems
of sickle-cell anemia. When chemist Linus Pauling (1901–1994) discovered
this fact in the early 1950s, the science of molecular biology began.
Some diseases develop through random mutation to act as protection
against prevalent conditions in certain areas. In the 1940s Anthony
Allison from Kenya studied the blood of people living in malarial areas
of Africa. He found that those individuals who had sickle cells, and even
those with only one defective gene and therefore have the sickle-cell trait
and not the disease, were less likely to contract malaria. Thus those individuals
with a mutant gene have a survival advantage when they live in
malarial areas. This correlation explains why the mutant gene is so prevalent
in those areas. However, the gene was carried to the United States by
slaves and persists in the U.S. population in the early 2000s even though
malaria is extremely rare in the United States. Likewise, Tay-Sachs disease
evolved among the Ashkenazi Jews from a single town in Poland. The
mutations offered the carriers protection against tuberculosis but had a
lethal effect on some of the population. Current incidences of sickle-cell
disease and Tay-Sachs disease are a high price to pay for diseases of the
past. Fortunately, because of genetic testing and counseling, Tay-Sachs
disease is very rare in the early 2000s. Because of preimplantation genetic
diagnosis, fertilized ova that lack the mutated gene for sickle cell can be
selected for implantation thus ensuring a normal child.
Genetic birth defects can be dominant or recessive single-gene traits,
X-linked disorders, multigene traits, or aberrations in chromosomes.

• Single-gene dominant or Mendelian inheritance. The simplest
  patterns of birth defect inheritance are named for the Austrian monk
  Gregor Mendel (1822–1884), who observed them in the nineteenth
  century. In Mendelian inheritance, traits (both normal and defects)
  can be transmitted by way of dominant or recessive genes. A child
  inherits two copies of each gene, one from the mother and one from
  the father. If a defective gene is dominant, a child who inherits even
  one copy will have the defect. That is because the defective copy
  “dominates,” or overwhelms, the normal copy inherited from the
  other parent. Many of these defects may not show up until later life.
  Examples are Huntington’s disease and Lou Gehrig’s disease (nervous
  system disorders). Marfan syndrome is characterized by tallness
  and heart disorders, and certain kidney and cholesterol diseases.
  
• Single-gene recessive traits. According to Mendelian law, if a defective
  gene is recessive, the child must inherit two defective copies—
  one from the mother and one from the father—in order to have
  the defect. A person who inherits only one defective copy is healthy
  but can pass the defective copy on to his or her own children. Many
  of these disorders are rare but can be traced to specific mutations
  in the genes that produce enzymes. For example, thalassemia, a
  condition common in populations bordering the Mediterranean
  Sea, affects the hemoglobin. Cystic fibrosis affects the lungs and
  pancreas and is one of the most damaging of genetic disorders.
• X-linked disorders. Genes located on the X chromosome (the X and
  Y chromosomes determine the sex of an infant: two X chromosomes
  produce a female and an X and a Y chromosome produces a male)
  can cause birth defects. Such abnormalities are said to be X-linked.
  Hemophilia, a blood disorder; X-linked severe combined immunodeficiency
  (SCID), an immune disorder; Duchenne muscular dystrophy;
  and color blindness are examples of X-linked birth defects.
  
• Multigene traits. Interaction of several genes can cause certain
  defects. Many of these occur later in life and include Parkinson’s
  disease, epilepsy*, and Alzheimer’s disease.
  
• Chromosome abnormalities. Extra, missing, incomplete, or misshapen
  chromosomes cause some birth defects. Down syndrome, a
  condition caused by three copies of chromosome 21 or Trisomy 21,
  is one of the most common birth defects. Down syndrome produces
  mental retardation*, short stature, and distinctive facial features.
  Defects involving the sex chromosomes can produce problems in
  sexual development, including sterility, which is an inability to have
  children.
• Inborn errors of metabolism. Metabolic disorders are related to
  faulty genes that control chemical reactions of the body, producing
  energy from food or supporting the growth of the body. The two
  most common metabolic disorders are phenylketonuria (PKU), a
  disorder in which the child cannot handle the chemical phenylanaline;
  the child must be placed on a special diet for life. The second
  metabolic disease is hypothyroidism in which the thyroid gland
  under produces hormones necessary for body funct