It is common knowledge that a person's appearance -- height, hair color, skin color, and eye color -- are determined by genes. Mental abilities and natural talents are also affected by heredity, as is the susceptibility to acquire certain diseases.

An inherited, abnormal trait or "anomaly" may:

• Have no consequence to a person's health or well being (for example, a white patch of hair or an extended ear lobe).
• Be of minor consequence (for example, color blindness).
• Have dramatic effect on the quality or length of life.
  For most genetic disorders, genetic counseling is advised. Many people may also want to seek prenatal diagnosis.

The terms anomaly, abnormality, disorder, defect, disease, and syndrome are not used consistently, and do not have precise definitions.
Description
Human beings have cells with 46 chromosomes --2 sex chromosomes and 22 pairs of autosomal (non-sex) chromosomes. Males are "46, XY" and females are "46, XX". These chromosomes are made up of extremely long DNA molecules in combination with chromosomal proteins.
Genes are defined by intervals along one of the DNA molecules. The location of the gene is called the locus. Most genes carry information which is necessary to synthesize a protein.
The pairs of autosomal chromosomes (one from the mother and one from the father) carry basically the same information. That is, each has the same genes, but there are slight variations in the DNA sequence of nucleotide bases in each gene.
These slight variations occur in less than 1% of the DNA sequence and produce different variants of a particular gene that are called alleles.

The information contained in the nucleotide sequence of a gene is transcribed to mRNA (messenger RNA) by enzymes in the cell's nucleus and then translated to a protein in the cytoplasm. This protein may be a structural constituent of a given tissue. It may be an enzyme which catalyzes a chemical reaction, or it may be a hormone. There are also many other potential functions for proteins.

If a gene is abnormal, it may code for an abnormal protein or for an abnormal amount of a normal protein. Since the autosomal chromosomes are paired, there are 2 copies of each gene. If one of these genes is defective, the other may code for sufficient protein, so that no disease is clinically apparent. This is called a recessive disease, and the gene is said to be inherited in a recessive pattern.
In the case of a recessive disease, if one abnormal gene is inherited, the child will not show clinical disease, but they will pass the abnormal gene to 50% (on average) of their offspring. If one abnormal gene produces disease, this is called a dominant hereditary disorder. In the case of a dominant disorder, if one abnormal gene is inherited from mom or dad, the child will likely show the disease.
A person with one abnormal gene is termed HETEROZYGOUS for that gene. If a child receives an abnormal recessive disease gene from both parents, the child will show the disease and will be HOMOZYGOUS for that gene.
If two parents are each heterozygous for a particular recessive disease gene, then each child has a 25% chance of being homozygous for that gene and therefore, of showing the disease. If one parent is homozygous and the other heterozygous, then each child has a 50% chance of being homozygous.

Genetic disorders

Almost all diseases have a genetic component, but the importance of that component varies. Disorders where genetics play an important role, so-called genetic diseases, can be classified as single gene defects, chromosomal disorders, or multifactorial.

A single gene disorder (also called Mendelian disorder) is one that is determined by a single genetic locus and the specific allele on one or both members of a chromosome pair. Single gene defects are rare, with a frequency of less than 1 in 200 births. But since there are about 6,000 known single gene disorders, their combined impact is significant.
The incidence of serious single gene disorders is estimated to be about 1 in 200 births.

Single-gene disorders are characterized by the pattern of transmission in families -- this is called a pedigree. The term "kindred" includes the relatives outside of the immediate nuclear family. The affected individual that initially comes to light (or is of immediate interest) is called the proband. The brothers and sisters of the proband are called siblings.

There are only 5 basic patterns of single gene inheritance:

• Autosomal dominant
• Autosomal recessive
• X-linked dominant
• X-linked recessive
• Maternal (mitochondrial) inheritance
• The observed effect of an abnormal gene (the appearance of a disorder) is called the abnormal phenotype. A phenotype expressed in the same way (in both homozygotes and heterozygotes) is dominant. A phenotype expressed only in homozygotes (or, for X-linked traits, expressed in males but not females) is recessive.
  Heterozygotes for a recessive gene are called carriers. They usually don't express the phenotype clinically, but it can frequently be identified by sensitive laboratory methods.

In AUTOSOMAL DOMINANT INHERITANCE, the abnormality or abnormalities usually appear in every generation. Every affected child has an affected parent and each child of an affected parent has a 50% chance of inheriting the disease.
Normal members of the family do not transmit the disease. Males and females are equally likely to have the disease and to transmit the disease. Male-to-male transmission can occur (unlike with X-linked inheritance), and males can have unaffected daughters (unlike with X-linked dominant inheritance).

In AUTOSOMAL RECESSIVE INHERITANCE, the parents of an affected individual may not express the disease. On average, the chance of an affected child's brothers or sisters having the disease are 1 in 4. Males and females are equally likely to be affected. For a child to have symptoms of an autosomal recessive disorder, the child must receive the defective gene from BOTH parents.
Because most recessive disorders are rare, a child is at increased risk of a recessive disease if the parents are related. Related individuals are more likely to have inherited the same rare gene from a common ancestor.

In X-LINKED RECESSIVE INHERITANCE, the incidence of the disease is much higher in males than females. Since the abnormal gene is carried on the X chromosome, males do not transmit it to their sons -- they do transmit it to their daughters.
The presence of one normal X chromosome masks the effects of the X chromosome with the abnormal gene. So, almost all of the daughters of an affected man appear normal, but they are all carriers of the abnormal gene. The sons of these daughters then have a 50% chance of receiving the defective gene.

In X-LINKED DOMINANT INHERITANCE, the presence of the defective gene appears in females even if there is also a normal X chromosome present. Since males pass the Y chromosome to their sons, affected males will not have affected sons, but all of their daughters will be affected. Sons or daughters of affected females will have a 50% chance of getting the disease.
What Is Mental Retardation?

Mental retardation (say: ree-tar-day-shen) is a condition in which people have lower than normal intelligence and are unable to function at the level expected for their age. People with mental retardation are usually born with it, or it develops early in their life. They may also have some difficulty with daily living skills such as learning to read and write and caring for themselves.
Doctors and other professionals determine that a person has mental retardation based on their intelligence and how well they can do everyday activities. Intelligence is the ability to learn and understand. Levels of intelligence are measured by special tests called intelligence tests. The score a person gets on one of these tests gives a numerical measure of a person's intelligence. This is called an intelligence quotient (say: kwo-shunt) or IQ.
An average score on an IQ test is about 90 to 110. A person with mental retardation will usually score below 75 on an IQ test.
The IQ test alone does not determine whether someone is mentally retarded. A person must also have trouble with everyday activities such as getting dressed, eating, or washing or learning basic reading, writing, and arithmetic skills.