Genes reside within long strands of DNA which in turn make up the chromosomes. Some diseases, such as sickle cell anemia, can be caused by a change in a single gene (one out of the approximately 30,000 genes which make up the plan for the entire human body).

Genes are arranged in order along the DNA strand within the chromosome (similar to beads on a string). In most cases matching genes from each parent exist on matching chromosomes and matching positions along the DNA within the chromosome. These genes are paired, one from the mother and one from the father.

The exception is in males where only a single X chromosome has come from the mother and a non-matching Y chromosome came from the father. Genetic diseases are described as DOMINANT or RECESSIVE.

DOMINANT genetic traits are those that result from one gene in the gene pair being able to control the trait which that gene pair codes for. RECESSIVE traits require both genes in the gene pair to work together to control the trait.

RELATED TOPICS:
Autosomal dominant
Autosomal recessive
Genetic counseling and prenatal diagnosis
Sex-linked dominant
Sex-linked recessive

For detailed information, see heredity and disease (genetics).

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