Dominant
An allele that is almost always expressed, even if only one copy is present.
2. A characteristic of gene expression in which only one copy of a gene is needed in order for there to be manifestations of its presence.
3. The property of an allele where by possessing either one or
two copies of the allele results in the same phenotype. Contrast with recessive.

Dominant trait
A trait controlled by an allele that expresses itself even when heterozygous.

Dosage compensation
A mechanism by which the activity of a gene is increased or decreased depending on the number of copies of that gene present in the cell. Most commonly seen in genes found on the X chromosome, which must be regulated so that males and females have the same amount of gene product.

Dysplasia
Abnormality of development of particular tissue types; alteration in size, shape, or organization of cells into tissue.

Draft Sequence
The sequence generated by the HGP as of June 2000 that, while incomplete, offers a virtual road map to an estimated 95% of all human genes. Draft sequence data are mostly in the form of 10,000 base pair-sized fragments whose approximate chromosomal locations are known.