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Glossary Molecular Biology and Computational Biology

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H H is the relative entropy of the target and background residue frequencies. (Karlin and Altschul, 1990). H can be thought of as a measure of the average information (in bits) available per position that distinguishes an alignment from chance. At high values of H, short alignments can be distinguished by chance, whereas at lower H values, a longer alignment may be necessary. (Altschul, 1991)

Haploid A single set of chromosomes (half the full set of genetic material) present in the egg and sperm cells of animals and in the egg and pollen cells of plants. Human beings have 23 chromosomes in their reproductive cells.

Haplotype A way of denoting the collective genotype of a number of closely linked loci on a chromosome.

Hemizygous Having only one copy of a particular gene. For example, in humans, males are hemizygous for genes found on the Y chromosome.

Hereditary cancer Cancer that occurs due to the inheritance of an altered gene within a family.

Heterozygosity The presence of different alleles at one or more loci on homologous chromosomes. (ORNL)


Highly conserved sequence DNA sequence that is very similar across several different types of organisms.

High-throughput sequencing

Homeobox A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. Homeoboxes have been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development.

Homolog A member of a chromosome pair in diploid organisms or a gene that has the same origin and functions in two or more species.

Homologous chromosome Chromosome containing the same linear gene sequences as another, each derived from one parent.

Homologous recombination Swapping of DNA fragments between paired chromosomes.

Homology Similarity attributed to descent from a common ancestor.

Homology Similarity in DNA or protein sequences between individuals of the same species or among different species.

Homology Evolutionary descent from a common ancestor due to gene duplication.

Homozygote An organism that has two identical alleles of a gene.


HSP High-scoring segment pair. Local alignments with no gaps that achieve one of the top alignment scores in a given search.

Human gene therapy

Human Genome Initiative Collective name for several projects begun in 1986 by DOE to create an ordered set of DNA segments from known chromosomal locations, develop new computational methods for analyzing genetic map and DNA sequence data, and develop new techniques and instruments for detecting and analyzing DNA. This DOE initiative is now known as the Human Genome Program. The joint national effort, led by DOE and NIH, is known as the Human Genome Project. (ORNL)

Human Genome Project (HGP) Formerly titled Human Genome Initiative.

Hybrid The offspring of genetically different parents.

Hybridization The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule.

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