# H

##### H （相对熵值）
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)

##### Half-bits
Some scoring matrices are in half-bit units. These units are logarithms to the base 2 of odds scores times 2.

##### Heuristic（启发式方法）
A procedure that progresses along empirical lines by using rules of thumb to reach a solution. The solution is not guaranteed to be optimal.

The base 16 counting system that uses the digits O-9 followed by the letters A-F.

##### HGMP （人类基因组图谱计划）
Human Genome Mapping Project.

##### Hidden Markov Model (HMM)（隐马尔可夫模型）
In sequence analysis, a HMM is usually a probabilistic model of a multiple sequence alignment, but can also be a model of periodic patterns in a single sequence, representing, for example, patterns found in the exons of a gene. In a model of multiple sequence alignments, each column of symbols in the alignment is represented by a frequency distribution of the symbols called a state, and insertions and deletions by other states. One then moves through the model along a particular path from state to state trying to match a given sequence. The next matching symbol is chosen from each state, recording its probability (frequency) and also the probability of going to that particular state from a previous one (the transition probability). State and transition probabilities are then multiplied to obtain a probability of the given sequence. Generally speaking, a HMM is a statistical model for an ordered sequence of symbols, acting as a stochastic state machine that generates a symbol each time a transition is made from one state to the next. Transitions betweenstates are specified by transition probabilities.

##### Hidden layer（隐藏层）
An inner layer within a neural network that receives its input and sends its output to other layers within the network. One function of the hidden layer is to detect covariation within the input data, such as patterns of amino acid covariation that are associated with a particular type of secondary structure in proteins.

##### Hierarchical clustering（分级聚类）
The clustering or grouping of objects based on some single criterion of similarity or difference.An example is the clustering of genes in a microarray experiment based on the correlation between their expression patterns. The distance method used in phylogenetic analysis is another example.

##### Hill climbing
A nonoptimal search algorithm that selects the singular best possible solution at a given state or step. The solution may result in a locally best solution that is not a globally best solution.

##### Homology（同源性）
A similar component in two organisms (e.g., genes with strongly similar sequences) that can be attributed to a common ancestor of the two organisms during evolution.

##### Horizontal transfer（水平转移）
The transfer of genetic material between two distinct species that do not ordinarily exchange genetic material. The transferred DNA becomes established in the recipient genome and can be detected by a novel phylogenetic history and codon content com-pared to the rest of the genome.

##### HSP （高比值片段对）
High-scoring segment pair. Local alignments with no gaps that achieve one of the top alignment scores in a given search.

##### HTGS/HGT（高通量基因组序列）
High-throughout genome sequences

##### HTML（超文本标识语言）
The Hyper-Text Markup Language (HTML) provides a structural description of a document using a specified tag set. HTML currently serves as the Internet lingua franca for describing hypertext Web page documents.

##### Hyperplane
A generalization of the two-dimensional plane to N dimensions.

##### Hypercube
A generalization of the three-dimensional cube to N dimensions.

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