遗传学名词解释H

H

H substance

-- The carbohydrate group present on the surface of red blood cells.

When unmodified, it results in blood type O; when modified by the addition of

monosaccharides, it results in type A, B, and AB.

haploid

-- A cell or organism having a single set of unpaired chromosomes. The

gametic chromosome number.

haplotype

-- The set of alleles from closely linked loci carried by an individual and

usually inherited as a unit.

Hardy-Weinberg law

-- The principle that both gene and genotype frequencies will

remain in equilibrium in an infinitely large population in the absence of mutation,

migration, selection, and nonrandom mating.

heat shock

-- A transient response following exposure of cells or organisms to

elevated temperatures. The response involves activation of a small number of loci,

inactivation of some previously active loci, and selective translation of heat shock

mRNA. Appears to be a nearly universal phenomenon observed in organisms ranging

from bacteria to humans.

helicase

-- An enzyme that participates in DNA replication by unwinding the double

helix near the replication fork.

helix-turn-helix motif

-- The structure of a region of DNA-binding proteins in which a

turn of four amino acids holds two alpha helices at right angles to each other.

hemizygous

-- Conditions where a gene is present in a single dose in an otherwise

diploid cell. Usually applied to genes on the X chromosome in heterogametic males.

hemoglobin (Hb)

-- An iron-containing, oxygen-carrying protein occurring chiefly in

the red blood cells of vertebrates.

hemophilia

-- An X-linked trait in humans associated with defective blood-clotting

mechanisms.

heredity

-- Transmission of traits from one generation to another.

heritability

-- A measure of the degree to which observed phenotypic differences for

a trait are genetic.

heterochromatin

-- The heavily staining, late replicating regions of chromosomes

that are condensed in interphase. Thought to be devoid of structural genes.

heteroduplex

-- A double-stranded nucleic acid molecule in which each

polynucleotide chain has a different origin. These structures may be produced as

intermediates in a recombinational event, or by the in vitro reannealing of

single-stranded, complementary molecules.

heterogametic sex

-- The sex that produces gametes containing unlike sex

chromosomes.

heterogeneous nuclear RNA (hnRNA)

-- The collection of RNA transcripts in the

nucleus, representing precursors and processing intermediates to rRNA, mRNA, and

tRNA. Also represents RNA transcripts that will not be transported to the cytoplasm,

such as snRNA (small nuclear RNA).

heterokaryon

-- A somatic cell containing nuclei from two different sources.

heterosis

-- The superiority of a heterozygote over either homozygote for a given

trait.

heterozygote

-- An individual with different alleles at one or more loci. Such

individuals will produce unlike gametes and therefore will not breed true.

Hfr

-- A strain of bacteria exhibiting a high frequency of recombination. These

strains have a chromosomally integrated F factor that is able to mobilize and transfer

part of the chromosome to a recipient F

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cell.

histocompatibility antigens

-- See HLA.

histones

-- Proteins complexed with DNA in the nucleus. They are rich in the basic

amino acids arginine and lysine and function in the coiling of DNA to form

nucleosomes.

HLA

-- Cell surface proteins, produced by histocompatibility loci, involved in the

acceptance or rejection of tissue and organ grafts and transplants.

hnRNA

-- See heterogeneous nuclear RNA.

holandric

-- A trait transmitted from males to males. In humans, genes on the Y

chromosome are holandric.

Holliday structure

-- An intermediate in bidirectional DNA recombination seen in the

transmission electron microscope as an X-shaped structure showing four

single-stranded DNA regions.

homeobox

-- A sequence of about 180 nucleotides that encodes a 60-amino-acid

sequence called a homeodomain, which is part of a DNA- binding protein that acts as a

transcription factor.