| H-Y antigen |
| The histocompatibility Y-antigen, a protein
found on the cell surfaces of male mammals. |
| haemoglobin (hb) |
| The oxygen-transporting blood cell protein
in most animals. |
| haemophilia |
| A disease in which the blood fails to clot.
The most common form, primarily affecting males, is caused by a
mutation in a gene coding for a clotting protein (factor VIII) inherited
as an X-linked recessive phenotype. |
| half-chromatid conversion
|
| A type of gene conversion that is inferred
from the existence of non-identical sister spores in a fungal octad
showing a non-Mendelian allele ratio. |
| haplodiploidy |
| The sex-determining mechanism found in some
insect groups among which males are haploid and females are diploid.
|
| haploid |
| The state of having one copy of each chromosome
per nucleus or cell. A cell having one chromosome set, or an organism
composed of such cells. |
| haploidization |
| Production of a haploid from a diploid by
progressive chromosome loss. |
| haplotype |
| A set of closely linked genetic markers
present on one chromosome which tend to be inherited together (not
easily separable by recombination). Some haplotypes may be in linkage
disequilibrium. |
| Hardy-Weinberg equilibrium
|
| The stable frequency distribution of genotypes,
AA, Aa, and aa, in the proportions p2, 2pq, and q2 respectively
(where p and q are the frequencies of the alleles, A and a) that
is a consequence of random mating in the absence of mutation, migration,
natural selection, or random drift. |
| harlequin chromosome
|
| Sister chromatids that stain differently,
so that one appears dark and the other light (harlequin-like). |
| HAT medium |
| A selection medium for hybrid cell lines;
contains hypoxanthine; aminopterin; thymidine. Only cell lines expressing
both hypoxanthine phosphoribosyl transferase (HPRT+) and thymidine
kinase (TK+) can survive in this medium. Aminopterin inhibits de
novo synthesis of nucleosides, while HPRT and TK supply them from
hypoxanthine and thymidine. |
| heat-shock protein |
| One of a number of proteins appearing in
a cell after the cell has been subjected to elevated temperatures.
|
| helicase |
| A protein that unwinds DNA at replication
forks. |
| helix-turn-helix (HLH)
protein |
| A protein in which a domain which forms
two helices separated by a loop that acts as a sequence-specific
DNA binding domain. HLH proteins are thought to act as transcription
factors. |
| helix-turn-helix motif
|
| Configuration, found in some DNA-binding
proteins, consisting of a recognition helix and a stabilizing helix
separated by a short loop. |
| hemizygous |
| The condition of loci on the X chromosome
of the heterogametic sex of a diploid species. Or more generally
when one part of the genome, in a normally diploid species, is present
in only one copy. |
| hemizygous gene |
| A gene present in only one copy
in a diploid organism for example, X-linked genes in a male mammal.
|
| heredity |
| The biological similarity of offspring and
parents. |
| heritability |
| A measure of the degree to which the variance
in the distribution of a phenotype is due to genetic causes. In
the broad sense it is measured by the total genetic variance divided
by the total phenotypic variance. In the narrow sense it is measured
by the genetic variance due to additive genes divided by the total
phenotypic variance. |
| heritability in the narrow
sense |
| The proportion of phenotypic variance that
can be attributed to additive genetic variance. |
| hermaphrodite |
| An individual with both male and female
genitalia. (1) A plant species in which male and female organs occur
in the same flower of a single individual (compare monoecious plant).
(2) An animal with both male and female sex organs. |
| heterochromatin |
| Densely staining condensed chromosomal regions,
believed to be for the most part genetically inert. chromatin that
remains tightly coiled (and darkly staining) throughout the cell
cycle. See euchromatin |
| heteroduplex |
| A DNA double helix formed by annealing single
strands from different sources; if there is a sequence difference
between the strands, the heteroduplex may show single strand loops
or bubbles (unpaired regions). |
| heteroduplex analysis
|
| Duplex DNA formed by strands from different
sources, referred to as a heteroduplex, will have loops and bubbles
in regions where the two DNAs differ. Electron microscopic observation
(analysis) of this DNA has been a useful tool in recombinant DNA
work. |
| heteroduplex DNA model
|
| A model that explains both crossing over
and gene conversion by assuming the production of a short stretch
of heteroduplex DNA (formed from both parental DNAs) in the vicinity
of a chiasma. |
| heteroduplex mapping
|
| The use of heteroduplex analysis to determine
the location of various insertions; deletions; or heterogeneities
between two DNA molecules. |
| heterogametic sex |
| The sex with (usually) two heteromorphic
(differently shaped) sex chromosomes (for example, X and Y). During
meiosis it produces two kinds of gametes in regard to these sex
chromosomes. |
| heterogeneous nuclear
mRNA (hnRNA) |
| The original RNA transcripts found in eukaryotic
nuclei before post-transcriptional modifications. A diverse assortment
of RNA types found in the nucleus, including mRNA precursors (pre-mRNA)
and other types of RNA. |
| heterokaryon |
| A cell that contains two or more nuclei
from different origins. A cell composed of two different nuclear
types in a common cytoplasm. |
| heterokaryon test |
| A test for cytoplasmic organelle mutations
(eg. mitochondrial or chloroplast mutations), based on new associations
(recombination) of phenotypes in cells derived from genetically
marked haploid heterokaryons. Since the heterokaryons are haploid
and produce haploid spores by a process that does not involve diploid
nuclei normal genetic recombination does not occur. Therefore evidence
of recombination is evidence of reassociation by some other means
such as reassociation of an organelle with nucleus. |
| heteromorphic chromosomes
|
| A chromosome pair with some homology but
differing in size, shape, or staining properties. Homologous chromosome
pair which are not morphologically identical (eg the sex chromosomes).
|
| heteroplasmon |
| A cell containing a mixture of genetically
different cytoplasms, generally different mitochondria or different
chloroplasts. |
| heteroplasmy |
| The existence within an organism of genetic
heterogeneity within the populations of mitochondria or chloroplasts.
|
| heterothallic |
| A botanical term used for organisms in which
the two sexes reside in different individuals. |
| heterothallic fungus
|
| A fungus species in which two different
mating types must unite to complete the sexual cycle. |
| heterotroph |
| Organism requiring an organic form of carbon
as a carbon source. |
| heterozygosity |
| A measure of the genetic variation in a
population; with respect to one locus, stated as the frequency of
heterozygotes for that locus. |
| heterozygote |
| An individual having a heterozygous gene
pair. A diploid or polyploid with different alleles at a particular
locus. |
| heterozygote advantage
|
| A selection model in which heterozygotes
have the highest fitness. |
| heterozygous gene pair
|
| A gene pair having different alleles in
the two chromosome sets of the diploid individual for example, Aa
or, A1A2. |
| hexaploid |
| A cell having six chromosome sets, or an
organism composed of such cells. |
| Hfr |
| High frequency of recombination. A strain
of bacteria that has incorporated an F factor into its chromosome
and can then transfer the chromosome during conjugation. In Escherichia
coli, a cell having its fertility factor integrated into the bacterial
chromosome; a donor (male) cell. |
| Himalayan |
| A mammalian temperature-dependent coat phenotype,
generally albino with pigment only at the cooler tips of the ears,
feet, and tail. |
| His |
| Histidine (an amino acid). |
| histocompatibility antigen
|
| antigen that determines acceptance or rejection
of a tissue graft by the immune system. |
| histocompatibility gene
|
| A gene encoding an histocompatibility antigen.
|
| histone |
| A type of basic protein that forms the unit
around which DNA is coiled in the nucleosomes of eukaryotic chromosomes.
Arginine and lysine rich basic proteins making up a substantial
portion of eukaryotic nucleoprotein. |
| holandric trait |
| Trait controlled by a locus found only on
the Y chromosome. Involves father to son transmission. |
| holoenzyme |
| The complete enzyme including all subunits.
Often used in reference to RNA and DNA polymerases. |
| homeo-domain |
| An approximately sixty-amino acid protein
domain translated from the homeo-box. A highly conserved family
of protein domain sequences 60 amino acids in length found within
a large number of transcription factors that can form a helix-turn-helix
structure and bind DNA in a sequence-specific manner. |
| homeo-box |
| A consensus sequence of about 180 base pairs
discovered in homeotic genes in Drosophila. Also found in other
developmentally important genes from yeast to human beings. A family
of quite similar l80base-pair DNA sequences that encode a polypeptide
sequence called a homeo-domain, a sequence-specific DNA binding
sequence. While the homeobox was first discovered in all homeotic
genes, it is now known to encode a much more widespread DNA-binding
motif. |
| homeologous chromosomes
|
| Partially homologous chromosomes, usually
indicating some original ancestral homology. |
| homeosis |
| The replacement of one body part by another.
Homeosis can be caused by environmental factors leading to developmental
anomalies, or by mutation. |
| homeotic mutations (genes)
|
| Mutations (defining genes) with a phenotype
in which a given cell develops along a pathway normally followed
by a different cell type. Mutations that can change the fate of
an imaginal disk in insect development. |
| homogametic sex |
| The sex with homomorphic (similarly shaped)
sex chromosomes (for example, XX), producing only one kind of gamete
in regard to the sex chromosomes. |
| homologue |
| A member of a pair of homologous chromosomes.
|
| homologous chromosomes
|
| Members of a pair of essentially identical
chromosomes that synapse during meiosis. Chromosomes that pair with
each other at meiosis or chromosomes in different species that have
retained most of the same genes during their evolution from a common
ancestor. |
| homologous recombination
|
| Breakage and reunion between homologous
lengths of DNA mediated in Escherichia coli by products of the genes
RecA and RecBCD, and functionally equivalent genes in other organisms.
|
| homomorphic chromosomes
|
| Morphologically identical members of a homologous
pair of chromosomes. |
| homoplasmy |
| The existence within an organism of only
one type of plastid; usually referring to genetic identity of mitochondria
or chloroplasts. |
| homothallic |
| A botanical term used for groups whose individuals
are not of different sexes. |
| homothallic fungus |
| A fungus species in which a single sexual
spore can complete the entire sexual cycle (compare heterothallic
fungus). |
| homozygote |
| An individual having a homozygous gene pair.
A diploid or a polyploid with identical alleles at a locus. |
| homozygous gene pair
|
| A diploid gene pair having identical alleles
in both copies for example, AA or, aa. |
| hormone response element
(HRE) |
| An HRE is a cis-regulatory DNA sequence
for a hormone that acts by binding to a receptor that can act as
a transcription factor, that is a binding site for the hormone-receptor
complex. |
| hormone |
| Chemical secreted by one type of cell and
acting on a second type of cell. |
| host range |
| 1. The spectrum of strains of bacterial
species that a given strain of phage can infect. |
| 2. The range of cells that can act as a
host to a virus or bacteriophage. |
| hot spot |
| A region of a genome exhibiting an increased
frequency of some genetic phenomenon such as mutation, recombination
and transposition. |
| Huntington's disease
|
| A late but variable age onset lethal human
disease of nerve degeneration. Inherited as an autosomal dominant
phenotype. Shows imprinting individuals inheriting HD from their
father show significantly earlier onset than those inheriting a
maternal allele. Shows genetic anticipation, in that the severity
increase and age of onset decreases in affected individuals of later
generations in a pedigree. The HD gene, encoding a protein called
huntingtin, has been cloned. |
| hybrid |
| (1) A heterozygote. |
| (2) A progeny individual from any cross
involving parents of differing genotypes. Offspring of unlike parents.
|
| (3) A duplex polynucleotide formed by hybridization
of two single stranded polynucleotides of different origin. |
| hybrid DNA |
| DNA whose two strands have different origins.
|
| hybrid dysgenesis |
| A syndrome of effects including sterility,
mutation, chromosome breakage, and male recombination in the hybrid
progeny of crosses between certain laboratory and natural isolates
of Drosophila. |
| hybrid plasmid |
| A plasmid that contains an inserted piece
of foreign DNA. |
| hybrid vector/hybrid
vehicle |
| An episome or plasmid containing an inserted
piece of foreign DNA. |
| hybrid zone |
| Geographical region in which previously
isolated populations that have evolved differences come into contact
and form hybrids. |
| hybridization in situ
|
| Finding the location of a gene or gene product
by adding specific radioactive or chemically tagged probes for the
gene and detecting the location of the radioactivity or chemical
on the chromosome or in the cell after hybridization. |
| hybridize |
| (1) To form a hybrid by performing a cross. |
| (2) To anneal nucleic acid strands from
different sources. |
| hybridoma |
| A cell resulting from the fusion of a spleen
cell and myeloma cell. These cells can be cloned and maintained
indefinitely in cell culture and produce monoclonal antibodies.
|
| hydroxyapatite |
| A form of calcium phosphate that binds double-stranded
DNA. |
| hypermorph |
| A mutant with more than the normal amount
of some gene product. |
| hyperploid |
| Aneuploid containing a small number of extra
chromosomes. |
| hypervariable locus |
| locus with many alleles; especially those
whose variation is due to variable numbers of tandem repeats. |
| hypervariable region
|
| The part of a variable region that actually
determines the specificity of an immunoglobulin. |
| hypha (plural hyphae)
|
| A thread-like structure (composed of cells
attached end to end) that forms the main tissue in many fungus species.
|
| hypomorph |
| A mutant with less than the normal amount
of some gene product. |
| hypoploid |
| Aneuploid with a small number of chromosomes
missing. |
| hypostatic gene |
| A gene whose expression is masked by an
epistatic gene. |
