H-2 complex: The major histocompatibility complex (MHC) of the mouse. It is the first MHC discovered in 1937 by Peter Gorer.
Habitat: The immediate space where an animal or plant lives and has food, water and protection. Habitat loss, which includes the destruction, degradation, or fragmentation of habitats, is the primary cause of decreasing biodiversity.
Haeckel’s 'Biogenetic Law': Proposed by Ernst Haeckel in 1874 as an attempt to explain the relationship between ontogeny and phylogeny. It claimed that ontogeny recapitulates phylogeny, i.e., an embryo repeats in its development the evolutionary history of its species as it passes through stages in which it resembles its remote ancestors (embryos, however, do not pas through the adult stages of their ancestors; ontogeny does not recapitulate phylogeny. Rather, ontogeny repeats some ontogeny - some embryonic features of ancestors are present in embryonic development (L. Wolpert: The Triumph of Embryo. Oxford University Press, 1991).
Hair pin loop: Binding of complementary sequences to each other to form a hair pin loop (also called stem loop). If happens in a PCR primer, it will not function.
Haldane's Hypothesis (on recombination and sex): Selection to lower recombination on the Y-chromosome causes a pleiotropic reduction in recombination rates on other chromosomes [hence, the recombination rate is lower in heterogametic sex such as males in humans, females in butterflies].
Hamilton’s Rule (theory of kin selection): In an altruistic act, if the donor sustains cost C, and the receiver gains a benefit B as a result of the altruism, then an allele that promotes an altruistic act in the donor will spread in the population if B/C >1/r or rB-C>0 (where r is the coefficient of relatedness).
Hamilton's Altruism Theory: If selection favored the evolution of altruistic acts between parents and offspring, then similar behavior might occur between other close relatives possessing the same altruistic genes which were identical by descent. In other words, individual may behave altruistically not only to their own immediate offspring but to others such as siblings, grandchildren and cousins (as happens in the bee society).
Haploid:Vegetative or gametic cells that have one set of chromosomes (1N).
Haploid number (n): The number of chromosomes in the gamete after meiosis. In humans, the haploid number is 23.
Haploinsufficiency: Situation where one normal copy of a gene alone is not sufficient to maintain normal function. It is observed as a dominant mutation on one allele (or deletion of it) resulting in total loss-of-function in a diploid cell because of the insufficient amount of the wild-type protein encoded by the normal allele on the other haplotype (see also dominant negative). A recent example of haploinsufficiency by Kurotaki et al, 2002 in Sotos syndrome.
Haplotype: The particular combination of alleles in a linked group encoded by genes in close vicinity on the same chromosome.
Hapteron: The area of the holdfast that resembles roots.
Hardy-Weinberg equilibrium (HWE): In an infinitely large population, gene and genotype frequencies remain stable as long as there is no selection, mutation, or migration. For a bi-allelic locus where the gene frequencies are p and q: p2+2pq+q2 = 1 (Online HWE Analysis; lectures on HWE: 1 & 2).
Hardy-Weinberg Law: In an infinitely large population, gene and genotype frequencies remain stable as long as there is no selection, mutation, or migration. In a panmictic population in infinite size, the genotype frequencies will remain constant in this population. For a biallelic locus where the allele frequencies are p and q: p2 + 2pq + q2 = 1 (see Notes on Population Genetics for more).
head -- That part of the body at the "front" end, where the brain, mouth, and most sensory organs are located.
Heart: Muscular pump which circulates the blood.
Heat Shock Response: Heat shock response is ubiquitous and highly conserved defense mechanism for protection of cells from harmful conditions such as heat shock, UV irradiation, toxic chemicals, infection, transformation and appearance of mutant and misfolded proteins. Heat Shock Proteins (HSPs) also function as accessory molecules in antigen presentation. HSP70 genes are within the MHC in most vertebrates. High levels of HSP70 prevent stress-induced apoptosis, and may have a transforming potential.
Helicase: An enzyme that unwinds the double DNA helix near the replication fork before DNA polymerase acts on it. Replication fork moves from 3' to 5' of the leading strand. Unwinding is also necessary for DNA repair. Mutations in the helicase genes on chromosome 2q and 19q are one group of causes of the DNA repair defect xeroderma pigmentosum (an autosomal recessive disease). See also primosome.
Hemizygous: As in any X-linked trait in males, absence of a homologous counterpart for an allele. It may also result from deletion.
hepatotoxic - Toxic to the liver.
Herbivore: A plant-eating animal. Sea urchins re a good example of a marine herbivore as they feed on kelp. See also heterotroph, primary consumer.
Heritability: The proportion of the total phenotypic variance that is attributable to genetic causes (h2= genetic variance / total phenotypic variance).
Hermaphrodite: An animal that has both male and female reproductive organs. Nudibranchs (sea slugs) are a good example o a hermaphrodite.
Hermaphroditism: Having both male and female sexual organs in one individual. Most invertebrates and plants are hermaphrodites. Union of the gametes of the same individual (self-fertilization) is the most extreme example of inbreeding.
Heterogametic sex: The sex which has the two different sex chromosomes (XY). Human and Drosophila males are the heterogametic sex, whereas, in birds, moths, some fish and amphibians, females are the heterogametic sex (ZW).
Heterogeneous nuclear RNA (hnRNA): RNA products immediately synthesized from the DNA template in the nucleus (sometimes called DNA-like RNA or dRNA). This RNA species has a short half-life, is very heterogeneous and very large (molecular weight in excess of 107). hnRNA molecules are processed to generate the mRNA molecules (molecular weight generally less than 2x106) before leaving the nucleus.
Heterokaryon: A cell containing more than one genetically different nuclei. Naturally occurs in fungi as long as their fungal (heterokaryon) incompatibility types are identical (see also dikaryotic).
Heterothallic: Organisms (fungi, algae, plants) which can only undergo sexual reproduction with another bearing a different mating/compatibility type (self- incompatible). See also homothallic.
Heterothallism: Sexual cycle in dinoflagellates which involves opposite mating types; e.g. Gymnodinium catenatum.
Heterotroph: An organism that is unable to synthesize organic compounds (and thus its energy) from the environment and therefore fulfils its energy requirements by feeding on other organisms or organic matter.
Heterotrophy: Nutritional mode in which absorption of organic matter is required for growth, metabolism and reproduction; e.g. auxotrophy, mixotrophy, myzocytosis, phagotrophy and organotrophy.
Heterozygosity: Presence of two different alleles at a locus in a diploid organism (see homozygosity). It is the result of inheritance of different alleles from parents.
Hfr: A male bacterial cell that has the F factor integrated into its chromosome is an Hfr (high frequency of recombination) cell. Crosses between Hfr cells and F- females produce far more recombinant progeny than do crosses between F+ males and F- females.
Histones: Basic proteins that are involved in the packing of DNA. They bind to the phosphate groups of DNA. There are five major types of histone proteins.
HLA complex: The human major histocompatibility complex (MHC). An HLA haplotype has been totally sequenced in 1999.
Holandric gene: A gene carried on the Y chromosome and therefore transmitted from father to son.
Holdfast: organ of a benthic alga that attaches the alga to the seafloor
Holoplankton: Organisms living their entire life cycle in the floating (planktonic) state
Homeobox: Conserved protein sequence which forms a DNA-binding domain in a class of transcription factors.
Hominid: A member of the Hominidae family.
Homogeneous: Similar or uniform structure or composition throughout.
Homologous chromosomes: Chromosomes that occur in pairs one having come from the male parent and the other from the female parent. They pair participate in crossing-over during meiosis. Homologous chromosomes contain the same array of genes but may contain different alleles at those loci.
Homology: Two structures are considered homologous when they are inherited from a common ancestor which possessed the structure. This may be difficult to determine when the structure has been modified through descent.
Homothallic: Organisms (fungi, algae, plants) which can undergo sexual reproduction with a similar strain including the self (self-compatible) (see also heterothallic).
Homozygosity: Presence of two identical alleles at a locus in a diploid organism (see heterozygosity). It is the result of inheritance of identical alleles from both parents.
horn - In armoured cells an extension of the apical or antapical plates.
House-keeping genes: Genes which are constitutively expressed in most cells because they provide basic functions.
Htf island: Hpa Tiny Fragment island which are unmeth island which are unmethylated CpG-rich regions in the genome. Eighty percent of these occur at or near genes, particularly housekeeping genes. Many of the MHC genes discovered were not near Htf islands.
Hybrid vigor (heterosis): Unusual growth, strength, and health of heterozygous offspring from two less vigorous homozygous parents.
Hybrid: The offspring of two distinct species.
Hybridization: The specific reassociation of complementary strands of nucleic acids.
Hydrozoa: A class of phylum Cnidaria. The primary life stage is nonmotile and has a sac-like body composed of two layers of cells and a mouth that opens directly into the body cavity. A second life stage, the free-living medusa, often resembles the common jellyfish.
Hyper saline: Water with a salt concentration over 40l.
Hypnozygote: A thick-walled zygote formed following fusion of two motile gametes (diploid-2N).
Hypotheca: The posterior part of a dinokont-type cell below the cingulum.
Hypothesis: A concept or idea that can be falsified by various scientific methods. An unproven but testable scientific proposition. A theory is a statement with some confirmation.
Hypotrich: A protozoan of the Ciliate order which reproduces sexually or by asexual binary division. Sexual reproduction can be via conjugation (nuclear exchange) or gamete fusion. In the former, multiple mating types are involved. In gametic fusion there are only two types one of which is the only source of intracellular organelles (see also mating type).
hystrichosphere - A fossilized dinoflagellate cyst.