alkyl group a univalent radical having the general formula CnH2n+1 derived from a saturated aliphatic hydrocarbon by removal of one atom of hydrogen. Named by replacing the ending -ane of the hydro-carbon with -yl (e.g., methane becomes methyl). allantois a saclike outgrowth of the ventral side of the hindgut present in the embryos of reptiles, birds, and mammals. The allantois represents a large and precocious development of the urinary bladder.
allatum hormones hormones synthesized by the insect corpus allatum. The titer of allatum hormones influences the qualitative properties of each molt in holometabolous insects. At high concentrations, lar-val development ensues; at lower levels, the insect undergoes pupal metamorphosis, and in the absence of the allatum hormones adult differentiation takes place. The allatum hormones thus have a juveniliz- ing action and for this reason have also been called juvenile hormones (JHs). The structural formulas for three of the juvenile hormones are illustrated on page 14. In adult females, the allatum hormone is required for vitellogenesis. The JH analog, ZR515 (q.v.), is often used as a substitute for natural JHs in Drosophila experiments. See Appendix C, 1966, Roller et al.; ring gland, status quo hormones.
allele a shorthand form of allelomorph, one of a series of possible alternative forms of a given gene (cistron, q.v.), differing in DNA sequence, and af- fecting the functioning of a single product (RNAand/or protein). If more than two alleles have been identified in a population, the locus is said to show multiple allelism. See heteroallele, homoallele, isoal-lele, null allele, silent allele. allele-specific oligonucleotide testing a tech-nique used to identify a specific mutation in a collec- tion of DNA fragments isolated from a mutant or- ganism. An oligonucleotide is synthesized that has a base sequence complementary to the segment under study, and it is used as a probe. All segments binding to the probe are then collected and analyzed.
allelic complementation the production of nearly normal phenotype in an organism carrying two dif- ferent mutant alleles in trans configuration. Such complementation is sometimes caused by the recon- struction in the cytoplasm of a functional protein from the inactive products of the two alleles. When such a phenomenon can be demonstrated by mixing extracts from individuals homozygous for each al- lele, the term in vitro complementation is used. Syn- onymous with intra-allelic complementation. See Appendix C, 1963, Schlesinger and Levinthal; alco- hol dehydrogenase, alkaline phosphatase, transvec- tion.
allelic exclusion the situation in a diploid nucleus where either the parental or the maternal allele, but not both, is expressed, even though both parental alleles are capable of being transcribed and may even be identical. This situation is seen during recombina-
tion within the segmented Ig genes of immature lymphocytes. In any one B lymphocyte (q.v.), a light chain or heavy chain can be synthesized from a ma- ternal or paternal homolog, not both. See immuno-globulin genes, isotype exclusion, somatic recombina- tion.
allelic frequency the percentage of all alleles at a given locus in a population gene pool represented by a particular allele. For example, in a population containing 20 AA, 10 Aa, and 5 aa, the frequency of the A allele is [2(20) + 1(10)]/2(35) = 5/7 = 0.714. See gene frequency.
allelism test complementation test (q.v.).
allelomorph commonly shortened to allele (q.v.).
See Appendix C, 1900, Bateson.
allelopathy an interaction involving two different species in which chemicals introduced into the en-vironment by one suppress the growth or reproduc- tion of the other.
allelotype the frequency of alleles in a breeding population.
allergen a substance inducing hypersensitivity.
allergy an immune hypersensitivity response to an agent that is nonantigenic to most of the individuals in a population. allesthetic trait any individual characteristic that has an adaptive function only via the nervous sys- tems of other organisms, for example, odors, display of color patterns, mating calls, etc., which are im- portant components of courtship in various species. See courtship ritual, pheromone. Allium the genus that includes A. cepa, the onion; A. porrum, the leek; A. sativum, the garlic; and A. schoenoprasum, the chive—all classic subjects for cy- tological studies of mitotic chromosomes.
alloantigen an antigen (q.v.) that elicits an im- mune response (q.v.) when introduced into a geneti- cally different individual of the same species. Anti- bodies produced in response to alloantigens are called alloantibodies. See histocompatibility mole- cules. allochromacy the formation of other coloring agents from a given dye that is unstable in solution. Nile blue (q.v.) exhibits allochromacy.
allocycly a term referring to differences in the coiling behavior shown by chromosomal segments or whole chromosomes. Allocyclic behavior character- izes the pericentric heterochromatin, the nucleolus organizer, and in some species entire sex chromo- somes. If a chromosome or chromosomal segment is tightly condensed in comparison with the rest of the chromosomal complement, the chromosome or chromosomal segment is said to show positive hetero- pycnosis (q.v.).
Allocycly is also used to describe asynchronous separation of bivalents during the first anaphase in meiosis. In man, for example, the X and Y chromosomes segregate ahead of the autosomes and are said to show positive allocycly.
allogeneic disease See graft-versus-host reaction. allogeneic graft a graft of tissue between geneti- cally different members of the same species, espe- cially with regard to alloantigens (q.v.). See allograft, heterograft. Compare with xenograft. allograft a graft of tissue from a donor of one ge- notype to a host of a different genotype but of the same species. allolactose See lactose.
allometry the relation between the rate of growth of a part of an individual and the growth rate of the whole or of another part. In the case of isometry, the relative proportions of the body parts remain constant as the individual grows; in all other cases, the relative proportions change as total body size in- creases. See heterauxesis. allomone any chemical secreted by an organism that influences behavior in a member of another spe- cies, benefiting only the producer. If both species benefit, it is a synamone. If only the receiver benefits, it is a kairomone.
alloparapatric speciation a mode of gradual spe- ciation in which new species originate through pop- ulations that are initially allopatric, but later become parapatric before completely effective reproductive isolation has evolved. Natural selection may enhance incipient reproductive isolating mechanisms in the zone of contact by character displacement (q.v.), and other mechanisms.
Compare with parapatric spe- ciation. allopatric speciation the development of distinct species through differentiation of populations in geographic isolation. Such populations are called al- lopatric. allopatry referring to species living in different geographic locations and separated by distance alone or by some barrier to migration such as a mountain range, river, or desert. Compare with sympatry.