stocks (Pisum is self-fertile) pure-breeding for one or more characters; stocks which could be selfed and crossed in large numbers, reciprocally when necessary, whose seeds and offspring could be scored for several pairs of contrasting characters used. He carried out both monohybrid and dihybrid crosses. Since, as is now realized, the pairs of characters studied were determined by unlinked loci, he was able to obtain offspring in ratios enabling the subsequent formulation of laws of inheritance. His greatest conceptual innovation was to regard heritable factors determining characters as atomistic and material particles which neither fused nor blended with one another ~ a conclusion inescapable in the light of the experimental results he obtained. Mendel was elected Abbot of Brno monastery in 1868. His
century, after the 'rediscovery' of his laws.
Mendelian heredity (m. inheritance, mendelism). The view, expressed here in modern terminology, that in eukaryotic genomes alleles segregate (separate into different nuclei) during he iosi s, after which any-member of a pair of alleles has equal probability of finding itself in a nucleus with either of the members of any other pair (if the loci are unlinked). As a result of chromosome behaviour during meiosis and fertilization, and of dominance and recessiveness among characters, ratios of characters among offspring phenotypes are predictable, given knowledge of the parental genotypes.
men del was unaware of the genetic role of chromosomes (see G en i , w e i Smann), and studied inheritance of variation determined by unlinked allelic differences of major effect. He was also unaware of polygenic Inheritance and linkage, both of which liable to' cause departures from Mendelian ratios in breeding work. Linkage provides a clear exception to the law of independent assortment (see men del's la ws). Mendelian ratios may also be distorted by MUTATION, SEX-LINKAGE, MEIOTIC DRIVE, CYTOPLASMIC inheritance, maternal "effect, EPISTASIS and by selection among embryos or gamete types, male HA PLOI D Y will also result in distortions; but even here, as with sex-linkage, alleles behave in a basically' Mendelian way. It is simply that the genetic system produces non-Mendelian ratios in breeding work.
Mendel's Laws. This account of the laws uses terminology which Mendel did not employ himself.
First Law, of Segregation: during meiosis, the two members of any pair of alleles possessed by an individual separate (segregate) into different gametes and subsequently into different offspring, neither having blended with nor altered the other in any way while together in the same cell (but see gene conversion) . The law asserts that alleles retain their integrities (barring mutation) during replication from generation to generation. None of the cells normally produced , by meiosis contains two alleles from any locus.
Second Law, of Independent Assortment: asserts that during meiosis 1 all combinations of alleles are distributed, to daughtei nuclei with equal' probability, distribution of 'members of one pair having no " influence on the distribution of members of any other pair. It holds, in effect, that during meiosis random reassortment occurs between alleles at different loci. Thus if one locus is represented by the two alleles A and a, while another locus is represented by alleles B and b, then all four haploid nuclei AB, aB, Ab and ab will be formed in 'equal frequency by meiosis. Mendel's second law is refuted by l i n k - , age. Genes at linked loci tend to 'retain their linear Sequences on chromosomes during meiosis and therefore tend to be inherited as blocks (but see crossover value).
somes during meiosis. His second law is a consequence of the inde. pendent behaviour of non-homologous chromosomes,during meiosis.
meninges. Three membranous coverings of the vertebrate brain, spinal cord, and spinal nerves as far as their exits from between the vertebrae.- Innermost is the vascular pia mater, Separated from the arachnoid by the fluid-filled arachnoid space's in which a fine web of fibres and villi reabsorb the cerebrospinal fluid back into the venous system. Outermost membrane, the dura hater, is a thick, fibrous membrane lining the skull atnd separated from the arachnoid below it by the dural sinus draining blood from the brain. The pia mater supplies capillaries to the ventricles of the btain.
Menstrual cycle. See Fig. 47. Modified oestrous cyc.j£ of catarrhine primates; characterized by sudden breakdown of uterine endometrium, producing bleeding (menstruation), and by absence of a period of heat (oestrous) in which the animal is particularly sexually receptive. Controlled and coordinated nervously and hormonally, in particular by the hypothalamus and pituitary gland and the gonads. A mature human cycle has the following main sequence of events: a , hypothalamic releasing factor (FS H-RF) causes release of follicle-stimulating hormone from the pituitary, initiating growth of one graafian follicle in the ovaries and its consequent increased production of oestrogens, especially oestradiol. High oestrogen levels eventually inhibit release of both hypothalamic FSH- RFand LH-RF, decreasing FSH and LH output; but about two days prior to ovulation a. marked rise in output of. pituitary lute I NIZING hormone occurs. Through a positive feedback mechanism, rising levels of oestrogens induce LH release (via LH R F) from the pituitary, which has increased sensitivity at this time. A mid-cycle surge of L H induces rupture of the Graafian follicle (ovulation) with release of the egg and development of the Graafian follicle into a c cr pus l uteu m, which secretes oestradiol and increasing amounts of progesterone, which maintains the vascularization of the
uterine endometrium begun by oestrogens. Progesterone is secreted only so long as small amounts of pituitary LH maintain the corpus luteum, about 10-12 days in women unless pregnancy occurs. With atrophy of the corpus luteum, progesterone and oestradiol levels drop sharply, causing menstruation - the shedding of the uterine lining each month in the absence of pregnancy. If there is no pregnancy, the cycle repeats immediately as hypothalamic F S H > RF is released again, having been inhibited by the negative feedback effect of steroid sex hormones. If pregnancy occurs, maintenance of the corpus luteum is sustained for a short period by LH, then by HCG (human chorionic gonadotrophin) secreted by the implanted blastocyst and later by the placenta. See contracepTIVE PILL. MATURATION OF GERM CELLS.
MERICARP. Single-seeded portion of SCHI zocar p.
MERISTELE. individual vascular'unit of dicty ostele .
MERISTEM. Localized region of active mitotic cell division in plants, from which permanent tissue is derived. New cells formed by activity of a meristem become variously modified to form characteristic tissues of the adult (e.g. epidermis, cortex, vascular tissue, etc.), A meristem may have its origin in a single cell (e.g. in ferns), or in a group of cells (e.g. in flowering plants). The principal meristems in latter group occur at tips of stems and roots (apical meristems, or growing points), between xylem and phloem of vascular bundles (ca m b i u m) in cortex (ccr K ca mb I u m) , in young leaves and (e.g. in many grasses) at bases of internodes (intercalary meristems). Meristems may also arise in response to wounding._
MERISTEMATIC ACTIVITY. State of active mitosis in a meristem.
Outer meristematic cell layer in certain brown algae.
Merocrine gland. Gland whose cells secrete their product while remaining intact: no portion is pinched off (see a po c r i n e gland), nor do cells have to disintegrate in order to release their product (see h OLOCRINE gland) . Examples include vertebrate salivary glands and exocrine cells of the pancreas.
Meroplanktonic. Term describing organisms that spend part of their life cycle in the plankton and part in the benthos -as a resting stage.
Merostomata. Class of aquatic arthropods (formerly the order Xiphosura of the class Arachnida). Includes the extinct eury -pter ida. The only living forms (king crabs, e.g. Limulus) have a broad cephalothorax (prosoma) covered dorsally by carapace which covers limbs and in which is located a pair of eyes. Chelicerae chelate. Pedipalps resemble walking limbs. Gnathobases (spiny basal segments of legs) function as mandibles, which are absent. Opis-thosoma represented by fused tergites forming a single dorsal plate, with a long caudal spine hinged to its posterior border. Gill books serve for gaseous exchange. The trilobite larva of Limulus has cheli-cerae and lacks antennae; however the chelicerate arthropods could have been derived from a pro-trilobite. See arthropoda.
-merous. As a suffix, referring to the number of parts, e.g. corolla pentamerous, consisting of five petals.
Mesarch. Type of maturation of primary xylem from a central point outwards; i.e. the oldest xylem elements (protoxylem) are surrounded by later-forming metaxylem.
Mesencephalon. See midbrain.
Mesenchyme. Embryonic mesoderm comprising widely scattered tissue giving rise to connective tissue, blood, cartilage, bone, etc.
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