GENETICS Glossary and Indexing Instructions Index Section Bibliographic Services Division National Library of Medicine 1968 acrocentric CHROMATIDS acrocentric syn. subtcrmlnal type of chromosome in which the cen- tromere is located near one end. At mctaphase, it has the appearance of a "wishbone" (Illinois Med J 132:31, Jul 67) Index CHROMOSOMES (68) ALLELES (Gl) r 1968 syn, allelomorphs alternative forms of the same gene, occupying the same place (=locus) on homologous chromosomes, governing the some characteristic of the phenotype (eye color, enzyme formation), and always segregating in meiosis (MeSH definition) allelomorphs see ALLELES allocyciy the specific differential staining reactivity of heterochromatin which permits its cytological recognition and distinction from the remaining euchromatic portion of the chromosome (Illinois Med J 132:31, Jul 67) Index CHROMOSOMES (IM) (68) MITOSIS (NIM) (68) anaphase phase of cell division characterized by the movement of chromosomes from the mctaphase plate towards the opposite poles (Illinois Med J 132:32, Jul 67) Index MITOSIS (68) ANEUPLOIDY (Gl) 1968 syn. aneusomy the chromosomal constitution of cells with an addition or subtraction of chromo- somes to or from the normal 6et. Includes monosomy (symbol: 2N-1), TRISOMY (symbol: 2N+1), tetrasomy (symbol: 2N+2) (MeSH definition) aneusomy see ANEUPLOIDY autosome 6yn. heterosomes a non-sex chromosome. Man has twenty- two pairs of autosomes and two sex chromosomes (XX or XY) (Illinois Med J 132:33, Jul 67) Index CHROMOSOMES (68) carrier, female sec female carrier centric fusion a special type of reciprocal trans- location in which each of the involved chromosomes is broken very close to its centromere so that exchange of ■ virtually entire, or even entire chromosome arms takes place ("whole- arm" translocation) (Illinois Med J 132:33, Jul 67) Index CHROMOSOMES (IM) (68) CROSSING OVER (NIM) (68) centromere syn. kinetochore a non-staining apparently structureless constriction visible in chromosomes during prophase, metaphase and anaphase (Illinois Med J 132:34, Jul 67) Index CHROMOSOMES (68) chiasma the point of contact between non-sister chromatids of paired homologous chromosomes, seen during the prophase of the first meiotic division.. It is the cytologic expression of completed genetic crossing over, the latter providing for the exchange of .genic material between two chromosomes before they separate (Illinois Med J 132:34, Jul 67) Index CHROMOSOMES (IM) (68) MEIOSIS (NIM) (68) CROSSING OVER (NIM) (68) chimera an'organism, usually a grafted plant, exhibiting a mixture of genetically different types lying adjacent to one another. This term has been applied to human subjects (twins) possessing more than one blood group: i.e., mixtures of A and 0 cells, who there- fore exhibit erythrocyte mosaicism (Illinois Med J 132:34, Jul 67) Index MOSAICISM (68) CHROMATIDS (All) 1968 the two microscopic subunits of chromosomes. Distinguish from DNA strand (MeSH definition; Illinois Med J 132:34, Jul 67) chromaLin 2 DNA chromatin the substance in nuclei and chromosomes which stain intensely with basic dyes. It is composed of DNA combined with proteins. In the fixed inter- mitotic nucleus, it usually takes the form of an irregular network of long coiled threads. Larger and denser chromatin particles are known as chromocenters (Illinois Med J 132:34, Jul 67) Index CHROMOSOMES (68) chromntin-negative refers to nuclei which lack Barr's sex-chromatin mass. This state is characteristic of the normal human male (Illinois Med J 132:34, Jul 67) Index SEX CHROMATIN (68) chromatin-positive refers to nuclei containing the distinctive sex-chromatin body of Barr. It is present in the normal human female (Illinois Med J 132:34, Jul 67) Index SEX CHROMATIN (68) chromocenter 6yn. sex-chromatin body of Barr these are deeply staining clumps of chromatin material present in inter- mitotic nuclei. A chromocenter of characteristic size and shape lying in contact with the nuclear membrane is present in females of many species, including man(Illinois Med J 132:34, Jul 67) Index SEX CHROMATIN (68) CHROMOSOME ABERRATIONS (Gl) 1968 deviations from the normal number or structure of chromosomes, not necess- arily causing a disease. Structural aberrations include translocations, inversions, deletions, duplications, iso-chromosomes, and ring chromosomes (MeSH definition; Illinois Med J 132:35, Jul 67) CHROMOSOMES (D2) 1968 one of the finite number of small bodies occurring in pairs into which the chromatin material of a cell nucleus resolves itself prior to cell division (Illinois Med J 132:34, Jul 67) clone a group of cells constituting the progeny of asexual reproduction from a single sexually-produced cell. The clone is of importance in mosaicism in which genotypically different ancestral cells, give rise to genetically dissimilar stem-lines of daughter cell6 (Illinois Med J 132:35, Jul 67) Index CLONE CELLS (68) codominant (combinant) refers to alleles whose effects are recognizable side by side, without one's being influenced by the other (Illinois Med J 132:35, Jul 67) Index GENES, DOMINANT (68) CONSANGUINITY (Gl) 1968 blood relationship, this is, possession of common ancestors in the preceding few generations of the ascending line (Illinois Med J 132:35, Jul 67) cross see hybrid CROSSING OVER (GENETICS) (Gl.) 1968 an exchange of segments between chromatids of homologous chromosomes (MeSH definition; Illinois Med J 132:35, Jul 67) deletion • a chromosomal aberration characterized by detachment and loss of a portion , of a chromosome by breakage. In this manner, one or more genes may be I removed from the organism (Illinois Med J 132:35, Jul 67) Index CHROMOSOME ABERRATIONS (68) deletions, chromosomes see CHROMOSOME ABERRATIONS DIPLOIDY (Gl) 1968 the chromosomal constitution of somatic cells: two sets of chromosomes (symbol: 2N) (MeSH definition; Illinois Med J 132:35, Jul 67) DNA (D10) 1968 essential component of the cell nucleus which carries the genetic information (Illinois Med J 132:35, Jul 67) dominant 3 euchromatin dominant this classically refers to a gene which produces its effect despite the presence of an opposite or contrasting gene. Dominance and recessivity are complementary concepts. A gene is not dominant or recessive in itself but only in its behavior with respect to a certain allele. In human genetics, the homozygous state of "dominant" anomalies is usually unknown (Illinois Med J 132:35, Jul 67) Index GENES, DOMINANT (68) dominant, completely applies to a gene whose heterozygous state cannot be distinguished from its homozygous state (Illinois Med- J 132:35, Jul 67) Index GENES, DOMINANT (68) dominant, conditionally applies to a gene only the heterozygous state of which is known, this state being markedly different, however, from the homozygous state of the other allele (Illinois Med J 132:35, Jul 67) Index GENES, DOMINANT (68) dominant, intermediately applies to a gene which has a weaker effect in the heterzygous state than in the homozygous state (the expression has the same meaning as "semidominant, partially dominant, or incompletely dominant") (Illinois Med J 132:35, Jul 67) Index GENES, DOMINANT (68) dominant, irregularly applies to a gene whose heterozygous state is not always recognizable. The greater the irregularity of the gene effect, the more questionable it is whether this is really due to a "dominant" gene and not rather to a more complicated genetic mechanism (Illinois Med J 132:35, Jul 67) Index GENES, DOMINANT (68) duplication a type of chromosomal aberration characterized by the presence of an extra segment of chromosome which may exist as a separate fragment, ol may be attached to one of the members of the ordinary chromosomal complement. This is to be distinguished from aneupioidy and polyploidy, in which variations in somatic chromosome numbers are irregular or exact multiples, respectively, of the basic chromosomal number (Illinois Med J 132:35, Jul 67) Index CHROMOSOME ABERRATIONS (68) equationai division the second meiotic division, essenti- ally mitotic in type, characterized by the separation of sister chromatids,* The latter are genetically identical since they are longitudinally-split reduplications of individual .chromosomes (Illinois Med J t32:36, Jul 67) Index MEIOSIS (68) equatorial plate syn. metaphase plate a circular disc-like area situated at right angles to the spindle fibres of the dividing cell. On it are arranged the chromosomes preparatory to their separation (in meiosis I) or to the separation of their chromatid (potential daughter chromosomes) in • mitosis, and in meiosis II. (Illinois Med J 132:36, Jul 67) Index MITOSIS (68) euchromatin the chromatin material composing a chromosomal segment which exhibits no variation in staining intensity (iso-pyenosis) or in condensation at any time during the mitotic cycle. Most if not all genes directly res- ponsible for hereditary transmission are believed to be situated in this type of chromatin. Contrasted with heterochrony tin (Illinois Med J 132:36, Jul 67) Index CHROMDSOMES (68) euploidy 4 gynandromorphism euploidy a state in which there is a balanced set of chromosomes. This includes variations in the somatic chromosomal number, characterized by the presence of the exact multiples of the basic number of the species. Accordingly, these may be the haploid (monoploid), diploid, triploid, tetraploid and higher multiple numbers. Complements higher than diploids are called polyploids (Illinois Med J 132:36, Jul 67) Index GENES (68) expressivity the regularity in different individuals with which a gene produces a trait. Fluctuation in the degree of distinct- ness of a feature are explained by the "expressivity of the responsible gene." Low expressivity (low distinct- ness of a feature) passes continuously into "missing penetrance" (the feature is no longer recognizable) (Illinois Med J 132:36, Jul 67) Index GENOTYPE (IM) (68) GENES (IM) (68) female carrier a woman who is heterozygous for a recessive X-chromosomal gene. Half of the sons of a female carrier 6how the recessive X-chromosomal feature (Illinois Med J 132:160, Aug 67) Index SEX CHROMOSOMES (IM) (68) GENOTYPE (NIM) (68) gamete in bisexual organisms, the mature germ cell of either sex (spermatozoa or ova) containing the haploid ("n") number of chromosomes (Illinois Med J 132:160, Aug 67) Index OVUM (IM) (68) SPERMATOZOA (IM) (68) if not specified GERM CELLS (IM) (68) gene mutation a mutation which involves only a single gene (Illinois Med J 132:160, Aug 67) Index GENES (IM) (68) MUTATION (IM) (68) gene product the specific protein or polypeptide molecules which depend on the pre- sence of a gene are called primary gene products, frequently enzymes. Enzymes can synthesize most* of the other molecules, such as complex polysaccharides,lipids, and even deoxy- ribonucleic acid. However, enzymes cannot synthesize proteins. For this, v ribonucleic acid is required, which translates the information contained in the genes into the synthesis of the gene products (Illinois Med J 132:160, Aug 67) Index GENES (IM) (68) GENETICS, BIOCHEMICAL (NIM) (68) or specific enzyme or other chemical GENES (Gl) 1968 the basic ultramicroscopic intra- molecular units of heredity, arranged linearly at definite points (loci) in a chromosome. They occur in pairs (allelic genes) situated at correspond- ing loci of a pair of homologous chromosomes and are capable of pro- ducing externally visible body traits composing the phenotype. Apparently, genes function by controlling enzymatic 'action (Illinois Med J 132:160,' Aug 67) genomic mutation a mutation which alters the normal numbe'r of chromosomes of an individual. If the result of this is a whole multiple of the haploid set of the chromosomes, then one also speaks of "ploidic mutation" (Illinois Med J 132:160, Aug 67) Index MUTATION (IM) (68) CHROMOSOME ABERRATION (IM) (68) GENOTYPE (Gl) 1968 the genetic constitution of the individual; the characterization of the genes (MeSH definition; Illinois Med J 132:160, Aug 67) gynandromorphism the condition of sexual mosaicism in an organism of a bisexual species (Illinois Med J 132:160, Aug 67) Index MOSAICISM (IM) (68) SEX CHROMOSOMES (NIM) (68) HAPLOIDY 5 holandric HAPLOIDY (Gl) 1968 the chromosomal constitution of gametes (in humans spermatozoon and ovum): one set of chromosomes (symbol: N) (MeSH definition; Illinois Med J 132:164, Aug 67) hemizygous normal diploid cells, with their pairs of homologous chromosomes and genes, are either homozygous or heterozygous for each pair of genes, depending on whether the alleles of a pair are identical or contrasting. The hemi- zygous condition refers to the presence of an unpaired gene or chromosome. Hemizygosity, as an abnormality, exists in monosomic cells in which only one member of a pair of homologous chromosomes is present (Illinois Med J 132:161, Aug 67) Index CHROMOSOME ABERRATIONS (IM) (68) HAPLOIDY (NIM) (68) hermaphrodite, pseudo- see pseudo-hermaphrodite hermaphrodite, true an intersex that has gonadal tissue of both sexes present (Illinois Med J 132:161 Aug 67) Index HERMAPHRODITISM (68) HERMAPHRODITISM (C6,C7) 1968 the intersexual condition in which male and female genital structures exist in an individual of a bisexual species (Illinois Med J 132:161, Aug 67) heterochromatln the chromatin material occupying a chromosomal segment which exhibits variations in condensation and in staining intensity (heteropyenosis). Heterochromatln is generally believed to be lacking in genes directly con- cerned with hereditary transmission, although it probably exerts a modifying effect on near-by genes in the cuchromatic zone (Illinois Med J 132:162, Aug 67) Index CHROMOSOMES (68) he te ro game t i c this refers to the XY or XO sex which produces two kinds of garnets (X, and Y or 0) in equal numbera, whereas the XX series is said to be homogametic, since it produces only one kind of gamete (X) (Illinois Med J 132:162, Aug 67) Index SEX CHROMOSOMES (68) heteropyenosis the phenomenon exhibited by the heterochromatic region of a chromosome, characterized by variations in the degree of condensation and in the intensity of staining (Illinois Med J 132:162, Aug 67) Index CHROMATIN (68) heteropyenosis, negative occurs when the heterochromatic zone appears thinner and more faintly stained than the remainder of the chromosome (Illinois Med J 132:162, Aug 67) Index CHROMATIN (68) heteropyenosis, positive occurs when the heterochromatic segment appears thicker and stains .more intensely than the rest of the chromosome (Illinois Med J 132:162, Aug 67) Index CHROMATIN (68) heterosomes see autosome heterozygous this refers to a zygote having a pair of dissimilar or contrasting alleles controlling a certain trait or characteristic (Illinois Med J 132:162, Aug 67) Index HETEROZYGOTE (68) holandric this refers to genes situated in that portion of the Y-chromosome which does not pair with the homologous segment of the X-chromosome (Illinois Med J 132:162, Aug 67) Index SEX CHROMOSOMES (68) hologynic 6 isoalleles hologynic Inheritance from a woman to all female descendants. This occurs in Drosophila as a consequence of non-disjunction of the X-chromosome. In man, hologynic heredity is not known (Illinois Med J 132:162, Aug 67) Index PHENOTYPE (68) homogamy occurs if chance does not prevail in matings within a population, but if like genes are joined with higher-than-chance frequency through mating (Illinois Med J 132:162, Aug 67) Index BREEDING (68) homozygous this refers to a zygote having two members of a pair of genes alike for a certain trait or characteristic (Illinois Med J 132:162, Aug 67) Index HOMOZYGOTE (68) human genetics .see GENETICS, HUMAN hybrid syn. cross the progeny of two parents who differ genetically in any way (Illinois Med J.132:162, Aug 67) Index HYBRIDIZATION (68) hypcrploid the aneuploid condition in which there is more than the normal diploid number of chromosomes, as exemplified by 2"n" plus 1, or 2"n" plus 2 in' man with 47 or 48 chromosomes (Illinois Med J 132:162, Aug 67) Index ANEUPLOIDY (68) hypoploid the aneuploid condition in which there is less than the normal diploid number of chromosomes, for example,^ the 2"n" minus 1 state in man with 45 chromosomes (Illinois Med J 132:162, Aug 67) Index ANEUPLOIDY (68) idiogram a diagrammatic rrproscntntion of the chromosome constitution of the cells of the individual. Customarily, the chromosomes arc systematically arranged according to size and centro- mere position (Illinois Med J 132:163, Aug 67) Index KARYOTYPING (68) intermitotic syn. interphase the state of the resting nucleus, i.e., when the cell is not undergoing division. Chromosomes, as such, are not visible during this phase. Actually, the nucleus is not "resting" since biochemical activity is at its height at this time (Illinois Med J 132:163, Aug 67) Index MITOSIS (68) interphase see intermitotic intersex (Illinois Med J 132:163, Aug 67) Index HERMAPHRODITISM (68) inversion a structural chromosomal aberration characterized by the rearrangement ' (reversal) of a portion of the gene sequence. It occurs as a result of the breakage of a chromosome at two points followed by a reversal of the replaced fragment with respect to the rest of the chromosomes (Illinois Med J 132:163, Aug 67) Index CHROMOSOME ABERRATIONS (68). inversions, chromosome see CHROMOSOME ABERRATIONS isoalleles alleles which, though so nearly iden- tical that their effects cannot usually be distinguished, may have different modifying effects on genes inherited with them. This may influence the expressivity of the gene so that the disease varies slightly in different generations-in the father the abnormal gene will have been partnered by a different normal allele. The normal isoalleles 7 metaphase alleles in the father and in his children are said to be isoalleles (Illinois Med J 132:163, Aug 67) Index ALLELES (68) isochromosome an abnormal chromosome arising through the misdivision of the centromere. If the centromere misdivides, i.e., in a transverse rather than in a longitudinal plane, each of the two separated centromere fragments remains attached to the chromatids of one arm. Thus, the two new chromosomes (iso- chromosomes) differ from one another (Illinois Med J 132:164, Aug 67) Index CHROMOSOME ABERRATIONS (68) karyotype a group of characteristics (number, size and form) used in identifying a particular chromosome constituion. For purposes of demonstration, a karyotype is a systematized array of the meta- phase chromosomes of a single cell, typifying the chromosomes of an indi- vidual. It is customary to arrange the chromosomes in descending order of size, and according to the position of their centromeres (Illinois Med J 132:164, Aug 67) Index KARYOTYPING (68) kinetochore see centromere lethal factors disorders of the genetic material .. which leads to the death of the zygote, or of the individual developing from it before attaining the age of repro- duction. This can be a matter of lethal genes or deficiencies due to missing sections (Illinois Med J 132:164, Aug 67) Index GENES, LETHAL (68) lethal gene a mutant gene which acts as a lethal factor (Illinois Med J 132:164, Aug 67) Index GENES, LETHAL (68) linkage this term applies to two different genetic phenomena: linkage between a gene and a chromosome Index GENES (IM) (68) CHROMOSOMES (IM) (68) and linkage between two genes (Illinois Med J 132:164, Aug 67) Index GENES (68) linkage non-allelic genes which lie in the same pair of chromosomes arc called linked. If they are in the same^ chromosome they are transmitted'to- gether to the descendents (exception: crossing over); if they lie in two different chromosomes of a homologous pair, a child obtains either one or the other gene (exception:crossing over has the result that a child may re- ceive both genes). "Crossing over" is so frequent that no great practical significance Is attached to linkage (Illinois Med J 132:164, Aug 67) Index GENES (68) median syn. metagentric this refers to the location of the centromere of a chromosome midway between the two extremities. The arms of the chromosomes are therefore of equal length (Illinois Med J 132:164, Aug 67) Index CHROMOSOMES (68) MEIOSIS (Gl) 1968 a special method of cell division, occurring in maturation of the sex cells, by means of which each daughter nucleus receives half the number of chromosomes characteristic of the somatic cells of the species (MeSH definition; Illinois Med J 132:164, Aug 67) metagentric see median metaphase phase of cell division characterized by complete disappearance of the nuclear membrane and by the formation- of the spindle. At this time, the chromosomes are maximally contracted and are arranged on the equatorial plate preparatory to separation. In mitosis, the metaphase chromosome has a single centromere which becomes functionally doubleonly at anaphase. In the first meiotic division each bivalent has two centromeres. In the second meiotic division, metaphase is similiar to that of ordinary mitosis (Illinois Med J 132:165, Aug 67) Index CELL DIVISION (68) metaphase plate 8 phene, phenic metaphase plate see equatorial plate MITOSIS (Gl) 1968 indirect division of a cell, consisting of a complex of various processes, by means of which the two daughter nuclei normally receive Identical complements of the number of chromo- somes characteristic of the somatic cells of the species (MeSH definition) monosomic an aneuploid individual in whom there is a loss of one of the two members of a pair of homologous chromosomes* When it occurs spontaneously, this condition results from the meiotic non-disjunction, i.e., failure of the members of a pair to separate at anaphase (Illinois Med J 132:166, Aug 67) Index ANEUPLOIDY (68) mosaic the presence of genetically dissimilar cells in adjacent tissue of an organism (Illinois Med J 132:166, Aug 67) Index MOSAICISM (68) MUTATION (Gl) 1968 a change in a gene. The mutant gene is capable of transmission and thus of producing hereditary variations when it is present in germ cells. Mutation may also take place in somatic cells, in which case the mutant gene cannot be sexually transmitted. Mutant genes are usually recessive to the wild type (Illinois Med J 132:166, Aug 67) mutation, gene see gene mutation 2N see DIPLOIDY 3N see POLYPLOIDY 4N see POLYPLOIDY negative heteropyenosis 6ee heteropyenosis, negative non-disjunction a type of abnormal chromosomal behavior during nuclear division. It may occur in mitosis or in meiosis. As a result, one daughter cell receives both chromosomes while the other aquires none (Illinois Med J 132:166, Aug 67) Index CHROMOSOMES (IM) (68) CELL DIVISION (NIM) (68) non-disjunction, primary occurs in one or both parents, with the production of gametes containing an abnormal chromosomal constitution (Illinois Med J 132:166, Aug 67) Index CHROMOSOMES (IM) (68) CELL DIVISION (NIM) (68) non-disjunction, secondary refers to the occurrence of non- disjunction during meiosis in an individual who is abnormal from the beginning, being a product of %\ie union of gametes, one of which "was non-disjunctional (Illinois Med J 132:166, Aug 67) Index CHROMOSOMES (IM) (68) CELL DIVISION (NIM) (68) penetrance the regularity with which a gene pro- duces its effect. Penetrance is "com- plete" if a dominant gene produces its effect in each person carrying it, or if a recessive gene causes its effect in each individual homozygous'for it. It is "reduced" if some but not all persons exhibit the trait even though the gene is present in the required heterozygous or homozygous stage (Illinois Med J 132:166, Aug 67) Index GENES (68) phene, phenic the features which are assigned to certain genes or configuration of genes are called phenes. The expression is not equivalent to phenotype (Illinois Med J 132:166, Aug 67) Index PHENOTYPE (68) phenocopy 9 polysomic phenocopy a reproduction or copy of a gene- producing phenotype which is visibly indistinguishable from it. It occurs in the offspring of pure normal indi- viduals as a non-hereditary modification produced by certain experiemental conditions (Illinois Med J 132:166, Aug 67) Index PHENOTYPE (68) phenogenetics the science which attempts to explain the chain of causality between geno- type and phenotype. Phenogenetics is more a part of developmental physiology than of genetics (Illinois Med J 132:168, Aug 67) Index GENETICS (68) PHENOTYPE (Gl) 1968 the outward appearance of the indi- vidual; the product of interactions between genes and between the genotype and the environment (MeSH definition; Illinois Med J 132:168, Aug 67) pleiotropism a gene which determines several characteristics is called pleiotropic or polyphenic. A gene might have a major effect, such as that controlling the production of a blood group antigen and also play a minor role in pre- disposing to duodenal ulcer (Illinois Med J 132:168, Aug 67) Index GENES (68) ploidic mutation see genomic mutation point mutation a mutation which is not accompanied by a morphologically demonstrable change in the chromosome. In practice, it may be regarded as equivalent to a gene mutation (Illinois Med J 132:168, Aug 67) Index MUTATION (68) polygenic a feature, usually a quantitatively variable character, is polygenic if it is dependent on numerous genes. Most quantitatively variable physiological characteristics have polygenic inheri- tance. The involved genes are also designated as polygenes (Illinois Med J 132:168, Aug 67) Index PHENOTYPE (IM) (68) GENES (NIM) (68) polymerism (genetics) polygenic effect in which the parti- cipating genes are of equally strong effectiveness (Illinois Med J 132:168, Aug 67) Index PHENOTYPE (IM) (68) GENES (NIM) (68) polymorphism the simultaneous occurrence of two or more different genotypes in the population whose ratio Is kept in equilibrium by means of selection (Illinois Med J 132:168, Aug 67) Index GENOTYPE (68) polyphenin (Illinois Med J 132:169, Aug 67) Index GENES (68) POLYPLOIDY (Gl) 1968 the chromosomal constitution of a cell containing multiples of the normal chromosomal set; includes triploidy (symbol: 3N), tetraploidy (symbol: 4N) etc. (MeSH definition; Illinois Med J 132:168, Aug 67) polysomic an individual, an aneuploid in whom one or more chromosomes are redupli- cated and are found represented three or four times. The abnormal number of chromosomes results from meiotic non-disjunction. The resulting zygote has a 2"n" plus 1 chromosomal consti-^ tution. Such a polysomic oi0anisra is also designated trisomic for the non- disjunctional chromosome (Illinois Med J 132:168, Aug 67) Index ANEUPLOIDY (68) population genetics 10 segregation population genetics see GENETICS, POPULATION positive heteropyenosis see heteropyenosis, positive primary non-disjunction see non-disjunction, primary proband syn. propositus the index patient, an individual with an abnormality whose relatives are studied in an effort to ascertain the hereditary or genetic aspects of the trait (Illinois Med J 132:168, Aug 67) Index GENOTYPE (68) prophase the first phase of cell division characterized by the transformation of the irregular network of chromatin material of the interphase nucleus into spirally coiled threads, which grad- ually become doubled, shortened, and thickened to form individual chromo- somes (Illinois Med J 132:168, Aug 67) Index CELL DIVISION (68) propositus see proband pseudo-hermaphrodite an intersex that has the gonads containing either testicular or ovarian tissue respectively, but not both (Illinois Med J 132:161, Aug 67) Index HERMAPHRODITISM (68) recessive this refers to a gene which fails to produce its effect in the presence of the opposite or contrasting gene. A recessive gene produces its effect only when both members of a pair are recessive, i.e., when the*individual is homozygous for this particular ^ene, having received it from both parents (Illinois Med J 132:168, Aug 67) Index GENES, RECESSIVE (68) recombination the union of two members of a pair of genes for a given character. This occurs at fertilization, each gamete contributing its single member of a pair of genes (Illinois Med J 132:168, Aug 67) Index REC0M3INATI0N, GENETIC (68) reductional division the first meiotic division, character- ized by the mere separation of member^ of homologous pairs of chromosomes, resulting in a reduction in the chromosome number from the somatic or diploid number to the haploid number (Illinois Med J 132:169, Aug 67) Index MEIOSIS (68) ring chromosome see CHROMOSOME ABERRATIONS satellite a small rounded body attached to the end of certain chromosomes by a deli- cate thread of chromatin. Satellites" and their filaments are constant in their size and form for each particular chromosome. Therefore, they provide landmarks?for the identification of these chromosomes (Illinois Med J 132:169., Aug 67) Index CHROMOSOMES (68) secondary non-disjunction see non-disjunction, secondary segregation the process in sexual organisms, constituting the Mendelian first law of inheritance, by which only one of the two genes of a pair gets into each mature germ cell. Then, when gametes unite at fertilization, two genes for each trait or character are brought together in the new individual. Segre- gation takes place during the reduc- tional stage of meiosis (Illinois Med J 132:169, Aug 67) Index MEIOSIS (68) sex-chromatin body of Barr 11 syngamy sex-chromatin body of Barr see chromocenter SEX CHROMOSOMES (All) 1968 a pair of chromosomes determining the sex of the individual. One sex has 2 equal sex chromosomes (XX in the human and Drosophila female), while the other has different sex chromosomes (XY in the human and Drosophila male). Distinguish from SEX CHROMATIN (MeSH definition; Illinois Med J 132:169, Aug 67) sex-influenced this refers to genes which are dominant in one sex and recessive in the other, such as those for baldness, and for one form of white forelock (Illinois Med J 132:169, Aug 67) Index SEX CHROMOSOMES (IM) (68) GENES (NIM) (68) sex-limited this refers to genes, the penetrance of which is completely reduced in one sex, i.e., they are capable of produc- ing traits in one sex but not in the other.(Illinois Med J 132:169, Aug 67) Index SEX CHROMOSOMES (IM) (68) GENES (NIM) (68) sex-linked this refers to genes located in a sex chromosome, X or Y. The best known sex-linked genes in man are those for red-green color blindness, and hemo- phila.(Illinois Med J 132:169, Aug 67) Index SEX CHROMOSOMES (IM) (68) GENES (NIM) . (68) somatic mutation a mutation which involves a somatic cell and its daughter cells, that is, only a part of the organism (Illinois Med J 132:276, Sept 67) Index MUTATION (68) speciricity a gene which can have quantitatively differentiated effects in different individuals has a low "specificity." Since this practically never occurs, the expression can be dispensed with. The high degree of specificity of gene effects is a matter of general ex- perience in human genetics (Illinois Med J 132:276, Sep 67) Index GENES "(68) spindle a fusiform bundle of delicate filaments appearing at the beginning of the metaphase stage of ceil division (Illinois Med J 132:276, Sep 67) Index CELL DIVISION (68) • submedian this refers to the location of the centromere of a chromosome at a point elsewhere than at the center, or near the end (Illinois Med J 132:276, Sep 67) Index CHROMOSOMES (68) subtcrminal' see acrocentric supernumerary chromosome one or more extra chromosomes found inconstantly in wild populations of certain species of animals. They are not homologous to members of the regular set of chromosomes and they apparently exert little influence on the phenotypic effect (Illinois Med J 132:276, Sep 67) Index CHROMOSOMES (68) synapsis the period in meiosis in which the two members of every pair of homo- logous chromosomes are in very precise and intimate apposition (Illinois Med J 132:276, Sep 67) Index MEIOSIS (68) syngamy in sexual reproduction, the union of germ cells (gametes) at fertilization to produce a new individual, a zygote (Illinois Med J 132:276, Sep 67) Index GERM CELLS (68) telophase 12 zygote telophase the final phase of cell division during which the chromosomes regroup into a nuclear structure and are gradually transformed into long thread- like fibers* enclosed by a nuclear membrane (Illinois Med J 132:276, Sep 67) Index CELL DIVISION (68) tetraploid an organism containing four complete sets of each kind of chromosome characteristic of the species. Thus, the tetraploid number in man is 92 (Illinois Med J 132:276, Sep 67) Index POLYPLOIDY (68) translocation a type of aberration characterized by fragmentation of a chromosome and transfer of the broken-off portion to another chromosome, often of a different pair (Illinois Med J 132:277, Sep 67) Index CHROMOSOME ABERRATIONS (68) translocations, chromosome see CHROMOSOME ABERRATIONS triploid an organism containing three complete sets of each kind of chromosome characteristic of the species. There- fore, the triploid number in man is 69 (Illinois Med J 132:278, Sep 67) Index POLYPLOIDY (68) TRISOMY (CI6) 1966 numerous congenital anomalies that are the result of the presence of a third chromosome in one of the pairs of the autosomes (trisomy), so that the total number of chromosomes in the karyotype of the affected individual is likewise 47. The three autosomal trlsomes known at the present time are: trisomy 21 or mongolism, with a small acrocentric chromosome in 21; trisomy 13-15 or Dj^ syndrome with a large acrocentric chromosome in 13-15: and trisomy 17-18 or E syndrome with a medium sized acrocentric chromosome in 17-18 (MeSH definition) Index all trisomies TRISOMY (IM) (68) CHROMOSOMES, HUMAN (IM) (68) and specific chromosome number trisomy 13-15 Index TRISOMY (IM) (68) CHROMOSOMES, HUMAN, 13-15 (IM) (68) trisomy 17-18 Index TRISOMY (IM) (68) CHROMOSOMES, HUMAN, 17-18 (IM) (68) trisomy 21 Index MONGOLISM (68) true hermaphrodite see hermaphrodite, true zygote the cell resulting from the fusion of two gametes; the fertilized ovum (Dorland) Index OVUM (IM) (68) FERTILIZATION (NIM) (68) trisomic an aneuploid (hyperploid) individual in whom a chromosome is present in triplicate, rather than in the normal double or disomic 'number, or condition.(Illinois Med J 132:276, Sep 67) Index TRISOMY (68)