GENETICS-IMPORTANT TERMS

• Acquired character. The alteration in the morphology or physiology of an organism in response to its ecological factors (environment) is known as acquired character. Acquired characters are usually not heritable.
• Albinism. Absence of colour in skin, hair and eyes or absence of chloroplast in a plant; an inherited trait.
• Albino. The animal without pigmentation in skin, hairsand eyes is called albino.
• Allele (Allelomorph). One of two or more forms that can exist at a single gene locus, distinguished by their differing effects on the phenotype. Alleles are genes controlling the same characteristic (e.g. hair colour) but producing different effects (e.g. black or red), and occupying corresponding positions on homologous chromosomes.
• Amniocentesis. Puncture of the uterine wall with a needle for the purpose of obtaining amniotic fluid, which can be analyzed to determine whether the foetus has a genetic abnormality. Amniotic fluid contains sloughed foetal cells.
• Aneuploidy. Karyotypic abnormality in which a specific chromosome(s) is present in too many or too few copies.
• Animal breeding. The practical application of genetic analysis for development of purebreeding lines of domestic animals suited to human purposes.
• Autosome. The chromosomes which are not associated with sex are known as autosomes. Except the sex chromosomes (X) and (Y) other chromosomes are the autosomes.
• Back cross. The cross of a progeny individual with its parents is known as back cross.
• Barr body. A densely staining mass that represents an X-chromosome inactivated by dosage compensation.
• Bead theory. The disproved hypothesis that genes are arranged on the chromosome like beads on a necklace, indivisible into smaller units of mutation and recombination.
• Bivalent. A pair of synapsed homologous chromosomes is known as a bivalent.
• Blending inheritance. A discredited model of inheritance suggesting
• that the characteristics of individual result from the smooth blending of fluid-like influences from its parents.
• Carrier. A heterozygous individual. An individual who possesses a mutant allele but does not
• express it in the phenotype because of a dominant allelic partner; thus, an individual of genotype Aa
• is a carrier of a if there is complete dominance of A on a.
• Chiasma (plural chiasmata). A cross-shaped structure commonly observed between nonsister chromatids during meiosis; the site of crossing-over.
• Chromatin. A DNA, RNA, histone and non-histone protein containing thread-like coiled
• structure of interphase nucleus is called chromatin.
• Chromosome. The nucleoprotein structure which are generally more or less rod-like during
• nuclear division. The genes are arranged on the chromosomes in a linear fashion. Each species has acharacteristic number of chromosomes. Chromosomes play most important role in inheritance.
• Cis arrangement. Linkage of dominants of two or more pairs of alleles on one chromosome
• and the recessive on the homologous chromosome.
• Cistron. A term equated with the term gene. It is a region of DNA that encodes a single polypeptide
• (or functional RNA molecule such as tRNA or rRNA).
• Clone. A group of genetically identical cells or individuals derived by asexual division from a
• common ancestor.
• Cloning. Asexual production of a line of cells or organisms or segments of DNA genetically
• identical to the original.
• Codominance. When both the alleles (dominant and recessive) are equally expressed in the
• hybrid, the phenomenon is known as codominance, e.g., when the red and white cattles are crossed, they produce a roan offspring which possess both red and white hairs on the skin.
• Crisscross inheritance. Transmission of a gene from male parent to female child to male
• grandchild—for example, X-linked inheritance.
• Cross. The deliberate mating of two parental types of organisms in genetic analysis.
• Crossing over. Crossing over is a phenomenon in which exchange of chromosomal segment or genetic material occurs between the non-sister homologous chromosomes by breakage and union.
• Culture. Tissues or cells multiplying by asexual division, grown for experimentation.
• Cytoplasmic inheritance. Inheritance via genes found in cytoplasmic organelles.
• Daltonism. Red and green colour blindness, a recessive trait known to be X-linked.
• Deoxyribonucleic acid (DNA). It is a genetic material of many viruses, bacteria, plants and
• animals. It is a double stranded, helically coiled, macromolecule which is composed of phosphoric acid, deoxyribose sugar, two pyrimidenes (cytosine and thymine) and two purines (adenine and guanine), It is found to be most stable biological molecule which contains encoded genetic informations.
• Dihybrid. An individual which is hybrid or hetrozygous in two pairs of alleles or allelomorphs
• is known as dihybrid. For instance, the cross between pea plants with yellow round seeds and green wrinkled seeds produces a dihybrid having yellow round seeds.
• Diploid. An individual or cell containing two complete haploid sets of chromosomes is known
• as diploid.
• Dominance. A phenomenon in which one member of a pair of allelic genes expresses itself as a whole (complete dominance) or in part (incomplete dominance).
• Dominant allele. An allele that expresses its phenotypic effect even when heterozygous with a recessive allele; thus if A is dominant over a; then AA and Aa have the same phenotype.
• Dominant phenotype. The phenotype of genotype containing the dominant allele; the parental phenotype that is expressed in a heterozygote.
• Dominant trait. When out of two contrasting characters or traits only one expresses or appears in a generation. That trait is known as dominant trait, e.g., in pea, round character of seed is dominant over wrinkled character of seed.
• Dosage compensation. (1) Inactivation of X-chromosome in mammalsso that no cell has more than one functioning X chromosome. (2) Regulation at some autosomal loci so that homozygous dominants do not produce twice as much product as the heterozygote.
• Down’s syndrome. An abnormal human phenotype including mental retardation, due to a trisomy of chromosome 21; more common in babies born to older mothers.
• Environment. The combination of all the conditions external to the genome that potentially affect its expression and its structure.
• Episome. A genetic element Down’s syndrome. (closed, circular DNA molecule) in bacteria that can replicate free in the cytoplasm or can be inserted into the main bacterial chromosome and replicate with the chromosome, e.g., F factor of E. coli.
• Epistasis. A situation in which an allele of one gene obliterates the phenotypic expression of all allelic alternatives of another gene. The masked gene is said to be hypostatic.
• Euchromatin. A type of chromatin that is non-condensed during interphase and condensed
• during nuclear division, reaching a maximum in metaphase. The banded segments of the polytene chromosomes of Drosophila larval salivary glands contain euchromatin.
• Euploidy. Variation in chromosome number by whole sets or exact multiples of the monoploid
• (haploid) number, e.g. diploid, triploid. Euploids above the diploid level may be referred tocollectively as polyploids.
• Eugenics. Controlled human breeding based on notions of desirable and undesirable genotypes.
• Exon. DNA sequence of a cistron that are transcribed into mRNA and are translated into protein.
• Expressivity. The degree to which a particular genotype is expressed in the phenotype under avariety of environmental conditions.
• F factor. The fertility factor in the bacterium, Escherichia coli. It is composed of DNA and
• must be present in a cell to function as a donor in conjugation.
• Filial generations. Successive generations of progeny in a controlled series of crosses, starting
• with two specific parents (the P generation) and selfing or intercrossing the progeny of each new (F1,F2 ............) generation.
• F1 or First filial generation. The word filial is derived from the Latin word filin, meaning the
• son. The first generation of a given cross is known as F1 generation.
• F2 or Second filial generation. The second generation which is resulted by interbreeding or
• selfing of F1 offsprings is known as second filial generation.
• Forward mutation. A mutation that converts a wild-type allele to a mutant allele.
• Frame-shift mutation. The insertion or deletion of a nucleotide pair or pairs, causing a
• disruption of the translational reading frame.
• Gamete. A sex cell having haploid set of chromosomes and arising due to meiotic cell division
• of diploid germ cell is known as gamete. The male gamete is known as pollen or sperm and female gamete is known as ovum or egg.
• Gene. The fundamental physical and functional unit of heredity, which carries information from
• one generation to the next; a segment of DNA, composed of a transcribed region and a regulatory sequence, that makes possible transcription.
• Gene interaction. The coordinated effect of two or more genes in producing a given phenotypic trait.
• Gene mutation. Change in the structure of a gene.
• Genetic engineering. Array of techniques that facilitate the manipulation and duplication of pieces of DNA for industrial,medical and research purposes.
• Genome. A complete set of chromosomes, or of chromosomal genes, inherited as a unit
• from one parent, or the entire genotype of a cell or individual.
• Genotype. The genetic makeup or constitution of an individual, with reference to the traits
• under consideration, usually expressed by a symbol,
• e.g., +, DD (tall), dd (short), etc.
• Gynandromorph or gynander. When the body of an individual exhibits both the male and female characters, this type of individual in known as gynandromorph. A sexual mosaic.
• Haploid (Monoploid) . An individual or cell containing a single complete set of chromosomes
• is known as haploid.
• Haemophilia. A metabolic disorder characterized by free bleeding even from slight wounds
• because of the lack of clot forming substances. It is associated with a sex- or X-linked recessive gene.
• Heterochromatin. Condensed chromatin. Constitutive heterochromatin occurs in centromeric region of chromosomes of a given species; it includes certain important, genetically active
• regions. Facultative heterochromatin is that chromatin that makes up the genetically inactive whole chromosomes (e.g., the X chromosome in human females).
• Heterogametic sex. A sex containing either only one or two different sex chromosomes such as XO, XY or ZW produces two types of gametes half with X or Z chromosome and other half with Y orW or no chromosomes. This type of sex is known as heterogametic sex.
• Heterozygote (Heterozygous). An individual containing both dominant and recessive genes or traits or characters of a allelic pair is known as heterozygous or hybrid.
• Holandric gene. A gene which occurs only in Y chromosome is known as holandric gene.
• Homogametic sex. The sex which possesses two identical sex chromosomes (XX or ZZ) and
• produces single type of gametes each with X or Z chromosome is known as homogametic sex
• Homologous chromosomes (Homolog). The identical male and female parent chromosomes occur in pairs and are known as homologous chromosomes. Each chromosome of a homologous pair is known as homolog.
• Homozygote (Homozygous). The organism having two similar genes for a particular character in a homologous pair of chromosomes is known as homozygous or genetically ‘pure’ for that particular character.
• Hybrid. (i) A heterozygote. (2) A progeny individual from any cross involving parents of
• differing genotypes.
• Hybridoma. A cell hybrid between an antibody-producing B cells and a tumour cell, which
• divides indefinitely and produces a single antibody (monoclonal antibody).
• Inbreeding. Mating between relatives more frequently than would be expected by chance.
• Incomplete dominance. The condition in heterozygotes where the phenotype is intermediate
• between the two homozygotes. In some plants the cross of red and white produces pink-flowered progeny.
• Independent segregation. The independent behaviour of genes occurs on different pairs of
• chromosomes. This phenomenon is known as independent segregation.
• Intron. A sequence of nucleotides in DNA that does not appear in mRNA; probably excised from hnRNA in its processing.
• Karyotype. The entire chromosome complement of an individual or cell, as seen during mitotic
• Klinefelter’s syndrome. A genetic disease due to the XXY karyotype. It produces sterile males with some mental retardation.
• Lawn. A continuous layer of bacteria on the surface of an agar medium.
• Lethal gene. A gene whose phenotypic effect is sufficiently drastic to kill the bearer. Death from different lethal genes may occur at any time from fertilization of the egg to advanced age. Lethal genes may be dominant, incompletely dominant, or recessive.
• Linkage. The occurrence of different genes on the same chromosome. They show nonrandom assortment at meiosis.
• Linkage group. All of the genes located physically on a given chromosome.
• Linkage map. A chromosome map; an abstract map of chromosomal loci, based on
• recombinant frequencies.
• Locus. The position or place on a chromosome occupied by a particular gene or one of its alleles.
• Maternal inheritance. A type of uniparental inheritance in which phenotypic differences in progeny occur due to factors such as chloroplasts and mitochondria transmitted by the female gamete.
• Meiosis. Meiosis is the reduction division in which the diploid or somatic chromosome
• numbers are reduced to half. The meiosis often produces haploid gametes or individuals.
• Mendelian ratio. A ratio of progeny phenotypes reflecting the operation of Mendel’s laws.
• Mendel’s first law. The two members of a gene pair segregate from each other during meiosis; each gamete has an equal probability of obtaining either member of the gene.
• Mendel’s second law. The law of independent assortment; unlinked or distantly linked
• segregating genes pairs behave independently.
• Mitosis. Process of cell division, whereby the genetic material is precisely divided and two new chromosome sets identical to the original are generated.
• Monohybrid. When the cross takes place between the parents differing in a single pair of
• contrasting characters resulted into a monohybrid individual.
• Monohybrid cross. The cross between the two parents differing in a single pair of contrasting
• characters is known as monohybrid cross.
• Monosomic. An individual lacking one chromosome of a set (2n – 1).
• Mutation. A spontaneous permanent change in a gene or chromosome which usually produces a detectable effect in the organism concerned and is transmitted to the offsprings.
• Nondisjunction. The failure of homologs (at meiosis) or sister chromatids (at mitosis) to
• separate properly to opposite poles.
• Norm of reaction. The pattern of phenotypes produced by a given genotype under different
• environmental conditions.
• Oncogene. A gene that induces uncontrolled cellular proliferation (i.e., cancer). Oncogenes may be either cellular or viral in origin.
• Pedigree. A “family tree”, drawn with standard genetic symbols, showing inheritance patterns
• for specific phenotypic characters.
• Penetrance. The proportion of individualsof a particular genotype that show
• the expected phenotype under a certain set of environmental conditions.
• Petite. A yeast mutation producing  small colonies and altered mitochondrial functions. In cytoplasmic petites (neutral and suppressive petites), the mutation is a deletion in mitochondrial DNA, in segregational petites, the mutation occurs in nuclear DNA.
• Phenocopies. When two genotypes produce the same phenotype due to different
• environments, then each one is called the phenocopy of the other, because they differ genotypically. For example, in Drosophila melanogaster, the normal (natural or wild) body colour is brown and a hereditary variant has yellow colour, when the larvae of wild type Drosophila with brown body colour are raised on food containing silver salts, they develop into yellow bodied flies. Thus, these flies are the phenocopies of yellow mutant, but would give rise to wild type brown flies in normal environment.
• Phenotype. The appearance or discernible character of an individual, which is dependent on its genetic makeup usually expressed in words, e.g., “tall”, “dwarf”, “wild type”, “albino”, “prolineless”.
• Philadelphia chromosome. A translocation between the long arms of chromosome 9 and 22,
• often found in the white blood cells of patients with chronic myeloid leukemia.
• Plant breeding. The application of genetic analysis to development of plant lines better suited
• for human purposes.
• Plasmagene. A self-replicating, cytoplasmically located gene.
• Plasmid. A closed, circular DNA molecule of restricted size (i.e., a few tens of thousands of
• nucleotide pairs), existing only in the cytoplasm (of bacteria); incapable of integration into the bacterial “chromosome”. Now plasmid is used to include both episomes and plasmids.
• Pleiotropic mutation. A mutation that has effects on several different characters.
• Pleiotropy. The influencing of more than one trait by a single gene.
• Point mutation. Change in a single base of the DNA molecule.
• Polygenes. Two or more different pairs of alleles, with a presumed cumulative effect that
• governs such quantitative traits as size, pigmentation, intelligence, among others. Those contributingto the trait are termed contributing (effective) alleles; those appearing not to do so are referred to as non-contributing or non-effective alleles.
• Position effect. A phenotypic effect dependent on a change in position on the chromosome of a gene or group of genes.
• Progeny. Offspring individuals.
• Pseudoalleles. Non-alleles so closely linked that they are often inherited as one gene, but shown to be separable by crossover studies.
• Pulse-chase experiment. An experiment in which cells are grown in radioactive medium for a
• brief period (the pulse) and then transferred to non-radioactive medium for a longer period (the chase).
• Punnett square. A “checkerboard” grid designed to determine all possible genotypes produced by a given cross. Genotypes of the gametes of one sex are entered across the top, those of the other downone side. Zygote genotypes produced by each possible mating are then entered in the appropriate squares of the grid.
• Pure line (pure breeding line). A strain of individuals homozygous for all genes being considered.
• Recessive allele. An allele whose phenotypic effect is not expressed in a heterozygote.
• Recessive phenotype. The phenotype of a homozygote for the recessive alleles; the parental phenotype that is not expressed in a heterozygote.
• Reciprocal cross. A second cross of the same genotypes in which the sexes of the parental generation are reversed. The cross AA X aa is the reciprocal of the cross aa X AA.
• Recombinant DNA. (1) A novel DNA sequence formed by the combination of two non-homologous DNA molecules. (2) A DNA molecule (in practice generally a bacterial plasmid) which has been enzymatically cut and DNA of another individual of the same or a different species inserted in the space so produced, then reannealed to the closed, circular form.
• Recombination. The new association of genes in a recombinant individual; this association arises from independent assortment of unlinked genes, from crossing over between linked genes, or from intracistronic crossing-over.
• Retrovirus. RNA viruses that replicate with reverse transcriptase. Since the enzyme produces
• a DNA copy of the viral RNA that is the reverse of transcription, the name retro, suggesting backward transcription, is used.
• Reverse transcriptase. An enzyme, carried within the coat of the retroviruses, that makes
• double-stranded DNA from a single-stranded RNA template.
• Segregation. The separation of allelomorphic (allelic) genes into different gametes at meiosis; or, separation in its offspring of traits which are combined in a hybrid.
• Sex chromosomes. Heteromorphic chromosomes that do not occur in identical pairs in both sexes in diploid organisms; in humans and fruit flies these are designated as X and Y chromosomes, respectively; in fowl, as the Z and W chromosomes.
• Sex linkage. The location of a gene on a sex chromosome.
• Sexduction. Incorporation of bacterial chromosomal genes in the fertility plasmid, with subsequent transfer to a recipient cell in conjugation.
• Sex-influenced trait. One in which dominance of an allele depends on sex of the bearer, e.g.,
• pattern baldness in humans in dominant in males, recessive in females.
• Sex-limited trait. One expressed in only one of the sexes; e.g., cock feathering in fowl is limited to normal males.
• Siblings or Sibs. The individuals having same maternal and paternal parents, e.g., brother sister relationship.
• Sickle-cell anaemia. Anaemia in humans inherited as an autosomal recessive and due to a single amino acid substitution in the beta-haemoglobin chain.
• Soma (adj., somatic). The body, cells of which in mammals and flowering plants normally have two sets of chromosomes, one derived from each parent.
• Somatic cell. A cell that is not destined to become a gamete; a “body cell”, “whose genes will not be passed on to future generations.
• Somatic-cell genetics. Asexual genetics, involving study of somatic mutation, assortment, and crossing-over, and of cell fusion.
• Splicing. The reaction that removes introns and joins together exons in RNA.
• Synapsis. The pairing of homologous chromosomes that occur in prophase-I of meiosis.
• Syngamy. The union of the nuclei of sex cells (gametes) in reproduction.
• Test cross. The cross of an individual (generally of dominant phenotype) with one having the recessive phenotype. Generally used to determine whether an individual of dominant phenotype is homozygous or heterozygous, or to determine the degree of linkage.
• Tetrad. The four monoploid (haploid) cells arising from meiosis of a megasporocyte or microsporocyte in plants; also, a group of four associated chromatids during synapsis.
• Trans arrangement. Linkage of the dominant alleles of one pair of gene and the recessive of another on the same chromosome.
• Transduction. Recombination in bacteria whereby DNA is transferred by a phage from one cell to another. Generalized transduction involves phages that have incorporated a segment of bacterial chromosome during packaging of the phage; specialized transduction involves temperate phages that are always inserted into the bacterial chromosome at a site specific for that phage.
• Trihybrid. The individual which is heterozygous for three pair of alleles is known as trihybrid.
• Trisomic An individual with one extra chromosome of a set (2n + 1).
• Turner’s syndrome. A series of abnormalities in humans due to monosomy for the X chromosome (XO). Individuals are phenotypically female, but are sterile.
• Uniparental inheritance. The transmission of certain phenotypes from one parental type to all the progeny; such inheritance is generally produced by organelle genes.
• Variation. The differences among parents and their offspring or among individuals in a population.
• Vector. In cloning, the plasmid or phage chromosome used to carry the cloned DNA segment.
• Wild type. The genotype or phenotype that is found in nature or in the standard laboratory stockfor a given organism.
• X linkage. The presence of a gene on the X chromosome but not on the Y.
• X- and -Y linkage. The presence of a gene on both the X and Y chromosomes.
• Y linkage. The presence of a gene on the Y chromosome but not on the X.
• Zygote. The cell formed by the fusion of an egg and a sperm; the unique diploid cell that willdivide mitotically to