Glossary
Genetics glossary
914 definitions · page 12 of 16.
- lawn
- A continuous layer of bacteria on the surface of an agar medium.
- leader peptide gene
- A small gene within the attenuator control region of repressible amino acid operons. Translation of a specific leader peptide tests for the concentration of a specific amino acid or set of amino acids in the cell. If the concentration is high transcription…
- leader sequence
- The sequence at the 5' end of an mRNA that is not translated into protein. The length of untranslated mRNA from the 5' end to the initiation codon AUG.
- leader transcript
- The mRNA transcribed by the attenuator region of repressible amino acid operons. The transcript is capable of several alternative stem-loop structures dependent on translation of a short leader peptide gene.
- leading strand
- Strand of DNA being replicated continuously. In DNA replication, the strand that is made in the 5' to 3' direction by continuous polymerization at the 3' growing tip. See also lagging strand.
- leaky mutant
- A mutant (typically an auxotroph) that results from a partial rather than a complete inactivation of the wild-type function.
- leptonema (leptotene stage)
- The first stage of prophase 1 of meiosis in which chromosomes become distinct.
- lesion
- A damaged area (a mutant site) in a gene, a chromosome, or a protein.
- lethal gene
- A gene whose expression results in the death of the individual expressing it.
- lethal-equivalent alleles
- alleles whose summed effect is that of lethality for example, four alleles each of which would be lethal 25% of the time (or to 25% of their bearers), are equivalent to one lethal allele.
- Leu
- Leucine (an amino acid).
- leucine zipper
- Configuration of a DNA-binding protein in which leucine residues on two protein alpha-helices interdigitate in zipper fashion to stabilize the protein.
- leukemia
- cancer of the bone marrow resulting in excess production of leukocytes.
- level of significance
- The probability value in statistics used to reject the null hypothesis.
- ligand-receptor interaction
- interaction between a molecule (usually of an extracellular origin) and a protein on or within a target cell. One type of ligand-receptor interaction can be between steroid hormones and their cytoplasmic or nuclear receptors. Another can be between secreted…
- ligase
- An enzyme that can rejoin a broken phosphodiester bond in a nucleic acid.
- LINE
- Long interspersed element; a type of large repetitive DNA segment found throughout the genome of Eukaryotes.
- linear tetrad
- A tetrad that results from the occurrence of the meiotic and postmeiotic nuclear divisions in such a way that sister products remain adjacent to one another in an ordered manner constrained within an ascus.
- linkage
- An association of gene loci on the same chromosome.
- linkage disequilibrium
- When the observed frequencies of haplotypes in a population does not agree with haplotype frequencies predicted by multiplying together the frequency of individual genetic markers in each haplotype.
- linkage group
- A group of gene loci known to be linked; a chromosome. There are as many linkage groups as there are homologous pairs of chromosomes. See synteny.
- linkage map
- An abstract map of chromosomal loci, based on recombinant frequencies.
- linking number
- The number of times one strand of a helix coils about the other.
- locus(Plural loci)
- The position of a gene, DNA marker or genetic marker on a chromosome. See gene locus.
- lod score method
- A technique (logarithmic odds) for determining the most likely recombination frequency between two loci from pedigree data.
- lymphoma
- cancer of the lymph nodes and spleen that causes excessive production of lymphocytes.
- Lyon hypothesis
- The hypothesis, named after Mary Lyon who stated it, suggesting that doseage compensation in mammals is by inactivation of all but one X chromosome in cells with more than one X chromosome. The Barr body, visible in some female mammalian cells, is an…
- Lys
- Lysine (an amino acid).
- lysate
- The contents released from a lysed cell.
- lysis
- The breaking open of a cell by the destruction of its wall or membrane. The rupture and death of a bacterial cell upon the release of phage progeny.
- lysogen
- See lysogenic bacterium.
- lysogenic
- The state of a bacterial cell that has an integrated phage (prophage) in its chromosome.
- lysogenic bacterium
- A bacterial cell capable of spontaneous lysis due, for example, to the release of a prophage from the bacterial chromosome.
- macromolecule
- A large polymer such as DNA, RNA, protein, lipid or polysaccharide.
- major histocompatibility complex (MHC)
- A group of highly polymorphic genes whose products appear on the surface of cells imparting the property of self (belonging to that organism). A genetic region found in all mammals whose products are primarily responsible for the rapid rejection of tissue…
- Manx
- Tailless phenotype in cats, caused by an autosomal dominant mutation that is lethal when homozygous.
- map unit (m.u.)
- The distance between two linked gene pairs where 1 percent of the products of meiosis are recombinant. A unit of distance in a linkage map. The distance equal to 1% recombination between two loci.
- mapping
- The study of the position of genes on chromosomes.
- mapping function
- The mathematical relationship between measured map distance and actual recombination frequency. A formula expressing the relationship between distance in a linkage map and recombinant frequency.
- Marfan s syndrome
- A human disorder of the connective tissue expressed as a range of symptoms, including very long limbs and digits, and heart defects; inherited as an autosomal dominant phenotype. Mutations in a gene called fibrillin.
- marker
- Any genetic element (locus, allele, DNA sequence or chromosome feature) which can be readily detected by phenotype, cytological or molecular techniques, and used to follow a chromosome or chromosomal segment during genetic analysis. See centromere marker…
- marker retention
- A technique used in yeast to test the degree of linkage between two mitochondrial mutations.
- mate-killer
- A phenotype of Paramecium induced by intracellular bacteria-like mu particles.
- maternal effect
- The effect of the maternal parent's genotype on the phenotype of her offspring.
- maternal effect lethal
- A recessive mutation that is viable in zygotes, but in which homozygous mutant mothers produce inviable offspring.
- maternal inheritance
- A type of uniparental inheritance in which all progeny have the genotype and phenotype of the female parent.
- mating type
- In many species of microorganisms individuals can be divided into two types. Mating can take place only between individuals of opposite mating types due to the interaction of cell surface components. The equivalent in lower organisms of the sexes in higher…
- matroclinous inheritance
- Inheritance in which all offspring have the phenotype of the mother.
- mean
- The arithmetic average; the sum of the data divided by the sample size.
- mean fitness of the population W(bar)
- The sum of the fitnesses of the genotypes of a population weighted by their proportions; hence a weighted mean fitness.
- medium
- Any material on (or in) which experimental cultures are grown.
- meiocyte
- Cell in which meiosis occurs.
- meiosis
- Two successive nuclear divisions (with corresponding cell divisions) that produce haploid gametes (in animals) or haploid sexual spores (in plants and fungi) having one-half of the genetic material of the original cell. The nuclear and cell division process…
- meiospore
- Cell that is one of the products of meiosis in plants.
- melting of DNA
- Denaturation of duplex DNA by heat or increased pH leading to strand separation.
- 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 pair.
- Mendel s second law
- The law of independent assortment; unlinked or distantly linked segregating gene pairs assort independently at meiosis.
- Mendelian ratio
- A ratio of progeny phenotypes reflecting the operation of Mendel's laws.
- merozygote
- A bacterial cell having a second copy of a particular chromosomal region in the form of an exogenote. A partially diploid Escherichia coli cell formed from a complete chromosome (the endogenote) plus a fragment (the exogenote).
- messenger RNA (mRNA)
- An RNA molecule transcribed from the DNA of a gene, and from which a protein is translated by the action of ribosomes. The basic function of the nucleotide sequence of mRNA is to determine the amino acid sequence in proteins.