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one of the different forms of a gene that can exist at a single locus


in a fungus, a sac that encloses a tetrad or an octad of ascospores


homologous chromosomes paired at meiosis


an attribute of individual members of a species for which various heritable differences can be defined


one of the two side-by-side replicas produced by chromosome division


the deliberate mating of two parental types of organisms in genetic analysis


a polymorphism with only two forms

dioecious species

a plant species in which male and famale organs are on separate plants


the phenotype shown by a heterozygote


a pair of sister chromatids joined at the centromere, as in the first division of meiosis

first filial generation

produced by crossing two parental lines

forward genetics

the classical approach to genetic analysis, in which genes are first identified by mutant alleles and mutant phenotypes and later cloned and subjected to molecular analysis


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 transcription possible

gene discovery

the process whereby geneticists find a set of genes affecting some biological process of interest by the single-gene inheritance patterns of their mutant alleles or by genomic analysis

genetic dissection

the use of recombination and mutation to piece together the various components of a given biological function


the allelic composition of an individual or of a cell-either of the entire genome or, more commonly, of a certain gene or let of genes

haploid number

the number of chromosomes in the basic genomic set of a species


describes a gene in which a null mutant allele will be dominant because, in a heterozygote, the single wild-type allele cannot provide enough product from normal function


describes a gene that, in a diploid cell, can promote wild-type function in only one copy (dose)


a gene present in only one copy in a diploid organism- for example, an X-linked gene in a male mammal

heterogametic sex

the sex that has heteromorphic sex chromosomes (e.g. XY) and hence produces two different kinds of gametes with respect to the sex chromosomes


an individual organism having a heterozygous gene pair

homogametic sex

the sex with homologous sex chromosomes (e.g. XX)


an individual organism that is homozygous

homozygous dominant

refers to a genotype such as A/A

homozygous recessive

refers to a genotype such as a/a

law of equal segregation (Mandel's first law)

the production of equal numbers (50%) of each allele in the meiotic products (e.g. gametes) of a heterozygous meiocyte

leaky mutation

a mutation that confers a mutant phenotype but still retains a low but detectable level of wild-type function


a cell in which meiosis takes place


two successive nuclear divisions (with corresponding cell divisions) that produce gametes (in animals) or sexual spores (in plants and fungi) that have one-half of the genetic material of the original cell


a type of nuclear division (occurring at cell division) that produces two daughter nuclei identical with the parent nucleus

molecular markers

a site of DNA heterozygosity, not necessarily associated with phenotypic variation, used as a tag for a particular chromosomal locus


a single-locus heterozygote of the type A/a

monohybrid cross

a cross between two individuals identically heterozygous at one gene pair- for example, A/a x A/a


one form of a genetic polymorphism; the morph can be either a phenotype or a molecular sequence


an organism or cell carrying a mutation


the process that produces a gene or a chromosome set differing from that of the wild type. or. the gene or chromosome set that results from such a process

null allele

an allele whose effect is the absence either of normal gene product at the molecular level or of normal function at the phenotypic level

parental generation

the two strands or individual organisms that constitute the start of a genetic breeding experiment; their progeny constitute the F1 generation

pedigree analysis

deducing single-gene inheritance of human phenotypes by a study of the progeny of matings within a family, often stretching back several generations


the form taken by some character (or group of characters) in a specific individual. or. the detectable outward manifestations of a specific genotype


the occurrence in a population (or among populations) of several phenotypic forms associated with alleles of one gene or homologs of one chromosome

product of meiosis

one of the (usually four) cells formed by the two meiotic divisions

product rule

the probability of two independent events both occurring is the product of their individual probabilities


a characteristic feature of an organism, such as size, color, shape, or enzyme activity


in a human pedigree, the person who first came to the attention of the geneticist

pseudoautosomal regions 1 and 2

small regions at the ends of the X and Y sex chromosomes; they are homologous and undergo pairing and crossing over at meiosis

pure line

a population of individuals all bearing the identical fully homozygous genotype

reverse genetics

an experimental procedure that begins with a cloned segment of DNA or a protein sequence and uses it (through directed mutagenesis) to introduce programmed mutations back into the genome to investigate function

second filial generation

the progeny of a cross between two individuals from the F1 generation


to fertilize eggs with sperms from the same individual

sex chromosome

a chromosome whose presence or absence is correlated with the sex of the bearer; a chromosome that plays a role in sex determination

sex linkage

the location of a gene on a sex chromosome

SRY gene

the maleness gene, residing on the Y chromosome


a cross of an individual organism of unknown genotype or a heterozygote (or a multiple heterozygote) with a tester


an individual organism homozygous for one or more recessive alleles; used in a testcross


four homologous chromatids in a bundle in the first meiotic prophase and metaphase. or. the four haploid product cells from a single meiosis


more or less synonymous with phenotype

wild type

the genotype or phenotype that is found in nature or in the standard laboratory stock for a given organism

X chromosome

one of a pair of sex chromosomes, distinguished from the Y chromosome

X linkage

the inheritance pattern of genes found on the Y chromosome but not on the X chromosome (rare)

Y chromosome

one of a pair of sex chromosomes, distinguished from the X chromosome

Y linkage

the inheritance pattern of genes found on the Y chromosome but not on the X chromosome (rare)


a cell formed by the fusion of an egg and a sperm; the unique diploid cell that will divide mitotically to create a differentiated diploid organism

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