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addition rule - A rule of probability stating that the probability of any one of two or more mutually exclusive events can be determined by adding their individual probabilities

alleles - Any of the alternative versions of a gene that may produce distinguishable phenotypic effects.

alternation of generations - A life cycle in which there is both a multicellular diploid form, the sporophyte, and a multicellular haploid form, the gametophyte; characteristic of some plants an algae

amniocentesis - A technique associated with prenatal diagnosis in which amniotic fluid is obtained by aspiration from a needle inserted into the uterus. Th fluid and fetal cells it contains are analyzed to detect certain genetic and congenital defects in the fetus

anaphase - The fourth stage of mitosis, in which the chromatids of each chromosome have seperated and the daughter chromosomes are moving to the poles of the cell.

anchorage dependence - The requirement that a cell must must be attached to a substratum in order to initiate cell division

aneuploidy - A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number

asexual reproduction - The generation of offspring from a single parent that occurs without the fusion of gametes (by budding, division of a single cell, or division of the entire organism into two or more parts). In most cases the offspring are genetically identical to the parent.

aster - A radial array of short microtubules that extends from each centrosome toward the plasma membrane in an animal cell undergoing mitosis

autosomes - A chromosome that is not directly involved in determining sex; not a sex chromosome

Barr body - A dense object lying inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome

benign tumor - A mass of abnormal cells with specific genetic and cellular changes such that the cells are not capable of surviving at a new site and generally remain at the site of the tumor's origin

binary fission - A method of asexual reproduction by "division in half." In prokaryotes, binary fission does not involve mitosis , in single-celled eukaryotes that undergo binary fission, mitosis part of .the process

carriers - In genetics, an individual who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring

cell cycle - An ordered sequence of events in the life cycle, from its origin

cell cycle control system - A cyclically operating set of molecules in the eukaryotic cell that both triggers and coordinates key events in the cell cycle

cell division - The reproduction of cells

cell plate - A membrane-bounded, flattened sac located at the midline of a dividing plant cell, inside the new cell wall forms during cytokinesis

centromere - In a duplicated chromosome, the region on each sister chromatid where it is most closely attached to the other chromatin by proteins that bind to the centromeric DNA. Other proteins condense the chromatin in that region, so it appears as a narrow "waist" on the duplicated chromosome.(An unduplicated chromosome has a single centromere, identified by the proteins bound there).

centrosome - A structure present in the cytoplasm of animal cells that functions as a a microtubule organizin center and is important during cell division. A centrosome has two centrioles

character - An observable heritable feature that may vary among individuals.

checkpoint - A control point in the cell cycle where stop and go-ahead signals can regulate the cycle

chiasmata - The X-shaped microscopically visible region whee crossing over has occurred earlier in prophase I between homologous nonsister chromatids. Chiasmata become visible after synapsis ends, with the two homologs remaining associated due to sister chromatid cohesion.

chorionic villus (CVS) - A technique associated with prenatal diagnosis i which a small sample of the fetal portion of the placenta is removed for analysis to detect certain genetic and congenital effects in the fetus

chromatin - The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a a mass of very long, thin fibers that are not visible with a light microscope

chromosome theory of inheirtance - A basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns

chromosomes - A cellular structure consisting of one DNA molecule and associated protein molecules. (in some contexts, such as genome sequencing, the term may refer to DNA alone.) A eukaryotic cell typically has multiple, linear chromosomes, which are located in the nucleus. A prokaryotic cell has often has a single, circular chromosome, which is found in the nucleoid, a region that is not enclosed by a membrane.

cleavage - (1) The process of cytokinesis in animal cells , characterized by pinching of the plasma membrane. (2) The succesion of rapid cell divisions without significant growth during early embryonic development that converts the zygote to a ball of cells

cleavage furrow - The first sign of cleavage in an animal cell; a shallow groove around the cell in the cell surface near the old metaphase plate

clone - (1) A lineage of genetically identical individuals or cells. (2) In popular usage, an individual that is genetically identical to another individual (3) As a verb, o make one or more genetic replicas of an individual or cell.

codominance - The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways

complete dominanace - The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable

crossing over - The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis

crossing over - The reciprocal exchange of genetic material between nonsister chromatids during prophase I of meiosis

cyclin - A cellular protein that occurs in a cyclically fluctuating concentration and that plays an important role in regulating the cell cycle

cyclin dependent kinases (Cdks) - A protein kinase that is active only when attached to a particular cyclin

cystic fibrosis - A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated

cytokinesis - The division of the the cytoplasm to form two separate daughter cells, immediately after mitosis, meiosis I or meiosis II.

deletion - (1) a deficiency in a chromosome resulting from the loss of a fragment through breakage (2) A mutational loss of one or more nucleotide pairs from a gene

density-dependent inhibition - The phenomenon observed in normal animal cells that causes them to stop dividing when they come in contact with one another

dihybrid cross - A cros between two organisms that are each heterozygous for both of the characters being followed (or the self-pollination of a plant that is heterozygous for both characters).

dihybrids - An organism that is heterozygous with respect to two genes of interest. All the offspring from a cros between parents doubly homozygous for different alleles are dihybrids. For example AABB and aabb produce a dihybrid of genotype AaBb.

diploid cell - A cell containing two sets of chromosomes (2n), one set inherited from each parent.

dominant allele - An allele that is fully expressed in the phenotype of a heterozygote

Down syndrome - A human genetic disease usually caused by the presence of an extra chromosome 21; characterized by developmental delays and heart and other defects that are generally treatable or non-life thratening

duplication - An abberation in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated

dystrophy - A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and loss of muscle tissue

epistasis - A type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene

F1 Generation - The first filial hybrid (heterozygous) offspring arising from the parental (P Generation) cross

F2 Generation - The offspring resulting from inbreeding (or self pollination) of the hybrid F1 Generation

fertilization - (1) the union of haploid gametes to produce a diploid zygote. (2) The addition of mineral nutrients to the soil

G0 (zero) phase - A non-dividing state occupied by cells that have left the cell cycle, sometimes reversibly

G1 Phase - The first gap, or growth phase, of the cell cycle, consisting of the portion of interphase before DNA synthesis begins

G2 Phase - The second gap, or growth phase, of the cell cycle, consisting of interphase after DNA synthesis occurs

gametes - A haploid reproductive cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote

gametes - A haploid reproductive cell, such as an egg or sperm. Gametes unite during sexual reproduction to produce a diploid zygote.

genes - Discrete units of hereditary information consisting of a specific nucleotide sequence in DNA (or RNA, in some viruses)

genetic map - An ordered list of genetic loci (genes or other genetic markers) along a chromosome

genetic recombination - General term for the production of offspring with combinations of traits that differ from those found in either parent

Genetics - The scientific study of heredity and hereditary variation

genome - The genetic material of an organism or virus; the complete complement of an organism's or virus's genes along with its noncoding nucleic acid sequences

genomic imprinting - A phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male or female parent

genotype - The genetic makeup or set of alleles of an oganism

genotype - The genetic makeup or set of alleles of an organism

growth factor - (1) A protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells. (2) A local regulator that acts on nearby cells to stimulate cell proliferation and differentiation

haploid cells - A cell containing only one set of chromosomes

Hemophilia - A human genetic disease caused by a sex-linked recessive allele resulting in the absence of one or more blood clotting proteins; characterized by excessive bleeding following injury

heredity - The transmission of traits from one generation to the next

heterozygous - Having two different alleles for a given gene

homologous chromosomes (or homologs) - A pair of chromosomes of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homolgous chromosome is inherited from the organism's father, the other from the mother. Also called a homologous pair.

Homologous pairs at the metaphase plate - A pair of chromosomes of the same length , centromere position, and staining pattern that possess genes for the same characters at the corresponding loci. One homologous chromosome is inherited from the organism's father the other from the organisms father. Also called a homologous pair. An imaginary structure at a a plane midway between the two poles of a cell in metaphase on which the centromeres of all the duplicated chromosomes are located.

homozygous - Having two identical alleles for a given gene

Huntington's Disease - A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms

hybridization - In genetics, the mating or crossing of two true-breeding varieties

incomplete dominance - The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele

interphase - The period of the cell-cycle when the cell is not dividing. During interphase, cellular metabolic activity is high, chromosomes and organelles are duplicated , and cell size may increase. Interphase accounts for about 90% of the cell cycle

inversion - An abberation in chromosome structure resulting from reattachment of a chromosome from which it originated

karyotype - A display of the chromosomes pairs of a cell arranged by size and shape

kinetochore - A structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle

law of independent assortment - Mendel's second law , stating that each pair of alleles segregatesor assorts , independently of ech other pair during gamete formation ; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.

law of segregation - Mendel's first law stating that two alleles in a pair segregate (seperate from each other) into different gametes during gamete formation

life cycle - The generation-to-generation sequence of stages in the reproduction history of an organism.

linkage map - A genetic map based o the frequencies of recombination between markers during crossing over of homologous chromosomes

linked genes - Genes located close enough together on a chromosome that they tend to be inherited together

locus - A specific place along the length of a chromosome where a given gene is located

malignant tumor - A cancerous tumor containing cells that have significant genetic and cellular changes and are capable of invading and surviving in new sites. Malignant tumors can impair the function of one or more organs

map units - A unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency

meiosis - A modified type of cell division in sexually reproducing organisms consisting of two rounds of cell division but only one round of DNA replication. It results in cells with half the number of chromosome sets as the original cell

Meiosis I - The first division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosome sets as the original cell.

Meiosis II - The second division of a two-stage process of cell division in sexually reproducing organisms that results in cells with half the number of chromosomes sets as the original cell

metaphase - The third stage of mitosis , in which the spindle is complete and the chromosomes attached to microtubules at their kinetechores, are all aligned at the metaphase plate

metaphase plate - An imaginary structure located at a plane midway between two poles at a cell in metaphase on which the centromeres of all the duplicated chromosomes are located

metastasis - The spread of cancer cells to locations distant from their original site

Mitosis - A process of nuclear division in eukaryotic cells conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis conserves replicated chromosome number by allocating replicated chromosomes equally to each of the daughter nuclei

Mitotic (M) phase - The phase of the cell cycle that includes mitosis and cytokinesis

mitotic spindle - An assemblage of microtubules and associated proteins that is involved in the movement of chromosomes during mitosis

monohybrid cross - A cross between two organisms that are heterozygous for the character being followed (or the self pollination of a heterozygous plant)

monohybrids - An organism that is heterozygous with respect to a single gene of interest . All the offspring from a cross between prents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa

monosomic - Referring to the diploid cell that has only one copy of a particular chromosome instead of the normal two

mpf - Maturation promoting factor (or M-phase promoting factor); a protein complex required for a cell to progress from late interphase to mitosis. The active form consists of cyclin and a protein kinase.

multifactorial - Referring to a phenotypical that is influenced by multiple genes and environmental factors

multiplication rule - A rule of probability stating that the probability of two or more independent events occurring together can be determined by multiplying their individual probabilities

nondisjunction - An error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other

origin of replication - Site where the replication of a DNA molecule begins, consisting of a specific of nucleotides

P generation - The true-breeding (homozygous) parent individuals from which F1 hybrid offspring are derived in studies of inheritance P stands for parental

parental types - An offspring that matches one of the true-breeding parental (P Generation) phenotypes; also refers to the phenotype itself

pedigree - A diagram of a family tree with conventional symbols , showing the occurrence of inheritable characters in parents and offspring over multiple generations

phenotype - The observable physical and physiological traits of an organism, which are determined by its genetic makeup

phenotype - The observable physical traits and physiological traits of an organism , which are determined by its genetic makeup

pleiotropy - The ability of a single gene to have multiple effects

polygenic inheritance - An additive effect of two or more genes on a single phenotypic character

polyploidy - A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accidental cell division

prometaphase - The second stage of mitosis, in which the nuclear envelope fragments and spindle microtubules attach to the kinetochores of the chromosomes

prophase - The first stage of mitosis, in which the chromatin condenses into discrete chromosomes visible with a light microscope, the mitotic spindle begins to form, and the nucleolous dissapears but the nucleus remains intact

Punnet square - A diagram used in the study of inheritance to show the predicted genotypic results of random fertilization in genetic crosses between individuals of known genotype

quantitative characters - A heritable feature that varies continuously over a range rather than either or fashion

recessive allele - An allele whose phenotypic effect is not observed in a heterozygote

recombinant chromosomes - A chromosome created when crossing over combines DNA from two parents into a single chromosome.

recombinant types or recombinants - An offspring whose phenotype differs from that of the true-breeding P generation parents; also refers to the phenotype itself

S Phase - The synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated

Separation of homologs -

sex chromosomes - A chromosome responsible for determining the sex of an individual.

sex-linked gene - A gene located on either sex chromosome. Most sex-linked genes are on the X- chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome

sexual reproduction - A type of reproduction in which two parents give rise to offspring that have unique combinations of genes inherited from both parents via the gametes

sickle cell disease - A recessively inherited human blood disorder in which a single nucleotide change in the beta-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals

sister chromatids - Two copies of a duplicated chromosome attached to each other by proteins at the centromere and sometimes, along the arms. While joined, two sister chromatids make up one chromosome. Chromatids are eventually seperated in Meiosis II.

somatic cells - Any cell in a multicellular organism except a sperm or egg or their precursors

somatic cells - Any cell in a multicellular organism except a sperm or egg or their precursors

synapsis - The pairing and physical connection of duplicated homologous chromosomes during prophase I of meiosis

Synapsis and crossing over - The pairing and physical connection of duplicated homologous chromosomes during prophase i of meiosis. The reciprocal exchange of genetic exchange of genetic material between nonsister chromatids during prophase of meiosis

synaptonemal complex - A zipper-like structure composed of proteins, which connects two homologous chromosomes tightly along their lengths

Tay-Sachs disease - A human genetic disease caused by a recessive allele for a dysfunctional enzyme leading to the accumulation of certain lipids in the brain. Seizures, blindness, and the degeneration of motor and mental performance usually become manifest a few months after birth, followed by death within a few years

telophase - The fifth and final stage of mitosis, in which daughter nuclei are forming and cytokinesis has typically begun.

transformation - (1) The conversion of conversion of a normal cell into a cell that is able to divide indefinitely in culture, thus behaving like cancer cell. (Malignant transformation may also describe the series of change in a normal cell in an organism that can change it to a malignant (cancerous cell) cell.) (2) A change in genotype and phenotype due to the assimilation of external DNA by a cell. When the external DNA is from a member of a different species, transformation results in horizontal gene transfer.

translocation - (1) An aberration in chromosome structure resulting from the attachment of a chromosome fragment to a nonhomologous chromosome (2) During protein synthesis, the third stage in the elongation cycle, when the RNA carrying the growing polypeptide moves from the A site to the P site on the ribosome. (3) The transport of organic nutrients to the phloem of vascular plants

trisomic - Referring to a diploid cell that has three copies of a particular chromosome instead of the normal two

true-breeding - Referring to organisms that produce offspring of the same variety over many generations of self pollination

variation - Differences between members of the same species

wild type - The phenotype most commonly observed in natural populations; also refers to the individual with that phenotype

X-linked gene - A gene located on the X chromosome ; such genes show a distinctive pattern of inheritance

zygote - The diploid cell produced by the union of the haploid gametes during fertilization; a fertilized egg