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Biology Key Terms

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Term
Definition
Gonads   Sex organs: ovaries in females, testes in males.  
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Autosomes   Paired chromosomes present in both males and females; all chromosomes except the X and Y chromosomes.  
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Sex Chromosomes   Paired chromosomes that differ between males and females, XX in females, XY in male.  
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X-Linked Trait   A phenotype determined by an allele on an X chromosome.  
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Pedigree   A visual representation of the occurrence of phenotype across generations.  
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Y-Chromosome Analysis   Comparing sequences on the Y chromosomes to examine paternity and paternal ancestry.  
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Incomplete Dominance   A form of inheritance in which heterozygotes have a phenotype that is intermediate between homozygous dominant and homozygous recessive.  
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Codominance   A form of inheritance in which both alleles contribute equally to the phenotype.  
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Continuous Variation   Variation in a population showing an unbroken range of phenotypes rather than discrete categories.  
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Polygenic Trait   A trait whose phenotype is determined by the interaction among alleles of more than one gene.  
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Multifactorial Inheritance   An interaction between genes and the environment that contributes to a phenotype or trait.  
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Aneuploidy   An abnormal number of one or more chromosomes (either extra or missing copies).  
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Nondisjunction   The failure of chromosomes to separate accurately during cell division; nondisjunction in meiosis leads to aneuploidy gametes.  
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Trisomy 21   Carrying an extra copy of chromosome 21; also known as Down syndrome.  
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Amniocentesis   A procedure that removes fluid surrounding the fetus to obtain and analyze fetal cells to diagnose genetic disorders.  
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Karyotype   The chromosomal makeup of cells. Karyotype analysis can be used to detect trisomy 21 prenatally.  
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Stem Cells   Immature cells that can divide and differentiate into specialized cell types.  
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Tissue   An organized group of different cell types that work together to carry out a particular function.  
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Adult (Somatic) Stem Cells   Stem cells located in tissues that help maintain and regenerate those tissues.  
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Cellular Differentiation   The process by which a cell specializes to carry out a specific role.  
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Differential Gene Expression   The process by which genes are "turned on," or expressed, in different cell types.  
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Multipotent   Describes a cell with the ability to differentiate into a limited number of cell types in the body.  
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Embryonic Stem Cells   Stem cells that make up an early embryo and which can differentiate into nearly every cell type in the body.  
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Blastocyst   The stage of embryonic development in which the embryo is a hollow ball of cells. Researchers can derive embryonic stem cell lines during the blastocyst stage.  
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Pluripotent   Describes a cell with the ability to differentiate into nearly any cell type in the body.  
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Totipotent   Describes a cell with the ability to differentiate into any cell type in the body.  
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Induced Pluripotent Stem Cell   A pluripotent stem cell that was generated by manipulation of a differentiated somatic cell.  
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Antibiotics   Chemicals that either kill bacteria or slow their growth by interfering with the function of essential bacterial cell structures.  
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Binary Fission   A type of asexual reproduction in which one parental cell divides into two.  
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Population   A group of organisms of the same species living together in the same geographic area.  
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Evolution   Change in allele frequencies in a population over time.  
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Fitness   The relative ability of an organism to survive and reproduce in a particular environment.  
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Natural Selection   Differential survival and reproduction of individuals in response to environmental pressure that leads to change in allele frequencies in a population over time.  
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Adaptation   The process by which populations become better suited to their environment as a result of natural selection.  
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Directional Selection   A type of natural selection in which organisms with phenotypes at one end of a spectrum are favored by the environment.  
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Stabilizing Selection   A type of natural selection in which organisms near the middle of the phenotypic range of variation are favored by the environment.  
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Diversifying Selection   A type of natural selection in which organisms with phenotypes at both extremes of the phenotypic range are favored by the environment.  
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Population Genetics   The study of the genetic makeup of populations and how the genetic composition of a population changes.  
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Gene Pool   The total collection of alleles in a population.  
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Allele Frequency   The relative proportion of an allele in a population.  
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Nonadaptive Evolution   Any change in allele frequency that does not by itself lead a population to become more adapted to its environment; the causes of nonadaptive evolution are mutation, genetic drift, and gene flow.  
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Genetic Drift   Random changes in the allele frequencies of a population between generations; genetic drift tends to have more dramatic effects in smaller populations than in larger ones.  
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Founder Effect   A type of genetic drift in which a small number of individuals leaves one population and establishes a new population; by chance, the newly established population may have lower genetic diversity than the original population.  
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Bottleneck   A type of genetic drift that occurs when a population is suddenly reduced to a small number of individuals, and alleles are lost from the population as a result.  
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Gene Flow   The movement of alleles from one population to another, which may increase the genetic diversity of a population.  
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Inbreeding   Mating between closely related individuals. Inbreeding does not change the allele frequency within a population, but it does increase the proportion of homozygous individuals to heterozygotes.  
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Inbreeding Depression   The negative reproductive consequences for a population associated with having a high frequency of homozygous individuals possessing harmful recessive alleles.  
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Hardy-Weinberg Equilibrium   The principle that, in a nonevolving population, both allele and genotype frequencies remain constant from one generation to the next.  
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Hardy-Weinberg Equation   A mathematical formula that calculates the frequency of genotypes and phenotypes one would expect to find in a nonevolving population.  
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Biological Species Concept   The definition of a species as a population whose members can interbreed to produce fertile offspring.  
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Reproductive Isolation   Mechanisms that prevent mating (and therefor gene flow) between members of different species.  
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Speciation   The genetic divergence of populations, leading over time to reproductive isolation and the formation of new species.  
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