| alleles | paired genes (alike or different) that affect a trait.
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| autism | pervasive developmental disorder of the brain; characterized by lack of normal social interaction, impaired communication and imagination, and repetitive, obsessive behaviors.
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| autosomes | the 22 pairs of chromosomes not related to sexual expression.
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| behavioral genetics | quantitative study of relative hereditary and environmental influences.
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| canalization | limitation on variance of expression of certain inherited characteristics.
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| chromosomes | coils of DNA that carry the genes
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| clone | (verb) to make a genetic copy of an individual; (noun) a genetic copy of an individual
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| concordant | term describing twins who share the same trait or disorder.
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| deoxyribonucleic acid (DNA) | chemical that carries inherited instructions for the formation and function of body cells.
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| dizygotic(two-egg) twins | twins conceived by the union of two different ova (or a single ovum that has split) with two different sperm cells; also called fraternal twins.
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| dominant inheritance | pattern of inheritance in which, when a child receives contradictory alleles, only the dominant one is expressed.
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| down syndrome | chromosomal disorder characterized by moderate-to-severe mental retardation and a downward sloping skin fold at the inner corner of the eyes.
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| fertilization | union of sperm and ovum fuse to produce a zygote; also called conception
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| genes | small segments of DNA located in definite positions on particular chromosomes
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| genetic code | sequence of base pairs within DNA, which determine inherited characteristics.
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| genotype | genetic makeup of a person, containing both expressed and unexpressed characteristics.
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| genotype environment correlation | tendency of certain genetic and environmental influences to reinforce each other ; may be passive, reactive, (evocative), or active. Also called genotype-environment covariance.
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| genotype-environment interaction | the portion of phenotypic variation that results from the reactions of genetically different individuals to similar environmental conditions.
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| heritability | statistical estimate of contribution of heredity to individual differences in a specific trait within a given population.
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| heterozygous | possessing differing alleles for a trait.
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| homozygous | possessing two identical alleles for a trait.
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| human genome | complete sequence or mapping of genes in the human body and their locations.
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| infertility | inability to conceive after 12 to 18 months of trying.
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| monozygotic (one-egg) twins | twins resulting from the division of a single zygote after fertilization; also called identical twins.
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| multifactorial transmission | combination of genetic and environmental factors to produce certain complex traits.
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| mutations | permanent alterations in genes or chromosomes that may produce harmful characteristics.
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| natural selection | according to Darwin's theory of evolution, process by which characteristics that promote survival of a species are reproduced in successive generations, and characteristics that do not promote survival die out.
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| niche picking | tendency of a person, especially after early childhood, to seek out environments compatible with his or her projects.
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| nonshared environmental effects | tendency of a person, especially after early childhood, to seek out environments compatible with his or her genotype.
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| obesity | extreme overweight in relation to age, sex, height, and body type; sometimes defined as having a mass body index (weight-for-height) at or above the 85th or 95th percentile of growth curves for children of the same age and sex.
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| phenotype | observable characteristics of a person.
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| quantitative trait loci (QLT) | interaction of multiple genes each with affects of varying size, to produce a complex trait.
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| reaction range | potential variability, depending on environmental conditions, in the expression of a hereditary trait.
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| recessive inheritance | pattern of inheritance in which a child receives identical recessive alleles, resulting in expression of a nondominant trait.
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| schizophrenia | mental disorder marked by loss of contact with reality; symptoms include hallucinations and delusions.
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| sex chromosomes | pair of chromosomes that determines sex XX in the normal female, XY in the normal male.
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| sex-linked inheritance | pattern of inheritance in which certain characteristics carried on the X chromosome inherited from the mother are transmitted differently to her male and female offspring.
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| temperament | characteristic disposition, or style of approaching and reacting to situations.
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| zygote | one-celled organism resulting from fertilization
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