| alleles | Two or more alternative forms of a gene that can occupy the same position on paired chromosomes and affect the same trait.
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| autosomes | In humans, the 22 pairs of chromosomes not related to sexual expression.
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| behavioral genetics | Quantitative study of relative hereditary and environmental influences on behavior.
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| canalization | Limitation on variance of expression of certain inherited characteristics.
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| chromosomes | Coils of DNA that consist of genes.
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| concordant | Term describing tendency of twins to share the same trait or disorder.
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| deoxyribonucleic acid (DNA) | Chemical that carries inherited instructions for the development of all cellular forms of life.
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| dominant inheritance | Pattern of inheritance in which, when a child receives different alleles, only the dominant one is expressed.
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| Down syndrome | Chromosomal disorder characterized by moderate-to-severe mental retardation and by such physical signs as a downward-sloping skin fold at the inner corners of the eyes.
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| epigenesist | Mechanism that turns genes on or off and determines functions of body cells.
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| fertilization | Union of sperm and ovum to produce a zygote; also called conception.
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| genes | Small segments of DNA located in definite positions on particular chromosomes; functional units of heredity.
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| genetic code | Sequence of bases within the DNA molecule; governs the formation of proteins that determine the structure and functions of living cells.
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| genetic counseling | Clinical service that advises couples of their probable risk of having children with hereditary defects.
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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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| incomplete dominance | Pattern of inheritance in which a child receives two different alleles, resulting in partial expression of a trait.
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| Infertility | Inability to conceive after 12 months of trying.
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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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| nonshared environmental effects | The unique environment in which each child grows up, consisting of distinctive influences or influences that affect one child differently than another.
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| obesity | Extreme overweight in relation to age, sex, height, and body type.
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| phenotype | Observable characteristics of a person.
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| polygenic inheritance | Pattern of inheritance in which multiple genes at different sites on chromosomes affect 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 | Neurological 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 human female, XY in the normal human 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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