| alleles | Two or more alternative forms of a gene that can occupy the same position on paired chromosomes and affect the same trait. 66
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| assisted reproductive technology (ART) | Methods used to achieve conception through artificial means. 60
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| autosomes | In humans, the 22 pairs of chromosomes not related to sexual expression. 65
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| behavioral genetics | Quantitative study of relative hereditary and environmental influences on behavior. 76
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| canalization | Limitation on variance of expression of certain inherited characteristics. 78
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| chromosomes | Coils of DNA that consist of genes. 65
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| concordant | Term describing the tendency of twins to share the same trait or disorder. 76
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| deoxyribonucleic acid (DNA) | Chemical that carries inherited instructions for the development of all cellular forms of life. 64
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| dominant inheritance | Pattern of inheritance in which, when a child receives different alleles, only the dominant one is expressed. 66
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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. 72
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| epigenesis | Mechanism that turns genes on or off and determines functions of body cells. 68
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| fertilization | Union of sperm and ovum to produce a zygote; also called conception. 59
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| genes | Small segments of DNA located in definite positions on particular chromosomes; functional units of heredity. 65
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| genetic code | Sequence of bases within the DNA molecule; a set of rules that govern the formation of proteins that determine the structure and functions of living cells. 65
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| genetic counseling | Clinical service that advises prospective parents of their probable risk of having children with hereditary defects. 74
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| genotype | Genetic makeup of a person, containing both expressed and unexpressed characteristics. 67
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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. 79
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| genotype–environment interaction | Effect of the interaction between genes and the environment on phenotypic variation. 79
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| heritability | Statistical estimate of contribution of heredity to individual differences in a specific trait within a given population at a particular time. 76
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| heterozygous | Possessing differing alleles for a trait. 66
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| homozygous | Possessing two identical alleles for a trait. 66
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| human genome | The complete sequence of genes in the human body. 65
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| incomplete dominance | Pattern of inheritance in which a child receives two different alleles, resulting in partial expression of a trait. 71
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| infertility | Inability to conceive after 12 months of trying. 59
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| multifactorial transmission | Combination of genetic and environmental factors to produce certain complex traits. 68
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| mutations | Permanent alterations in genes or chromosomes that usually produce harmful characteristics but provide the raw material of evolution. 67
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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. 80
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| obesity | Extreme overweight in relation to age, sex, height, and body type. 81
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| phenotype | Observable characteristics of a person. 67
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| polygenic inheritance | Pattern of inheritance in which multiple genes at different sites on chromosomes affect a complex trait. 67
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| reaction range | Potential variability, depending on environmental conditions, in the expression of a hereditary trait. 77
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| recessive inheritance | Pattern of inheritance in which a child receives identical recessive alleles, resulting in expression of a nondominant trait. 66
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| schizophrenia | Neurological disorder marked by loss of contact with reality; hallucinations and delusions; loss of coherent, logical thought; and inappropriate emotionality. 82
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| sex chromosomes | Pair of chromosomes that determines sex: XX in the normal human female, XY in the normal human male. 65
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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. 71
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| temperament | Characteristic disposition or style of approaching and reacting to situations. 82, 209
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| zygote | One-celled organism resulting from fertilization. 59
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