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al pair and has a normal allele), or is recessive (expressed only in homozygotes, when it is present on both chromosomes). Quantitative inheritance A quantitative trait is one that is under the control of many factors, both genetic and environmental, with each contrib- uting only a small amount to the total variability of the trait. The phenotype may show continuous variation (for example, height and skin color) or quasicontinuous variation (taking only integer values, such as the number of ridges in a fingerprint), or it may be discontinuous [a presence/absence trait, such as diabetes or intellectual disability (formerly termed mental retardation)]. With discontinuous traits, it is assumed that there exists an underlying continuous variable and that individuals having a value of this variable above (or below) a threshold possess the trait. A trait that "runs in families" is said to be familial. However, not all familial traits are hereditary because relatives tend to share common environments as well as common genes. It is the major task of quantitative genetics to disentangle the effects of environment and heredity, but this task is not easy in humans because the two factors are often confounded. A rough mea- sure of the relative importance of heredity and environment is an index called heritability. The total variance (a statistical measure of variability) of a trait can be partitioned into a ge- netic variance and an environmental variance, at least in simple cases. The quantitative model can be expressed as Heritability is then defined as For example, the heritability of height in humans is about 0.75; that is, about 75% of the total variance in height is due to vari- ability in genes that affect height and 25% is due to exposure to different environments. Hereditary diseases Medical genetics (that is, genetic counseling, including prenatal diagnostics) has become an integral part of preventive medi- cine. It has contributed increasingly to systematics of disease, diagnostics, and even therapy. Because many external causes of disease, including infections, have been brought under control in today's modern world, doctors can devote much of their skill to treating diseases from internal sources, that is, hereditary diseases, or those brought about by the interaction of genetic predispositions with certain stress factors in the environment. Hereditary diseases may be subdivided into three classes: chromosomal diseases; hereditary diseases with simple, mendelian modes of inheritance; and multifactorial diseases. Chromosomal diseases One of every 200 newborns suffers from an abnormality that is caused by a microscopically visible deviation in the number or structure of chromosomes. Such chromosomal aberrations are much more common (approximately 40%) among sponta- neous miscarriages. The most important clinical abnormality among the survivors is Down syndrome—a condition due to trisomy of chromosome 21, one of the smallest human chro- mosomes. The term trisomy means that this chromosome is present three times (instead of twice); the entire chromosome complement therefore comprises 47 (not 46) chromosomes. Down syndrome occurs one to two times in every 1000 births, and it is a good example of a characteristic pattern of abnor- malities that is produced by a single genetic defect. Other autosomal aberrations observed in living newborns that lead to characteristic syndromes include trisomies 13 and 18 (both very rare), as well as a variety of structural aberrations, including translocations (exchanges of chromosomal segments among different chromosomes) and deletions (losses of chro- mosome segments). Translocations normally have no influence on the health status of the individual if there is no gain or loss of chromosomal material (these are called balanced transloca- tions). However, carriers of balanced translocations usually run a high risk of having "unbalanced" offspring, that is, children in whom the same translocation causes the gain or loss of genetic material and who suffer from a characteristic malformation syndrome. Clinical syndromes caused by specific aberrations vary, but certain clinical signs are common: low birth weights; a peculiar face; delayed general, and especially mental, development, often leading to severe mental deficiency; and multiple malfor- mations, including the abnormal development of limbs, heart, and kidneys. Less severe signs than those caused by autosomal aberrations are found in individuals with abnormalities in the number (and structure) of sex chromosomes. This is because, in individuals having more than one X chromosome, the additional X chro- mosomes are inactivated early in pregnancy. For example, in women, one of the two X chromosomes is always inactivated. Inactivation occurs at random so that every normal woman is a mosaic of cells in which either one or the other X chromosome is active. Additional X chromosomes that an individual may have received will also be inactivated; thus, genetic imbalance Human Genetics (continued) + ward ' s science