Genetics Health Dictionary

A description that in GENETICS means circumstances in which two or more GENES lie near each other on a chromosome (see CHROMOSOMES) and so may well be inherited together.... linkage

A term used in genetics to describe one of the ways in which a gene is passed from parent to offspring. Many characteristics are determined by a single pair of genes, 1 of each pair being inherited from each parent. A dominant gene overrides an equivalent recessive gene. For example, the gene for brown eye colour is dominant, so if a child inherits the gene for brown eyes from 1 parent and the gene for blue eyes from the other, he or she will have brown eyes. Some genetic disorders are determined by a dominant gene. Examples include Marfan’s syndrome and Huntington’s disease. The child will have the disease if he or she inherits the gene from 1 or both parents.... dominant

The rod-shaped bodies to be found in the nucleus of every cell in the body. They contain the GENES, or hereditary elements, which establish the characteristics of an individual. Composed of a long double-coiled ?lament of DNA, they occur in pairs – one from the maternal, the other from the paternal – and human beings possess 46, made up of 23 pairs. The number of chromosomes is speci?c for each species of animal. Each chromosone can duplicate an exact copy of itself between each cell division. (See GENETIC CODE; GENETICS; HEREDITY; MEIOSIS; SEX CHROMOSOMES.)... chromosomes

The science of how the activity of DNA (deoxyribonucleic acid, which is the fundamental genetic material of cells) can be altered semi-permanently by chemical processes rather than by natural MUTATION. Genes contain instructions for making proteins. The natural process of implementing these instructions – gene expression – can be altered by chemical groups attaching themselves to the chemical bases that make up a strand of DNA. This, in turn, affects the generation of proteins from the genes so tagged. Some chemical groups can even stop a gene from being expressed. Recently, research in Australia showed that such a chemically induced alteration could be inherited, at least in mice. This points to the possibility that inherited epigenetic characteristics could cause ‘inherited diseases’ in the same way that natural genetic mutations do. These developments suggest that epigenetics will be an important part of genetic studies and research. Not all geneticists, however, believe that this developing aspect of genetics is so important and this debate will continue.... epigenetics

(1) Stopping a ?ow or discharge of, for example, blood from a wound. (2) In genetics the term describes a type of gene action (see GENES) where a gene is able to block the action of another one.... epistasis

An uninterrupted process of change from one condition, form or state to another. In biological evolution, all varieties of living things are seen as having developed by inheritable, incremental changes from unicellular structures to complex organisms such as humankind. Although the likelihood of some form of evolution had been postulated by scientists in the late 18th and early 19th centuries, the prime contribution to the development of biological evolutionary doctrine came from the British scientist, Charles Darwin, who argued in his book The Origin of Species by Means of Natural Selection (1859) that natural selection resulted in the survival of the ?ttest organisms. The precise biological mechanism of evolution was not unravelled until the 20th century, with the discovery of CHROMOSOMES and GENES and the development of the science of genetics. Charles Darwin’s theory was based on his studies of the varied and unique animal life in the Galapagos Islands in the 19th century. He believed that the diversity of life on the planet could be ascribed to the combined effects of random variation in living things, inherited by succeeding generations.... evolution

The procedure whereby advice is given about the risks of a genetic disorder and the various options that are open to the individual at risk. This may often involve establishing the diagnosis in the family, as this would be a prerequisite before giving any detailed advice. Risks can be calculated from simple Mendelian inheritance (see MENDELISM) in many genetic disorders. However, in many disorders with a genetic element, such as cleft lip or palate (see CLEFT PALATE), the risk of recurrence is obtained from population studies. Risks include not only the likelihood of having a child who is congenitally affected by a disorder, but also, for adults, that of being vulnerable to an adult-onset disease.

The options for individuals would include taking no action; modifying their behaviour; or taking some form of direct action. For those at risk of having an affected child, where prenatal diagnosis is available, this would involve either carrying on with reproduction regardless of risk; deciding not to have children; or deciding to go ahead to have children but opting for prenatal diagnosis. For an adult-onset disorder such as a predisposition to ovarian cancer, an individual may choose to take no action; to take preventive measures such as use of the oral contraceptive pill; to have screening of the ovaries with measures such as ultrasound; or to take direct action such as removing the ovaries to prevent ovarian cancer from occurring.

There are now regional genetics centres throughout the United Kingdom, and patients can be referred through their family doctor or specialists.... genetic counselling

The centralised collection and storage of information about the health of individuals. Recent advances in GENETICS have raised concerns about the potential for abuse of all health databases, whether maintained for scienti?c research – which has long used them – or for government or community health planning, or by groups of professionals (or individuals) to help in the treatment of patients. The public is concerned about whether their rights to privacy and con?dentiality are threatened by databases and whether information about them could be disclosed and misused.... health databases

The term applied to a law enunciated by G. J. Mendel that the o?spring is not intermediate in type between its parents, but that the type of one or other parent is predominant. Characteristics are classed as either dominant or recessive. The o?spring of the ?rst generation tend to inherit the dominant characteristics, whilst the recessive characteristics remain latent and appear in some of the o?spring of the second generation. If individuals possessing recessive characters unite, recessive characters then become dominant characters in succeeding generations. (See GENETICS.)... mendelism

Also called pharmacogenetics – the use of human genetic variations to optimise the discovery and development of drugs and the treatment of patients. The human race varies much more in its genetic make-up than has previously been realised; these variations in GENES and their PROTEIN products could be utilised to provide safer and more e?ective drugs. Genes affect drug absorption, distribution, METABOLISM and excretion. Drugs are designed and prescribed on the basis of a population’s needs, but patients comprise a diverse range of individuals. For example, nearly one-third of patients fail to respond to the cholesterol-reducing group of drugs, the STATINS. Around half do not respond to the tricyclic ANTIDEPRESSANT DRUGS. Over 80 per cent of patients’ responses to drugs depends on their genetics: this genetic variation needs to be identi?ed so as to make the prescription of drugs more e?ective, and technology for analysing genetic variants is progressing. Assessing drug e?ectiveness, however, is not simple because the health and diets of individuals are di?erent and this can affect the response to a drug. Even so, the genetic identi?cation of people who would or would not respond to a particular drug should bene?t patients by ensuring a more accurately targeted drug and by reducing the risks to a person of side-effects from taking a drug that would not work. There would also be substantial economic savings.... pharmacogenomics

A branch of genetics concerned with the alteration of the genetic material of an organism to produce a desired change in the organism’s characteristics. In practice, genetic engineering has mainly been used to mass-produce a variety of substances that are useful in medicine. A gene responsible for making a useful protein is identified and inserted into another cell (most often a bacterium or a yeast) that reproduces rapidly to form a colony of cells containing the gene. This colony produces the protein in large amounts. Some human hormones (notably insulin and growth hormone) and proteins such as factor VIII (used to treat haemophilia) are made in this way.... genetic engineering

A term used in genetics to describe one of the ways by which a gene is passed from parent to offspring. Many characteristics are determined by a single pair of genes, 1 of each pair being inherited from each parent. A recessive gene is overridden by an equivalent dominant gene. For example, the gene for blue eye colour is recessive; therefore, if a child inherits the gene for brown eyes from 1 parent and the gene for blue eyes from the other, the “blue eye” gene is overridden by the “brown eye” gene, and the child has brown eyes. The child must inherit the recessive blue eye gene from both parents in order to have blue eyes. Many genetic disorders are determined by a recessive gene. Examples include cystic fibrosis and sickle cell anaemia. The child will only have the disease if he or she inherits the gene from both parents.... recessive

n. (in genetics) a chromosome or fragment of a chromosome that has no *centromere. Since acentrics cannot attach to the *spindle they are usually lost during cell division. They are often found in cells damaged by radiation. —acentric adj.... acentric

n. (in genetics) a structure consisting of homologous chromosomes attached to each other by *chiasmata during the first division of *meiosis. —bivalent adj.... bivalent

n. 1. a person who harbours the microorganisms causing a particular disease without experiencing signs or symptoms of infection and who can transmit the disease to others. 2. (in genetics) a person who bears a gene for an abnormal trait without showing any signs of the disorder; the carrier is usually *heterozygous for the gene concerned, which is *recessive. 3. an animal, usually an insect, that passively transmits infectious organisms from one animal to another or from an infected animal to a human being. See also vector.... carrier

n. (pl. chiasmata) (in genetics) the point at which homologous chromosomes remain in contact after they have started to separate in the first division of *meiosis. Chiasmata occur from the end of prophase to anaphase and represent the point at which mutual exchange of genetic material takes place (see crossing over).... chiasma

(in genetics) the exchange of sections of chromatids that occurs between pairs of homologous chromosomes, which results in the recombination of genetic material. It occurs during *meiosis at a *chiasma.... crossing over

n. a science that links the study of inheritance (genetics) with that of cells (cytology); it is concerned mainly with the study of the *chromosomes, especially their origin, structure, and functions.... cytogenetics

n. (in genetics) see deletion.... deficiency

(deficiency) n. (in genetics) a type of mutation involving the loss of DNA. The deletion may be small, affecting only a portion of a single gene, or large, resulting in loss of a part of a chromosome and affecting many genes.... deletion

n. the alleged ‘science’ concerned with the improvement of the human race by means of the principles of genetics, strongly associated with ideas of selective breeding, discrimination, and immoral regimes, such as the Nazis in 20th-century Germany. Interventions used in reproductive medicine, such as *preimplantation genetic diagnosis, antenatal screening, diagnostic testing, and abortion, are regarded by some as potentially eugenic. —eugenic adj.... eugenics

a condition in which an individual belongs to one gender on the basis of physical appearance and genetics but identifies psychologically with the other gender. The name was introduced in DSM-5; in DSM-IV-TR it was called gender identity disorder. The condition is diagnosed only where there is evidence of strong and persistent cross-gender identification and discomfort about one’s sex, these cause significant distress and social impairment, and there is no concurrent endocrine disorder. Treatment may include counselling, hormone therapy, and gender reassignment surgery. See also transgender.... gender dysphoria

n. the study of the social, moral, and political implications of knowledge and practice in genetics and genomics.... genethics

n. the branch of genetics concerned with the study of genomes. It includes both the mapping of genomes – and ultimately producing a DNA sequence for any particular organism or individual – and understanding how gene expression is controlled and gene products change under different conditions, including disease states.... genomics

n. the process that causes the biological similarity between parents and their offspring. *Genetics is the study of heredity.... heredity

adj. 1. (in anatomy) describing organs or parts that have the same basic structure and evolutionary origin, but not necessarily the same function or superficial structure. Compare analogous. 2. (in genetics) describing a pair of chromosomes of similar shape and size and having identical gene loci. One member of the pair is derived from the mother; the other from the father.... homologous

n. 1. (in animal behaviour) a rapid and irreversible form of learning that takes place in some animals during the first hours of life. Animals attach themselves in this way to members of their own species, but if they are exposed to creatures of a different species during this short period, they become attached to this species instead. 2. (in genetics) a phenomenon whereby gene expression depends on whether the chromosome is maternal or paternal in origin. For example, both *Prader-Willi syndrome and *Angelman syndrome are caused by a *deletion of the same part of chromosome 15. When the deletion involves the chromosome 15 that came from the father, the child has Prader-Willi syndrome; inheritance of the same deletion from the mother results in Angelman syndrome.... imprinting

n. 1. (in anatomy) a region or site. The locus caeruleus is a small pigmented *nucleus in the floor of the fourth ventricle of the brain. 2. (in genetics) the region of a chromosome occupied by a particular gene.... locus

n. the condition in which excess fat has accumulated in the body, in both the subcutaneous and visceral tissues. Clinical obesity is defined as a *body mass index of 30 or over. A waist circumference of greater than 102 cm in men and 88 cm in women is a strong predictor of a person developing additional medical conditions associated with obesity. The accumulation of fat is usually caused by the consumption of more food than is required for producing enough energy for daily activities. There is ongoing research into how much obesity is due to genetics and how much to environmental factors. Hunger and satiety appear to be controlled by peptide messengers, encoded by specific genes and acting on the brain; an example is *leptin. Treatment includes traditional weight reduction diets; surgery, such as gastric banding or gastric bypass; and drug treatment (*orlistat). Obesity is now seen as the precursor of lethal conditions such as cancer and diabetes, making it a global public health issue. —obese adj.... obesity

n. the general medicine of childhood. Handling the sick child requires a special approach at every age from birth (or preterm birth) to adolescence and also a proper understanding of parents. It also requires detailed knowledge of genetics, obstetrics, psychological development, management of disabilities at home and in school, and effects of social conditions on child health. The preventive measures associated with all these aspects of paediatrics are the concern of *public health consultants and *community paediatricians. See also child health clinic. —paediatrician n.... paediatrics

(PCR) a technique of molecular genetics in which a particular sequence of DNA can be isolated and amplified sufficiently to enable genetic analysis. The technique may be utilized, for example, in the identification of viruses in tissue samples, e.g. human papillomavirus in cervical smears.... polymerase chain reaction

n. (in genetics) the occurrence of a chromosome or a genetic character in more than one form, resulting in the coexistence of more than one morphological type in the same population.... polymorphism

n. (in genetics) 1. the four cells resulting from meiosis after the second telophase. 2. the four chromatids of a pair of homologous chromosomes (see bivalent) in the first stage of meiosis.... tetrad

n. (in genetics) a type of chromosome mutation in which a part of a chromosome is transferred to another part of the same chromosome or to a different chromosome. This changes the order of the genes on the chromosomes and can lead to serious genetic disorders, e.g. chronic myeloid leukaemia.... translocation

1. (mother-to-child transmission) transmission of an infection, such as HIV, hepatitis B, or hepatitis C, from mother to fetus via the placental circulation. 2. (in population genetics) inheritance of an allele or condition from either the father or mother.... vertical transmission