This medical term were found from 1 different sources | Health Encyclopedia
Another term for gender and a commonly used term for sexual intercourse.
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Sometimes during cell division chromosomes may be lost or duplicated, or abnormalities in the structure of individual chromosomes may occur. The surprising fact is the infrequency of such errors. About one in 200 live-born babies has an abnormality of development caused by a chromosome, and two-thirds of these involve the sex chromosomes. There is little doubt that the frequency of these abnormalities in the early embryo is much higher, but because of the serious nature of the defect, early spontaneous ABORTION occurs.
Chromosome studies on such early abortions show that half have chromosome abnormalities, with errors of autosomes being three times as common as sex chromosome anomalies. Two of the most common abnormalities in such fetuses are triploidy with 69 chromosomes and trisomy of chromosome 16. These two anomalies almost always cause spontaneous abortion. Abnormalities of chromosome structure may arise because of:
Deletion Where a segment of a chromosome is lost.
Inversion Where a segment of a chromosome becomes detached and re-attached the other way around. GENES will then appear in the wrong order and thus will not correspond with their opposite numbers on homologous chromosomes.
Duplication Where a segment of a chromosome is included twice over. One chromosome will have too little nuclear material and one too much. The individual inheriting too little may be non-viable and the one with too much may be abnormal.
Translocation Where chromosomes of different pairs exchange segments.
Errors in division of centromere Sometimes the centromere divides transversely instead of longitudinally. If the centromere is not central, one of the daughter chromosomes will arise from the two short arms of the parent chromosome and the other from the two long arms. These abnormal daughter chromosomes are called isochromosomes.
These changes have important bearings on heredity, as the e?ect of a gene depends not only upon its nature but also upon its position on the chromosome with reference to other genes. Genes do not act in isolation but against the background of other genes. Each gene normally has its own position on the chromosome, and this corresponds precisely with the positon of its allele on the homologous chromosome of the pair. Each member of a pair of chromosomes will normally carry precisely the same number of genes in exactly the same order. Characteristic clinical syndromes, due to abnormalities of chromosome structure, are less constant than those due to loss or gain of a complete chromosome. This is because the degree of deletion, inversion and duplication is inconstant. However, translocation between chromosomes 15 and 21 of the parent is associated with a familial form of mongolism (see DOWN’S (DOWN) SYNDROME) in the o?spring, and deletion of part of an X chromosome may result in TURNER’S SYNDROME.
Non-disjunction Whilst alterations in the structure of chromosomes arise as a result of deletion or translocation, alterations in the number of chromosomes usually arise as a result of non-disjunction occurring during maturation of the parental gametes (germ cells). The two chromosomes of each pair (homologous chromosomes) may fail to come together at the beginning of meiosis and continue to lie free. If one chromosome then passes to each pole of the spindle, normal gametes may result; but if both chromosomes pass to one pole and neither to the other, two kinds of abnormal gametes will be produced. One kind of gamete will contain both chromosomes of the pair, and the other gamete will contain neither. Whilst this results in serious disease when the autosomes are involved, the loss or gain of sex chromosomes seems to be well tolerated. The loss of an autosome is incompatible with life and the malformation produced by a gain of an autosome is proportional to the size of the extra chromosome carried.
Only a few instances of a gain of an autosome are known. An additional chromosome 21 (one of the smallest autosomes) results in mongolism, and trisomy of chromosome 13 and 18 is associated with severe mental, skeletal and congenital cardiac defects. Diseases resulting from a gain of a sex chromosome are not as severe. A normal ovum contains 22 autosomes and an X sex chromosome. A normal sperm contains 22 autosomes and either an X or a Y sex chromosome. Thus, as a result of nondisjunction of the X chromosome at the ?rst meiotic division during the formation of female gametes, the ovum may contain two X chromosomes or none at all, whilst in the male the sperm may contain both X and Y chromosomes (XY) or none at all. (See also CHROMOSOMES; GENES.)... Medical Dictionary
form. In the absence of these hormones, a female reproductive tract develops. At puberty, another surge of hormones produces secondary sexual characteristics.
Chromosomal abnormalities or hormonal defects can lead to ambiguous sex (see genitalia, ambiguous), although true hermaphroditism is rare.... BMA Medical Dictionary
Sex education in schools is regarded as an e?ective way of reducing teenaged pregnancy, especially when linked with contraceptive services. Several studies have shown that it does not cause an increase in sexual activity and may even delay the onset of sexual relationships and lessen the number of partners. Programmes taught by youth agencies may be even more e?ective than those taught in the classroom – possibly because teaching takes place in small groups of volunteer participants, and the programmes are tailored to their target populations. Despite improvements in sex education, the United Kingdom has the highest incidence of teenaged pregnancies in the European Community.
Sex education, including information about AIDS/HIV and other sexually transmitted infections (STIs), is compulsory in all state-maintained secondary schools in England and Wales. The National Curriculum includes only biological aspects of AIDS/HIV, STIs and human sexual behaviour.
All maintained schools must have a written statement of their policy, which is available to parents. The local education authority, governing body and headteacher should ensure that sex education encourages pupils to have due regard to moral considerations and the value of family life. Sex-education policies and practices are monitored by the O?ce for Standards in Education (OFSTED) and the O?ce of HM Chief Inspector of Schools (OHMCI) as part of school inspections.... Medical Dictionary
In girls, breast enlargement is the first sign.
Shortly afterwards, pubic and underarm hair appears, and body fat increases around the hips, stomach, and thighs to produce the female body shape.
In boys, the first sign is enlargement of the testes, followed by thinning of the scrotal skin and enlargement of the penis.
Pubic, facial, axillary, and other body hair appears, the voice deepens, and muscle bulk and bone size increase.... BMA Medical Dictionary
Sexual dysfunction may be due to physical or psychiatric disease, or it may be the result of the administration of drugs. The main group of drugs likely to cause sexual problems are the ANTICONVULSANTS, the ANTIHYPERTENSIVE DRUGS, and drugs such as metoclopramide that induce HYPERPROLACTINAEMIA. The benzodiazepine TRANQUILLISERS can reduce libido and cause failure of erection. Tricyclic ANTIDEPRESSANT DRUGS may cause failure of erection and clomipramine may delay or abolish ejaculation by blockade of alpha-adrenergic receptors. The MONOAMINE OXIDASE INHIBITORS (MAOIS) often inhibit ejaculation. The PHENOTHIAZINES reduce sexual desire and arousal and may cause di?culty in maintaining an erection. The antihypertensive drug, methyldopa, causes impotence in over 20 per cent of patients on large doses. The beta-adrenoceptorblockers and the DIURETICS can also cause impotence. The main psychiatric causes of sexual dysfunction include stress, depression and guilt.... Medical Dictionary
The incidence of STDs rose sharply during World War II but the advent of PENICILLIN and subsequent antibiotics meant that syphilis and gonorrhoea could be treated e?ectively. The arrival of oral contraception and more tolerant public attitudes to sexual activities resulted in an increase in the incidence of sexually transmitted infections. The diagnosis of NONSPECIFIC URETHRITIS (NSU), once given to many patients whose symptoms were not due to the traditional recognised infections, was in the 1970s realised to be wrong, as the condition was proved to be the result of infection by chlamydia.
Most STDs are treatable, but herpes is an infection that could become chronic, while hepatitis B and, of course, AIDS/HIV are potentially fatal – although treatment of HIV is now proving more e?ective. As well as the treatment and subsequent monitoring of patients with STDs, one of the important functions of clinics has been the tracing, treatment and follow-up of sexual contacts of infected individuals, a procedure that is conducted con?dentially.
Apart from AIDS/HIV, the incidence of STDs fell during the 1980s; however in some countries the agents causing syphilis and gonorrhoea began to develop resistance to antibiotics, which showed the continued importance of practising safe sex – in particular by restricting the number of sexual partners and ensuring the regular use of condoms. In the United Kingdom the rates per million of the male population infected by syphilis rose from 8.8 in 1991 to 9.7 in 1999; in females the ?gures were 4.0 to 4.5, respectively. For gonorrhoea, the ?gures for men were 399.4 in 1991 and 385 in 1999, with women also showing a reduction, from
216.5 to 171.3. In 1991, 552.6 per million of men had chlamydia, a ?gure which rose to
829.5 in 1999; for women in the same period the incidence also rose, from 622.5 to 1,077.1 per million. For genital herpes simplex virus, the infection rate for men fell from 236.6 per million to 227.7, whereas the ?gures for women showed a rise, 258.5 to 357. The incidence of AIDS/HIV is given under the relevant entry. (These ?gures are based on information in United Kingdom Health Statistics, 2001 edition, UKHSI, published by the O?ce of National Statistics.)... Medical Dictionary
Practising safer sex can help prevent STIs.... BMA Medical Dictionary