Autosome Health Dictionary

In humans there are 23 pairs of CHROMOSOMES. Male and female di?er in respect of one pair. In the nucleus of female cells, the two members of the pair are identical and are called X chromosomes. In the male nucleus there is one X chromosome paired with a dissimilar, di?erently sized chromosome called the Y chromosome. In the sex cells, after MEIOSIS, all cells in the female contain a single X chromosome. In the male, half will contain an X chromosome and half a Y chromosome. If a sperm with an X chromosome fertilises an ovum (which, as stated, must have an X chromosome) the o?spring will be female; if a sperm with a Y chromosome fertilises the ovum the o?spring will be male. It is the sex chromosomes which determine the sex of an individual.

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.)... sex chromosomes

Humans possess around 30,000 genes which are the biological units of heredity. They are arranged along the length of the 23 pairs of CHROMOSOMES and, like the chromosomes, therefore come in pairs (see GENETIC CODE). Human beings have 46 chromosomes, comprising two sex chromosomes and 44 autosomes, but there is also a mitochondrial chromosome outside the cell nucleus (see CELLS) which is inherited from the mother.

Half of a person’s genes come from the father and half from the mother, and this mix determines the o?spring’s characteristics. (A quarter of a person’s genes come from each of the four grandparents.) Genes ful?l their functions by controlling the manufacture of particular proteins in the body. The power that genes have to in?uence the body’s characteristics varies: broadly, some are dominant (more powerful); others are recessive (less powerful) whose functions are overridden by the former. Genes are also liable to change or mutate, giving the potential for the characteristics of individuals or their o?spring to be altered. (See GENETIC CODE; GENETIC DISORDERS; GENE THERAPY; HUMAN GENOME.)... genes

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

Variations from normal in the number or structure of chromosomes contained in a person’s cells. The cause is generally a fault in the process of chromosome division, either during the formation of an egg or sperm, or during the first few divisions of a fertilized egg. Chromosomal abnormalities are classified according to whether they involve the 44 autosomes or the 2 X and Y sex chromosomes. A complete extra set of chromosomes per cell is called polyploidy and is lethal.

Autosomal abnormalities cause physical and mental defects of varying severity. Some types of autosomal abnormality, known as trisomy, consist of an extra chromosome on 1 of the 22 pairs of autosomes. The most common trisomy is Down’s syndrome. Sometimes, part of a chromosome is missing, as in cri du chat syndrome. In translocation, a part of a chromosome is joined to another, causing no ill effects in the person but a risk of abnormality in his or her children.

Sex chromosome abnormalities include Turner’s syndrome, in which a girl is born with a single X chromosome in her

cells instead of 2, causing physical abnormalities, defective sexual development, and infertility. A boy with 1 or more extra X chromosomes has Klinefelter’s syndrome, which causes defective sexual development and infertility. The presence of an extra X chromosome in women or an extra Y chromosome in men normally has no physical effect but increases the risk of mild mental handicap.

Chromosomal abnormalities are diagnosed by chromosome analysis in early pregnancy, using amniocentesis or chorionic villus sampling.... chromosomal abnormalities

adj. describing a gene (or its corresponding characteristic) whose effect is shown in the individual only when its *allele is the same, i.e. when two such alleles are present (the double recessive condition). Many hereditary diseases (including cystic fibrosis) are due to the presence of a defective gene as a double recessive. They are said to show autosomal recessive inheritance, since the gene is carried on an autosome (any chromosome other than a sex chromosome). Compare dominant. —recessive n.... recessive

a chromosome that is involved in the determination of the sex of the individual. Women have two *X chromosomes; men have one X chromosome and one *Y chromosome. Compare autosome.... sex chromosome

The transmission of characteristics and disorders from parents to their children through the influence of genes. Genes are the units of DNA (deoxyribonucleic acid) that are contained in a person’s cells; controls all growth and functioning of the body. Half of a person’s genes come from the mother, half from the father.

Genes are organized into chromosomes in the cell nucleus. Genes controlling most characteristics come in pairs, 1 from the father, the other from the mother. Everyone has 22 pairs of chromosomes (called autosomes) bearing these paired genes, in addition to 2 sex chromosomes. Females have 2 X chromosomes; males have an X and a Y chromosome.

Most physical characteristics, many disorders, and some mental abilitiesand aspects of personality are inherited. The inheritance of normal traits and disorders can be divided into those controlled by a single pair of genes on the autosomal chromosomes (unifactorial inheritance, such as eye colour); those controlled by genes on the sex chromosomes (sex-linked inheritance, such as haemophilia); and those controlled by the combination of many genes (multifactorial inheritance, such as height).

Either of the pair of genes controlling a trait may take any of several forms, known as alleles. For example, the genes controlling eye colour exist as 2 main alleles, coding for blue and brown eye colour. The brown allele is dominant over blue in that it “masks” the blue allele, which is called recessive to the brown allele. Only 1 of the pair of genes controlling a trait is passed to a child from each parent. For example, someone with the brown/blue combination for eye colour has a 50 per cent chance of passing on the blue gene, and a 50 per cent chance of passing on the brown gene, to any child. This factor is combined with the gene coming from the other parent, according to dominant or recessive relationships, to determine the child’s eye colour. Certain genetic disorders are also inherited in a unifactorial manner (for example, cystic fibrosis and achondroplasia).

Sex-linked inheritance depends on the 2 sex chromosomes, X and Y. The most obvious example is gender. Male gender is determined by genes on the Y chromosome, which is present only in males. Any faults in a male’s genes on the X chromosome tend to be expressed outwardly because such a fault cannot be masked by the presence of a normal gene on a 2nd X chromosome (as it can in females). Faults in the genes of the X chromosome include those responsible for colour vision deficiency, haemophilia, and other sex-linked inherited disorders, which almost exclusively affect males.

Multifactorial inheritance, along with the effects of environment, may play a part in causing certain disorders, such as diabetes mellitus and neural tube defects.... inheritance