Chromosomal Aberrations
Chromosomal Aberrations
Structural and numerical deviations from the norm that affect many genes at once are called chromosomal aberrations. They are sometimes called chromosomal mutations, but most cytogeneticists prefer to use the term “mutation”.
Despite the incredible precision of meiosis, chromosomal aberrations do occur, and they are more common than one might think. They are responsible for great economic benefit in agriculture. Unfortunately, they are also responsible for many human genetic malformations. It is estimated that five out of every 1000 humans are born with serious genetic defects attributable to chromosomal anomalies. An even greater number of embryos with chromosomal defects are aborted spontaneously, far more than ever reach term.
Changes in chromosome numbers are called euploidy when there is the addition or deletion of whole sets of chromosomes and aneuploidy when a single chromosome is added to or subtracted from a diploid set. A “set of chromosomes contains one member of each homologous pair as would be present in the nucleus of a gamete. The most common kind of euploidy is polyploidy, the carrying of one or more additional sets of chromosomes. Such aberrations are much more common in plants than in animals. Animals are much less tolerant of chromosomal aberrations, because sex determination requires a delicate balance between the numbers of sex chromosomes and autosomes. Many domestic plant species are polyploid (cotton, wheat, apples, oats, tobacco, and others), and perhaps 40% of flowering plants are believed to have originated in this manner. Horticulturists favor polyploids and often try to develop them because they have more intensely colored flowers and more vigorous vegetative growth.
Aneuploidy is usually caused by failure of chromosomes to separate during meiosis (nondisjunction). If a pair of chromosomes fails to separate during the first or second meiotic divisions, both members go to one pole and none to the other. This results in one gamete having n – 1 number of chromosomes and another having n + 1 number of chromosomes. If the n − 1 gamete is fertilized by a normal n gamete, the result is a monosomic animal. Survival is rare because the lack of one chromosome gives an uneven balance of genetic instructions. Trisomy, the result of the fusion of a normal n gamete and an n + 1 gamete, is much more common, and several kinds of trisomic conditions are known in humans. Perhaps the most familiar is trisomy 21, or Down syndrome. As the name indicates, it involves an extra chromosome 21 combined with the chromosome pair 21, and it is caused by nondisjunction of that pair during meiosis. It occurs spontaneously, and there is seldom any family history of the abnormality. However, the risk of its appearance rises dramatically with increasing age of the mother; it occurs 40 times as often in women over 40 years old as among women between the ages of 20 and 30. In cases where maternal age is not a factor, 20% to 25% of trisomy 21 is due to nondisjunction during spermatogenesis; it is paternal in origin and is apparently independent of the father’s age.
A syndrome is a group of symptoms associated with a particular disease or abnormality, although every symptom is not necessarily shown by every patient with the condition. An English physician, John Langdon Down, described the syndrome in 1866 that we now know is caused by trisomy 21. Because of Down’s belief that the facial features of affected individuals were mongoloid in appearance, the condition has been known as mongolism.The resemblances are superficial, however, and the currently accepted names are trisomy 21 and Down syndrome. Among the numerous characteristics of the condition, the most disabling is severe mental retardation. This, as well as other conditions caused by chromosomal aberrations and several other birth defects, can be diagnosed prenatally by a procedure involving amniocentesis. The physician inserts a hypodermic needle through the abdominal wall of the mother and into the fluids surrounding the fetus (not into the fetus) and withdraws some of the fluid, which contains some fetal cells. The cells are grown in culture, their chromosomes are examined, and other tests done. If a severe birth defect is found, the mother has the option of having an abortion performed. As an extra “bonus,” the sex of the fetus is learned after amniocentesis. How? Alternatively, determination of concentrations of certain substances in the maternal serum can detect about 50% of Down syndrome fetuses, which is less invasive than amniocentesis. Ultrasound scanning is not a reliable method.
Structural aberrations involve whole sets of genes within a chromosome. A portion of a chromosome may be reversed, placing the linear arrangement of genes in reverse order (inversion); nonhomologous chromosomes may exchange sections (translocation); entire blocks of genes may be lost (deletion); or an extra section of chromosome may attach to a normal chromosome (duplication). These structural changes often produce phenotypic changes. Duplications, although rare, are important for evolution because they supply additional genetic information that may enable new functions.
Structural and numerical deviations from the norm that affect many genes at once are called chromosomal aberrations. They are sometimes called chromosomal mutations, but most cytogeneticists prefer to use the term “mutation”.
Despite the incredible precision of meiosis, chromosomal aberrations do occur, and they are more common than one might think. They are responsible for great economic benefit in agriculture. Unfortunately, they are also responsible for many human genetic malformations. It is estimated that five out of every 1000 humans are born with serious genetic defects attributable to chromosomal anomalies. An even greater number of embryos with chromosomal defects are aborted spontaneously, far more than ever reach term.
Changes in chromosome numbers are called euploidy when there is the addition or deletion of whole sets of chromosomes and aneuploidy when a single chromosome is added to or subtracted from a diploid set. A “set of chromosomes contains one member of each homologous pair as would be present in the nucleus of a gamete. The most common kind of euploidy is polyploidy, the carrying of one or more additional sets of chromosomes. Such aberrations are much more common in plants than in animals. Animals are much less tolerant of chromosomal aberrations, because sex determination requires a delicate balance between the numbers of sex chromosomes and autosomes. Many domestic plant species are polyploid (cotton, wheat, apples, oats, tobacco, and others), and perhaps 40% of flowering plants are believed to have originated in this manner. Horticulturists favor polyploids and often try to develop them because they have more intensely colored flowers and more vigorous vegetative growth.
Aneuploidy is usually caused by failure of chromosomes to separate during meiosis (nondisjunction). If a pair of chromosomes fails to separate during the first or second meiotic divisions, both members go to one pole and none to the other. This results in one gamete having n – 1 number of chromosomes and another having n + 1 number of chromosomes. If the n − 1 gamete is fertilized by a normal n gamete, the result is a monosomic animal. Survival is rare because the lack of one chromosome gives an uneven balance of genetic instructions. Trisomy, the result of the fusion of a normal n gamete and an n + 1 gamete, is much more common, and several kinds of trisomic conditions are known in humans. Perhaps the most familiar is trisomy 21, or Down syndrome. As the name indicates, it involves an extra chromosome 21 combined with the chromosome pair 21, and it is caused by nondisjunction of that pair during meiosis. It occurs spontaneously, and there is seldom any family history of the abnormality. However, the risk of its appearance rises dramatically with increasing age of the mother; it occurs 40 times as often in women over 40 years old as among women between the ages of 20 and 30. In cases where maternal age is not a factor, 20% to 25% of trisomy 21 is due to nondisjunction during spermatogenesis; it is paternal in origin and is apparently independent of the father’s age.
A syndrome is a group of symptoms associated with a particular disease or abnormality, although every symptom is not necessarily shown by every patient with the condition. An English physician, John Langdon Down, described the syndrome in 1866 that we now know is caused by trisomy 21. Because of Down’s belief that the facial features of affected individuals were mongoloid in appearance, the condition has been known as mongolism.The resemblances are superficial, however, and the currently accepted names are trisomy 21 and Down syndrome. Among the numerous characteristics of the condition, the most disabling is severe mental retardation. This, as well as other conditions caused by chromosomal aberrations and several other birth defects, can be diagnosed prenatally by a procedure involving amniocentesis. The physician inserts a hypodermic needle through the abdominal wall of the mother and into the fluids surrounding the fetus (not into the fetus) and withdraws some of the fluid, which contains some fetal cells. The cells are grown in culture, their chromosomes are examined, and other tests done. If a severe birth defect is found, the mother has the option of having an abortion performed. As an extra “bonus,” the sex of the fetus is learned after amniocentesis. How? Alternatively, determination of concentrations of certain substances in the maternal serum can detect about 50% of Down syndrome fetuses, which is less invasive than amniocentesis. Ultrasound scanning is not a reliable method.
Structural aberrations involve whole sets of genes within a chromosome. A portion of a chromosome may be reversed, placing the linear arrangement of genes in reverse order (inversion); nonhomologous chromosomes may exchange sections (translocation); entire blocks of genes may be lost (deletion); or an extra section of chromosome may attach to a normal chromosome (duplication). These structural changes often produce phenotypic changes. Duplications, although rare, are important for evolution because they supply additional genetic information that may enable new functions.
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| Figure 5-10 Crossing over during meiosis. Nonsister chromatids exchange portions, so that none of the resulting gametes is genetically the same as any other. Gene X is farther from gene Y than Y is from Z; therefore gene X is more frequently separated from Y in crossing over than Y is from Z. |
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