1993 Woodrow Wilson Biology Institute
Chromosomes are arranged into seven groups based on size and centromere location. The centromeres can be found in the middle of the chromosome (median), near one end (acrocentric) or in between these first two (submedian).
Group B: chromosomes 4-5 are large with submedian centromere; Group C: chromosomes 6-12 are medium sized with submedian centromere;
Group D: chromosomes 13-15 are medium sized with acrocentric centromere;
Group E: chromosomes 16-18 are short with median or submedian centromere;
Group F: chromosomes 19-20 are short with median centromere;
Group G: chromosomes 21-22 are very short with acrocentric centromere; chromosome X is similar to group C and Y is similar to group G.
Chromosomes can also be identified by dark or light regions called bands. Bands are produced by staining the chromosomes with stains. Different stains produce different banding patterns (Levitan). See last page for chromosome grouping and banding diagram/template .
Through karyotyping, abnormalities in chromosome number and structure can be detected in the fetus. Using amniocentesis or chorionic villi sampling, the technician is able to obtain fetal cells for karyotyping. If chromosome abnormalities are discovered, a genetic counselor can educate the family about the disorder and its affects. These visible chromosome deviations are estimated to occur in 6-7 per 1000 live births and in about 1/2 of all spontaneous aborted pregnancies (Levitan). When cells have a normal number of chromosomes, they are called euploid. If on the other hand, the cells have an irregular number of chromosomes, the cells are called aneuploid. The aneuploid cells are caused by a nondisjunction during meiosis or by a nondisjunction during mitosis of the zygote. If a zygote exhibits nondisjunction after the first cleavage, then the organism is referred to as a mosaic since its cells exhibit both euploidy and aneuploidy.
Although any autosome can undergo nondisjunction, only a few of these aberrations are expressed in viable offspring. The most common autosome aberrations are Down Syndrome, Edward Syndrome and Patua Syndrome. Most other autosomal aberrations result in spontaneous abortions.
Down Syndrome or Trisomy 21 is the most common of the autosome abnormalities occurring in about 1.5 per 1,000 births. The rate for this trisomy appears to increase with maternal age. It has recently been established that the sperm can also carry this extra chromosome. Of those born with Down, 1/6 die within the first year and the average life span is 16.2 years (Levitan). The child usually has a round face, flattened nose bridge, a fold of skin above the eyes and short broad hands. The child can have an IQ between 25-74, septa defects in the heart, and a greater susceptibility to respiratory infection. Infections and heart defects increase the chance of infant death. Edward Syndrome or Trisomy 18 occurs about 1 per 6,000 births and this rate also increases with maternal age. Thirty percent of these children die within the first month and only 10% survive one year (Therman). Multiple malformations of most any organ are observed and there are severe mental, as well as developmental, handicaps. External manifestations can be an elongated skull, low-set malformed ears, deformed eyelids, possible webbed neck and unusually short sternum (Levitan).
Patua Syndrome or Trisomy 13 occurs about 1 per 15,000 births and this rate increases with maternal age. Forty five percent die within the first month, 90% by six months and less than 5% reach 3 years (Therman). External symptoms are similar to Edward's with the addition of cleft palate and/or lip, microcehphaly, polydactyl and deafness (Levitan).
Few additional autosome trisomies are seen in live births. Some have been reported for chromosome numbers 8 and 22 but these seem to be caused by nondisjunction during mitosis and thus are not expressed in all cells of the child.
Both the X and Y sex chromosomes exhibit nondisjunction. In females the most common is Trisomy X which occurs about 1 per 800 births. Symptoms range from normal physical and mental signs, to underdeveloped secondary sex characteristics (which can be treated with estrogen treatments), sterility and lowered intelligence. Most women with XXX who are fertile do not tend to pass the extra X to their offspring. This extra X chromosome tends to end up in a polar body after first meiosis. It is also possible to inherit 4 or 5 Xs. With each extra X there tends to be a corresponding increase in mental handicap, a chance of being sterile, and a lack of secondary sex characteristics (Levitan).
Turner Syndrome or X0 female occurs in about 1 per 2,700 births, is not linked to maternal age, and is thought to be a mosaic (Therman). The phenotype of this female includes short stature, short broad neck with webbing or looseness, a broad chest, lack of secondary sex characteristics and sterility. Intelligence does not seem to be affected (Levitan).
Klinefelter Syndrome or XXY occurs between 1 per 700 and 1 per 1,000 births. The man has small testes devoid of sperm, shows tendency for breast development, and has lower IQ which leads to mild mental handicap (Therman). One percent of institutionalized mentally handicapped males are XXY which is greater than what is seen in the birth rate (Levitan).
XYY males occur in the same rate of occurrence as XXY males. There are no major physical abnormalities that are indicative of this disorder. Any defects that do occur are usually confined to the genitals; such as undescended testes and defective development of seminiferous tubules. There can be some mental handicap due to the IQ range of 80-95 (Levitan).
Besides nondisjunction, chromosome mutations can be detected through karyotyping. The most common deletion, 1 per 45,000, is Cri du Chat (cat cry) Syndrome which is a deletion of a small arm of chromosome 5. During infancy the child has a cat-like cry which modifies with age. Other phenotype expressions are microcephaly, round face, low set ears, heart disease and mental handicap (Therman).
Wolf-Hirschhorn Syndrome is a deletion of a segment of chromosome 4's short arm. This affects scalp, nose, lips, palate and the penis in males. 13-Syndrome is a deletion of the long arm of chromosome 13. Severe malformations of face as well as retinoblastoma can occur (Levitan).
Two activities for karyotyping, a paper cut out method and a lab using human cells, will be described. Also if hypercard is available, refer to the American Biology Teacher, April 1992, article on "Karyotyping with HyperCard" by Jerry Jensen.
Premade karyotyping sheets available from: Carolina Biological