Meiotic errors are alterations of a cell's chromosome number or structure. Chromosomal alterations happen in both mitosis and meiosis. But in some ways meiosis provides more opportunities for errors. This is because the three sources of genetic variation in meiosis can also be opportunities for errors. Crossing over in prophase I and independent assortment in anaphase I and anaphase II are major sources of genetic variation among sexually reproducing organisms. Each sexually reproduced organism is genetically unique, unlike the genetically identical offspring in mitosis. The benefits of having multiple sources of genetic variation are diversity and enhanced adaptability to changing environments. But crossing over and independent assortment can also provide opportunities for errors, resulting in gametes with altered structures or numbers of chromosomes. The consequences of meiotic errors can be mild or fatal; they will be discussed in the last section.
Nondisjuncion, occurs when a pair of homologous chromosomes in anaphase I or a pair of sister chromatids in anaphase II (or in mitosis) are pulled together to the same pole and fail to separate. A nondisjunction at anaphase I will result in two gametes with a diploid number for the altered chromosome and two gametes with the chromosome missing altogether. A nondisjunction at anaphase II will result in two normal gametes, one gamete with two copies of the chromosome, and one gamete with no copies of the chromosome.
The abnormal gametes are called aneuploid; they have an excess or lack of one or more chromosomes or pieces of chromosomes. Aneuploid gametes that are fertilized by normal gametes will also result in aneuploidy. If the aneuploid gamete contains two copies of a chromosome, the zygote will be trisomic, having three copies of the chromosome. If the aneuploid gamete recieved no copy of the chromosome, the zygote will be monosomic, having only one copy the chromosome. When the sex chromosomes X or Y fail to separate, we call it sex chromosome aneuploidy. It usually produces milder defects than autosomal aneuploidy. The mechanisms responsible for nondisjunction are uncertain.
Sometimes a piece of a chromosome may break off , causing various meiotic and mitotic errors. A deletion is a deficiency of certain genes in a chromosome resulting from the loss of the broken fragment.
The brackets indicate where the deletion occurred. The arrow in chromosome 2 shows the normal chromosome. These pictures come from the wonderful people of Cytogenetics at the Waisman Center.
If the fragment is just lost only a deletion will result. But, the fragment may attach to its homologue which will then have duplicate genes of the fragment. This is called duplication.
Another possible alteration is an inversion which results when
the fragment reattaches to it^Òs original chromosome in reverse
orientation, reversing the order of genes within the fragment.
Theses pictures come from Cytogenetics at the
Waisman Center.
A translocation may result if the fragment attaches to a
nonhomologous chromosome.
Theses pictures come from Cytogenetics at the
Waisman Center.
Deletions and duplications may also result when crossing over in prophase I does not ocurr properly. One chromatid may break off a segment larger than that of the homologous chromosome it is crossing over with. After the exchange, one chromosome will have duplicate genes while the other chromosome will be missing some genes; these errors are called duplication and deletion respectively.
Reciprocal Translocations occur when nonhomologous chromosomes trade or exchange segments, like in crossing over. These types of translocations are associated with certain types of cancers, for example leukemia, and synoviosarcomas.
Meiotic errors, particularly aneuploidy, are suspected to occur quite frequently in human beings. If aneuploid gametes are successfully fertilized, the zygotes will also be aneuploid. Most aneuploid zygotes are thought to end in spontaneous abortions. In fact, at least one fifth of spontaneously terminated pregnancies are largely due to monosomies and trisomies. Some chromosomal alterations, however, are not fatal but instead result in various congenital diseases.
Down Syndrome is caused by monosomies and trisomies of chromosome 21. It can also result from the translocation of a large segment of a third chromosome 21 onto another chromosome. It affects about one in every 700 children. It is the most common serious birth defect in the United States, and the most common type of mental retardation. Older women have a higher incidence of bearing children with Down Syndrome. Recent research suggests that older women are more likely to carry Down Syndrome pregnancies to term, while younger women are more likely to spontaneously abort. The characteristics of Down Syndrome are mild to severe mental retardation, unusual facial features, a higher incidence of respiratory infections, leukemia (see Cancer and chromosome 21 below) and Alzheimer's disease, and heart defects. Individuals have a short stature and do not reach sexual maturity. They usually have a shortened life span.
Patau Syndrome results from a trisomy of chromosome 13. One in every 5000 new borns will inherit this disease characterized by harelip, cleft palate, severe defects to of the eyes, brain, and circulatory system. Most affected newborns die within a year.
Cri-du-chat Syndrome is caused by a specific deletion in chromosome 5. It results in severe mental retardation, abnormal facial features, a small head, and an abnormally developed larynx which causes the child's cry to sound like that of a cat. Affected individuals rarely survive past early childhood.
Edward's Syndrome results from a trisomoy of chromosome 18. It affects one in every 10,000 new borns, causing problems for almost every organ system. Most infants with the syndrome survive less than a year.
Turner syndrome, results from a monosomie of chromosome X and is the only viable monosomie known. About one in every 5000 newborns will inherit the disease. Their mental abilities are not affected, but they are usually sterile owing to underdeveloped sex organs.
Klinefelter syndrome, occurs in men with an extra X chromosome. Except for small testes, they have normal sex organs. However, they are sterile, and develop feminine body characteristics. They usually have normal intelligence. Less common is the occurrence of more than one extra X chromosomes. Males may have XXY, XXXY,XXXXY, even XXXXXY. These individuals have a higher incidence of mental retardation than XXY individuals.
Cancers are associated with certain chromosomal translocations. A reciprocal translocation between chromosome 21 and the tip of chromosome 9 is found in individuals with Leukemia. Synoviosarcomas are cancers of the synoviol cells in joints or tendon sheaths. Ninety percent of such cancers involve a balanced (equal size segments echanged) translocation between Chromosome X and chromosome 18, or a similar translocation. [picture icon]
Fluorescent In Situ Hybridization text and pictures
PAINT
Theses pictures come from Cytogenetics at the Waisman Center.
C-Banding text and pictures G-Banding text and pictures
Cellular and Molecular Biology Hypertext Self Quizes--biochemisty, large molecules, cell biology, prokaryotes genetics and, recombinant DNA
Cell Pictures from University of Utah
Encyclopedic Site on Chromosomal Abnormalitieas...and more
Specific Diagnosis Card Catalogue
Home Page on Klinefelter's Syndrome
Human Genome Project--all 23 human chromosomes with labeled loci and genes
More Genetics Web Sites and Resources
ch 9--chromosomal abnormalities--karyotypes, descriptions
University of Utah Picture Gallery--karyotypes, and cell slides of chromosomal abnormalities and cancers
General Pathology Images from The University of Utah--Caution: some images are graphic.
Human Cytogenetics--many related web sites
Bio Pictures--Links to various Biology Picture Web Sites
Selected Tropical Rainforest Pictures by Marco Bleeker
Selected European Garden Pictures by Marco Bleeker
