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Most members of diploid species contain two haploid sets of chromosomes, but there are variations from this pattern. These changes include a difference in chromosome number, deletion or duplication of genes, and rearrangement of genetic material among or within the chromosomes. All of these changes are considered chromosome mutations or aberrations.
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Minor alterations of genetic information will generally result in a phenotypic change, but substantial changes can be lethal.
8.1 Specific terminology describes variations in chromosome number
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Variations can range from addition or loss of one or more chromosomes to the addition of one or more sets of haploid chromosomes.
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Aneuploidy refers to the gain or loss of one or more chromosome, but not a complete set
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Monosomy is the loss of only one chromosome
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Trisomy is the gain of only one chromosome
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Euploidy is the gain of complete sets of haploid chromosomes
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Polyploidy is when two or more sets are present
8.2 Variation in the number of chromosomes results from nondisjunction
Chromosomal variation originates as a random error during the production of gametes. Nondisjunction is the failure of chromosomes to disjoin and move to opposite poles during division. The result is abnormal gametes containing either two members or none at all. Fertilization leads to monosomy or trisomy.
8.3 Monosomy, the loss of a single chromosome, may have severe phenotypic effects
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The loss of one chromosome is called monosomy. Although monosomy occurs for the X chromosome as in
Turner Syndrome (45, X), it is not usually tolerated in the autosomes (body chromosomes) of humans. The failure of monosomic individuals to survive puzzles people. The missing genes would be found on the remaining chromosome. Many ideas exist on why this is so.
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The unmasking of recessive lethal alleles is more tolerable in heterozygotes that have the wild type allele present in the second chromosome. If they only have one chromosome, then the lethal allele presents and leads to the death of the individual
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Another idea is that the expression of genes during early embryonic development is tightly regulated and certain gene products are necessary for correct development. The missing chromosome products are not produced; therefore, the organism does not develop properly and does not survive.
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Partial Monosomy in humans: Cri-du-Chat Syndrome – In humans, there are no reports of autosomal monosomic individuals surviving. There are survivors who have partial monosomy. These cases are called segmental deletions. Discovered by Jerome LeJeune in 1963, cri-du-chat (cry of the cat) was described. It includes a loss of part of the short arm on chromosome 5, meaning that even though all chromosomes are present, the p arm of 5 is missing parts.
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Symptoms include gastrointestinal or cardiac problems, mental retardation, abnormal development of the glottis and larynx (giving them a cry that sounds like a cat). Incidences are 1 in 50000 births. The amount of missing segment determines how severe effects are. Those who receive interventions early can develop self-care skills and communicate verbally.
8.4 Trisomy involves the addition of a chromosome to a diploid genome
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The effects of trisomy parallel those of monosomy. The addition of a chromosome is more likely to create a viable individual, and if it occurs on a sex chromosome rather than an autosome, then the effects are less dramatic.
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Down Syndrome – In 1866, Down Syndrome was first described by John Langdon Down. It is the only autosomal trisomy where a significant number of individuals survive longer than the first year. It involves a change in chromosome 21. Is found in 1 out of 800 births.
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Symptoms include an appearance that makes individuals similar to one another, generally short in stature, physical and mental retardation, and poor muscle tone. Individuals with this condition have a shortened life span, but have been known to survive to the 50s.
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In many cases, the incidence of leukemia and heart malformations is much higher. Those that survive to the 50s usually have an onset of Alzheimer’s which is much earlier than that of the general public.
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Patau Syndrome – In 1960, Klaus Patau observed malformations and a 47 chromosome karyotype. The chromosome appears in pair 13.

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Symptoms include a lack of mental alertness, cleft palate, probable deafness, and polydactyly.
Autopsies reveal malformations in most of the organ systems, which are believed to have started in the
5 th or 6 th week of embryonic development. Average survival is 3 months. Average age of the parents is around 32. It occurs roughly in 1 of 19,000 births.
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Edward Syndrome – John Edwards discovered in 1960. It is an extra chromosome on pair 18. Infants are smaller than average. The skull is elongated; the ears are low and malformed. The neck is webbed, the hips dislocated, and the chin is receding. Lifespan is usually around four months and death is caused by pneumonia or heart failure. About 1 in 8000 births will have this malformation. Maternal age is around 34 and the offspring are usually female.
8.7 A deletion is a missing region of a chromosome
When a chromosome breaks and a portion of it is lost, the missing piece is called a deletion (deficiency). Deletions can occur at the end (terminal) or in the interior (intercalary) of the chromosome. The part of the chromosome maintaining the centromere will be maintained during cell division, while the segment with no centromere will be lost during mitosis. A deletion does not have to be large to cause harmful effects (cri-du-chat). The more genetic information that is lost, the more likely the effects become lethal.
8.8 A duplication is a repeated segment of the genetic material
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When any part of the genetic material is present more than once in a genome, it is considered a duplication.
They can arise due to unequal crossing over during meiosis or through a replication error before meiosis. Three aspects can be considered when looking at duplication:
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It can result in genetic redundancy - Many gene products are not necessary, while others are essential to the cell. In some cases, multiple copies of one gene are required, such as those that code for rRNA
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It can produce phenotypic variations – the location of the duplication has a position effect. Where the change occurs will determine the phenotype. If they are present on the same homolog, then the effect will be more pronounced.
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In one theory, they are believed to be a source of genetic variation in evolution – when duplications occur, new phenotypes arise and give a reservoir to pull new genes from.
8.9 Inversions rearrange the linear gene sequence
Inversion is a chromosomal mutation where one gene segment is turned 180° within the chromosome. This does not involve a loss in material, just a rearrangement. Inversions seem to have little effect on the individuals who bear them, yet these individuals may produce gametes that bear the same inversions.
8.10 Translocations alter the location of chromosomal segments in the genome
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Translocation is the movement of a chromosome segment to a new location in the genome. In some cases, translocations can be reciprocal, where two NON homologous chromosomes exchange segments. The consequences are similar to those of an inversion because no material is gained or lost; only relocated. If incorporated into the gametes, it could lead to semisterility where meiotic products are genetically unbalanced and result in lethality. If the gametes pass on, they can lead to a monosomy or trisomy condition.
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Familial Down Syndrome – Involves breaks at the extreme end of the short arm of two chromosomes (13, 14,
15, 21, and 22). The small fragments are lost and the large pieces fuse at the centromere, producing a new chromosome. In a majority of these cases, the large portion of 21 translocates to 14. This causes the individual to be phenotypically normal, but only have 45 chromosomes. One-fourth of the gametes however, will have two copies of 21, and can pass that on to an offspring, resulting in trisomy 21, or Down Syndrome.
8.11 Fragile sites in humans are susceptible to chromosome breakage
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In cells from certain individuals, the chromosomes appear to have gaps. Such areas have become known as fragile sites. They seem to be susceptible to breakage when cultured or when there is an absence of certain chemicals such as folic acid. At first they seemed curious, but after research suggested they may be linked to some forms of mental retardation, studies were conducted. The cause of the fragility is unknown.
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Fragile X Syndrome (Martin-Bell Syndrome) – The most common form of inherited mental retardation.
Affects about 1 in 4000 males and 1 in 8000 females. In many cases, individuals also have ADD or may display autistic type symptoms.