Chapter 6: Chromosome Variation
paracentric inversion
(para meaning "next to") inversions that do not include the centromere.
pericentric inversions
(peri meaning "around") inversions that include the centromere.
What are ways in which aneuploidy can arise?
1. A chromosome may be lost in the course of mitosis or meiosis if, for example, its centromere is deleted. A loss of the centromere prevents the spindle microtubules from attaching, so the chromosome fails to move to the spindle pole and does not become incorporated into a nucleus after cell division. 2. The small chromosome generated by Robertsonian translocation may be lost in mitosis or meiosis. 3. Aneuploidy may arise through nondisjunction, the failure of chromosomes or sister chromatids to separate in meiosis or mitosis. Nondisjunction leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are missing a chromosome.
What are the two types of classifications of variations in chromosome number?
1. Aneuploidy, which is a change in the number of individual chromosomes 2. Polyploidy, which is an increase in the number of chromosome sets.
What are the four basic types chromosomes can be classified into?
1. Metacentric: The centromere is located approximately in the middle, so the chromosome has two arms of equal length. 2. Submetacentric: The centromere is displaced toward one end, creating a long arm and a short arm. (on human chromosomes, the short arm is designated by the letter p and the long arm by the letter d). 3. Acrocentric: The centromere is near one end, producing a long arm and a b knob or satellite, at the other end. 4. Telocentric: The centromere is at or very near the end of the chromosome.
8. List four major types of aneuploidy.
1. Nullisomy is the loss of both members of a homologous pair of chromosomes. It is represented as 2n-2, where n refers to the haploid number of chromosomes. Thus, among humans, who normally possess 2n=46 chromosomes, a nullisomic zygote has 44 chromosomes. 2. Monosomy is the loss of a single chromosome, represented as 2n-1. A human monosomic zygote has 45 chromosomes. 3. Trisomy is the gain of a single chromosome, represented as 2n+1. A human trisomic zygote has 47 chromosomes. The gain of a chromosome means that there are three homologous copies of one chromosome. Most cases of down syndrome result from trisomy of chromosome 21. 4. Tetrasomy is the gain of two homologous chromosomes, represented as 2n+2. A human tetrasomic zygote has 48 chromosomes. Tetrasomy is not the gain of any two extra chromosomes, but rather the gain of two homologous chromosomes, so that there are four homologous copies of a particular chromosome.
What are the three reasons individuals heterozygous for a deletion may have multiple defects?
1. The heterozygous condition may produce imbalances in the amounts of gene products, similar to the imbalances produced by extra gene copies. 2. normally recessive mutations on the homologous chromosome lacking the deletion may be expressed when the wild-type allele has been deleted ( and is no longer present to mask the recessive allele's expression). 3. Some genes must be present in two copies for normal function.
What are ways in which translocation can affect a phenotype?
1. They can physically link genes that were formerly located on different chromosomes. These new linkage relations may affect gene expression (position effect) which genes translocated to new locations may come under the control of different regulatory sequences or other genes that affect their expression. 2. The chromosomes breaks that bring about translocations may take place within a gene and disrupt its function. ex/ neurofibromatosis (translocation affecting chromosome 17) 3. Deletions frequently accompany translocations
A diploid organism has 2n=36 chromosomes. How many chromosomes will be found in a trisomic member of this species?
37 chromosomes.
A human karyotype consists of how many chromosomes?
46 chromosomes
17. The green-nose fly normally has six chromosomes: two metacentric and four acrocentric. A geneticist examines the chromosomes of an odd-looking green-nose fly and discovers that it has only five chromosomes; three of them are metacentric and two are acrocentric. Explain how this change in chromosome number might have taken place.
A Robertsonian translocation between two of the acrocentric chromosomes would result in a new metacentric chromosome and a very small chromosome that may have been lost.
How can you tell the difference from a male and female in a karyotype?
A male will typically only have an XY while a female will only have XX unless they have some sort of genetic abnormality.
Notch
A series of x-linked wing mutations in Drosophila. Their mutations often result from chromosome deletions. They behave in a dominant manner; when heterozygous from a notch deletion, a fly has wings that are notched at the tips and along the edges.
Why is polyploidy less common in animals than in plants?
Allopolyploids require hybridization between different species, which happens less frequently in animals than in plants. Animal behavior often prevents interbreeding among species and the complexity of animal development causes most interspecific hybrids to be nonviable. Many of the polypoid animals that do arise are in groups that reproduce through parthenogenesis (a type of fertilization in which the animal develops from an unfertilized egg). Thus asexual production may facilitate the development of polyploids, perhaps because of the perpetuation of hybrid individuals through asexual reproduction provides greater opportunities for nondisjunction than does sexual reproduction. Only a few human polypoid babies have been reported, and most died within a few days of birth. Polyploidy usually triploidy is seen in about 10% of all spontaneously aborted human fetuses.
Down syndrome
Also known as Trisomy 21, is the most common autosomal aneuploidy in humans. It occurs 1 in 700 human births but the chances increase among children born to older mothers. Characteristics are intellectually disabled, broad flat face, small nose, oval shaped eyes.
What happens in an individual heterozygous for a reciprocal translocation?
An individual heterozygous for this translocation would possess one normal copy of each chromosome and one translocated copy. Each of these chromosomes contains segments that are homologous to segments of two other chromosomes. Thus when the homologous sequences pair in prophase I of meiosis, crosslike configurations consisting of all four chromosomes form. Whether viable or nonviable gametes are produced depends on how the chromosomes in these crosslike configurations separate. Only about half of the gametes from an individual heterozygous for a reciprocal translocation are expected to be functional. so these individuals frequently exhibit reduced fertility.
11. Explain why autopolyploids are usually sterile, whereas allopolyploids are often fertile.
Autopolyploids contain various copies of each chromosome, thus, proper synapsis does not occur meaning viable gametes are not formed. On the other hand, allopolyploids contain different sets such that synapsis takes place (non-homologous chromosomes) and viable gametes are formed.
10. What is the difference between autopolyploidy and allopolyploidy? How does each arise?
Autopolyploidy appears when an individual has more than two sets of chromosomes, both of which from the same parental species. Allopolyploidy, on the other hand, occurs when the individual has more than two copies but these copies, come from different species. Autopolyploidy may arise through nondisjunction in an early 2n embryo or through meiotic nondisjunction that produces a gamete with extra sets of chromosomes. Allopolyploidy is usually preceded by hybridization between two different species, followed by chromosome doubling.
2. Why do extra copies of genes sometimes cause drastic phenotypic effects?
Because it has the presence of additional copies of normal sequences and it creates an imbalance in the number of gene products (abnormal gene dosage). Developmental processes often require the interaction of many genes. The development may be affected by the relative amount of gene products. Duplications and other chromosome mutations produce extra copies of some but not all genes, which alters the relative amounts (doses) of interacting products. If the number of one product increases but amounts of other products remain the same, developmental problems often result. So essentially... unbalanced gene doses lead to developmental abnormalities.
21. Red-green color blindness is a human x-linked recessive disorder. A young man with a 47, XXY karyotype (Klinefelter syndrome) is color blind. His 46,XY brother is also color blind. Both parents have normal color vision. Where did the nondisjunction that gave rise to the young man with Klinefelter syndrome take place? Assume that no crossing over took place in prophase I of meiosis.
Because the father has normal color vision, the mother must be the carrier for color blindness. The color-blind young man with Klinefelter syndrome must have inherited two copies of the X chromosome with the color-blindness allele from his mother. The nondisjunction event took place during meiosis II of the egg.
Primary Down syndrome
Caused by the presence of three copies of chromosome 21 (Trisomic for a complete chromosome 21). 92% of all people with down syndrome have primary down syndrome. This arises from spontaneous nondisjunction in egg formation. About 75% of the nondisjunction events that cause Down syndrome are maternal in origin, most arising in meiosis I.
displaced duplication
Chromosome rearrangement in which the duplicated segment is some distance from the original segment, either on the same chromosome or on a different one.
1. List the different types of chromosome mutations and define each one.
Chromosome rearrangements: alter the structure of chromosomes. For example, a piece of a chromosome may be duplicated, deleted, or inverted. Aneuploidy: The number of chromosomes is altered. One or more individual chromosomes are added or deleted. Polyploid: Any organism that has more than two sets of chromosomes (3n, 4n, 5n, or more).
Why are sex-chromosome aneuploidies more common than autosomal aneuploidies in humans and other mammals?
Dosage compensation prevents the expression of additional copies of x-linked genes in mammals, and there is little information in the Y chromosome, so extra copies of the X and Y chromosomes do not have a major effect on development. In contrast, there is no mechanism of dosage compensation for autosomes, and so extra copies of autosomal genes are expressed, upsetting development and causing the spontaneous abortion of aneuploid embryos.
segmental duplications
Duplications are greater than 1000 base pairs in length. The human genome contains numerous duplicated sequences and the average size of the duplication is 15,000bp.
Trisomy 18
Edwards syndrome. arises with a frequency of approximately 1 in 8000 live births. babies with Edward syndrome have sever intellectual disability, low-set ears, a short neck, deformed feet, clenched fingers, heart problems, and other disabilities. Few live for more than a year after birth.
25. Species I has 2n=16 chromosomes. How many chromosomes will be found per cell in each of the following mutants in this species? a. monosomic b. autotriploid c. autotetraploid d. trisomic e. double monosomic f. nullsomic g. autopentaploid h. tetrasomic
Given is that species is diploid and has 2n = 16. It means that there are 8 homologous pairs (16 chromosomes). Thus haploid (n) = 8. An example of aneuploidy is Monosomy, trisomy, and tetrasomy. The example of Polyploidy is auto triploid, autopentaploid and autotetraploid. The number of chromosomes in each type of ploidy is: a) Monosomic: 15 chromosomes In this type of ploidy, a diploid cell has a missing single chromosome. So, 2n=16-1=15. b) Autotriploid: 24chromosomes In this type of ploidy, 1 set from male and 2 set from the female is present. So, 2n=16 + 8 = 24. c) Autotetraploid: 32chromosomes In this type of ploidy, the doubling of the chromosome occurs. So, 2n = 16 x 2 = 32. d) Trisomic: 17 chromosomes In this type of ploidy, there will be the presence of an additional chromosome. So, 2n = 16 + 1 = 17. e) Autopentaploid: 40 chromosomes In this type of ploidy, there will be the presence of multiple copies that is 5n. So, 5xn= 5x8=40. f) Tetrasomic: 18 chromosomes In this type of ploidy, there will be the presence of a homologous pair of chromosomes in the diploid set. There are present two extra chromosomes. So, 2n+2= 18.
Familia Down Syndrome
Have an 46 chromosomes, but an extra copy of part of chromosome 21. It has tendency to run in families. The phenotypic characteristics are the same as those with primary down syndrome. 4% of all people with down syndrome have this. Familia down syndrome arises in offspring whose parents are carriers of chromosomes that have undergone a Robertsonian translocation, most commonly between chromosome 21 and chromosome 14. The long arm of 21 and the short arm of 14 exchange places.
dicentric bridge
In anaphase 1 of meiosis, the centromeres are pulled toward opposite poles and the two homologous chromosomes separate. This action stretches the dicentric chromatid across the center of the nucleus, forming a structure called the dicentric bridge.
7. How do translocations in which no genetic information is lost or gained produce phenotypic effects?
In translocations, parts of chromosomes move to other nonhomologous chromosomes or to other regions of the same chromosome. Translocations can affect the phenotype by causing genes to move to new locations, where they come under the influence of new regulatory sequences, or by breaking genes and disrupting their function.
4. What is the difference between a paracentric and a pericentric inversion?
Inversion is a process in which two breaks occur in the chromosome and that particular region within the break rotate and join, this leads to chromosomal mutation called an inversion. Inversions are classified into two categories: 1. Paracentric inversion: inversions that do not include the centromere. 2. Pericentric invasion: inversions that do include the centromere. The location at which inversion takes place determines the genetic behavior of the individual.
5. How can inversions in which no genetic information is lost or gained cause phenotypic effects?
Inversions may break a gene into two parts, and one part may move to a new location and destroy the function of the gene in that location. Even when the chromosome breaks lie between genes, phenotypic effects may arise from the inverted gene order. Many genes are regulated in a position-dependent order; if their positions are altered by an inversion their expression may be altered, an outcome referred to as a position effect. ex/ inversion moves a wild-type allele (that normally codes for red eyes) at the white locus in Drosophila to a chromosome region that contains highly condensed and inactive chromatin, the wild-type allele is not expressed in some cells, resulting in an eye consisting of red and white spots.
pseudodominance
Is the expression of a normally recessive mutation that indicates that one of the homologous chromosomes has a deletion.
Nondisjunction
Is the failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis. Nondisjunction leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are missing a chromosome.
chromosome inversion
It is the third type of chromosome rearrangement in which a chromosome segment is inverted turned 180 degrees. For an inversion to take place, the chromosome must break in two places.
What are the effects of aneuploidy?
It usually alters the phenotype drastically. Aneuploid mutations in plants and many animals are lethal. Aneuploidy affects the number of gene copies, but not their nucleotide sequences, which the effects of aneuploidy are most likely due to abnormal gene dosage.
position effect
Many genes are regulated in a position-dependent manner; if their positions are altered by an inversion, their expression may be altered thus results in the position effect.
Trisomy 13
Patau Syndrome- Has a frequency of about 1 in 15,000 live births and produces sever intellectual disability, a small hang, sloping forehead, small eyes, clef lip and palate, extra fingers and toes, and numerous other problems. About half of children with trisomy 13 die within the first month of life, and 95% die by the age of 3.
9. What is the difference between primary Down syndrome and familial Down syndrome? How does each type arise?
Primary Down syndrome is caused by spontaneous, random nondisjunction of chromosome 21, leading to trisomy 21. Familial Down syndrome most frequently arises as a result of a Robertsonian translocation of chromosome 21 with another chromosome, usually chromosome 14. Translocation carriers do not have Down syndrome, but their children have an increased incidence of Down syndrome. If the translocated chromosome segregates with the normal chromosome 21, the gamete will have two copies of chromosome 21 and result in a child with familial Down syndrome.
What is pseudodominance and how is it produced by a chromosome deletion?
Pseudodominance is the expression of a normally recessive mutation that is produced when the dominant wild-type allele in a heterozygous individual is absent due to a deletion on one chromosome.
Trisomy 8
Rarest trisomy. Which arises 1 in 25,000 to 1 in 50,000 live births. Characterized by intellectual disability, contracted fingers and toes, low-set malformed ears, and a prominent forehead. Many people who have this condition have a normal life expectancy.
16. The following represents two nonhomologous chromosomes: AB*CDEFG RS*TUVWX What type of chromosome mutation would produce each of the following groups of chromosomes? a. AB*CD RS*TUVWXEFG b. AUVB*CDEFG RS*TWX c. AB*TUVFG RS*CDEWX d. AB*CWG RS*TUVDEFX
TRANSLOCATION - Two nonhomologous chromosomes exchange their parts and the resulting chromosomal rearrangement is called translocation. Reciprocal translocation - a segment from one chromosome is exchanged with the segment from another non -homologous chromosome, so two translocated chromosomes generated simultaneously. Non- reciprocal translocation - transfer of a segment in one direction from one chromosome to another.
31. Nicotiana glutinosa (2n=24) and N. tabacum(2n=48) are two closely related plants that can be intercrossed, but the F1 hybrid plants that result are usually sterile. IN 1925, Roy Clausen, and Thomas Goodspeed crossed N. glutinosa and N. tabacum and obtained one fertile F1 plant (R.E. Clausen and T.H. Goodspeed. 1925. Genetics 10:278-284). They were able to self-pollinate the flowers of this plant to produce an F2 generation. Surprisingly, the F2 plants were fully fertile and produced viable seeds. When Clausen and Goodspeed examined the chromosomes of the F2 plants, they observed 36 pairs of chromosome in metaphase I and 36 individual chromosomes in metaphase II. Explain the origin of the F2 plants obtained by Clausen and Good-speed and the numbers of chromosomes observed.
The F1 plant was most likely an allotetraploid (4n = 72), and the F2 offspring were generated by fertilization of viable gametes produced by the F1.
6. Explain why recombination is suppressed in individuals heterozygous for paracentric inversions.
The frequency of crossing over within the inversion is not actually diminished, but when crossing over does take place, the result is abnormal gametes that do not give rise to viable offspring, and thus no recombinant progeny are observed. (look at fig. 6.13 in book)
What are the most common aneuploidies seen in living humans?
The most common aneuploidies seen in living humans are those that involve the sex chromosomes. Turner syndrome and Klinefelter syndrome result from aneuploidy of the sex chromosomes.
Translocation carries
They do not have down syndrome although they possess only 45 chromosomes. Their phenotypes are normal because they have two copies of the long arms of chromosomes 14 and 21, and apparently the short arms of the chromosomes carry no essential genetic information. Even though the carriers are healthy, they have increased change of producing children with down syndrome.
What happens with aneuploidy in humans?
Usually produces such serious developmental problems that results to spontaneous abortion. Only about 2% of all fetuses with chromosome defect survive to birth.
Copy number variations (CNVs)
Variations in the number of copies of particular DNA sequences. This includes duplications and deletions that range in length from thousands of base pairs to several million base pairs. Many of these variants encompass at least one gene and may encompass several genes.
haploid insufficient gene
When a single copy of a gene is not sufficient to produce a wild-type phenotype.
Are notch mutations haploinsufficient?
Yes they are haploinsufficient because flies can be heterozygous for the notch deletion and still have notches at the tips and along the edges of the wings.
Karyotype
a complete set of chromosomes from a cell that has been photographed during cell division and arranged in a standard sequence
Fragile X Syndrome (FXS)
a disorder that includes intellectual disability. It exhibits x-linked inheritance and arises with a frequency of about 1 in 5000 male births, has been shown to result from an increase in number of repeats of a CGG trinucleotide.
32. What would be the chromosome number of progeny resulting from the following crosses in wheat (see figure 6.26)? What type of polyploid (allotriploid, allotetraploid, etc.) would result from each cross? a. Einkorn wheat and emmer wheat b. Bread wheat and emmer wheat c. Einkorn wheat and bread wheat
a. Einkorn wheat and emmer wheat Einkorn is T.uratu= (2n=14) Emmer is T. turgidum= (4n=28) Gametes of einkorn is n=7 Gametes from Emmer have= 2n=14 chromosomes progeny from this cross is 3n=21 and allotriploid. b. Bread wheat and emmer wheat. Bread wheat tritcicum aestirum is (6n=42) is allohexaploid and it has 3n=21 chromosomes and emmer wheat has 2n=14 chromosomes. Progeny from this cross is (3n=21)+(2n=14) =5n= 35 chromosomes and allopentaploid c. Einkorn wheat and bread wheat. Einkorn wheat = n=7 bread wheat= 3n=21 progeny (n=7)+(3n=21) = 4n= 28 chromosomes and it is allotetraploid.
Chromosome duplications often result in abnormal phenotypes because a. developmental processes depend on the relative amounts of proteins encoded by different genes. b. extra copies of the genes within the duplicated region do not pair in meiosis. c. the chromosome is more likely to break when it loops in meiosis. d. extra DNA must be replicated, which slows down cell division.
a. developmental processes depend on the relative amounts of proteins encoded by different genes.
13. Which types of chromosome mutations a. increase the amount of genetic material in a particular chromosome? b. increase the amount of genetic material in all chromosomes? c. decrease the amount of genetic material in a particular chromosome? d. change the position of DNA sequences in a single chromosome without changing the amount of genetic material? e. move DNA from one chromosome to a nonhomologous chromosome?
a. duplication b. polyploidy c. deletions d. inversions e. translocations
27. Species I has 2n=8 chromosomes and species II has 2n=14 chromosomes. What would the expected chromosome numbers be in individuals with the following chromosome mutations? Give all possible answers. a. allotriploidy including species I and II b. autotetraploidy in species II c. Trisomy in species I d. Monosomy in species II e. tetrasomy in species I f. allotetraploidy including species I and II
a. such allotriploids could have 1n from species I and 2n from species II for 3n=18. alternatively, they could have 2n from species I and 1n from species II for 3n=15. b. 4n=28 c. 2n+1=9 d. 2n-1=13 e. 2n+2=10 f. allotetraploids must have chromosomes from both species, and total 4n. There are three possible combinations for such allotetraploids: 2n from each: 2(4) + 2(7)=22; 1n from species I + 3n from species II: 1(4) + 3(7)=25 ; 3n from species I + 1n from species II: 3(4) + 1(7)=19
14. A chromosome has the following segments, where * represents the centromere: AB*CDEFG What types of chromosome mutations are required to change this chromosome into each of the following chromosomes? (In some cases, more than one chromosome mutation may be required.) a. ABAB*CDEFG b. AB*CDEABFG c. AB*CFEDG d. A*CDEFG e. AB*CDE f. AB*EDCFG g. C*BADEFG h. AB*CFEDFEDG i. AB*CDEFCDFEG
a. tandem duplication of AB b. displaced duplication of AB c. paracentric inversion of DEF d. deletion of B e. Deletion of FG f. paracentric inversion of CDE g. pericentric inversion of ABC h. duplication and inversion of DEF i. duplication of CDEF, inversion of EF
Autopolyploidy
all chromosome sets are from a single species. This is caused by accidents of mitosis or meiosis that produce extra sets of chromosomes, all derived from a single species.
amphidiploid
allopolyploid consisting of two combined diploid genomes
26. Species I is diploid (2n=8) with chromosomes AABBCCDD; related species II is diploid (2n=8) with chromosomes MMNNOOPP. What types of chromosome mutations do individuals with the following sets of chromosomes have? a. AAABBCCDD b. MMNNOOOOPP c. AABBCDD d. AAABBBCCCDDD e. AAABBCCDDD f. AABBDD g. AABBCCDDMMNNOOPP h. AABBCCDDMNOP
b. MMNNOOOOPP: tetrasomy in species II (2 extra copies produced for chromosome O). a. AAABBCCDD: trisomy in species I (1 extra copy produced for chromosome A). c. AABBCDD: monosomy in species I (missing 1 copy of chromosome C) d. AAABBBCCCDDD: triploidy in species I (generation of 1 extra copy of all the chromosomes) e. AAABBCCDDD: double trisomy in species I (in this case for genes A and C). f. g. AABBCCDDMMNNOOPP: allotetraploidy of I & II h.
What is the outcome of a Robertsonian translocation? a. two acrocentric chromosomes b. one large chromosome and one very small chromosome with two very short arms. c. one large metacentric and one large acrocentric chromosome. d. two large metacentric chromosomes.
b. one large chromosome and one very small chromosome with two very short arms.
Species A has 2n=16 chromosomes and species B has 2n=14. How many chromosomes would be found in an allotriploid of these two species? a. 21 or 24 b. 42 or 48 c. 22 or 23 d. 45
c. 22 or 23
A dicentric chromosome is produced when crossing over takes place in an individual heterozygous for which type of chromosome rearrangement? a. duplication b. deletion c. paracentric inversion d. pericentric inversion
c. paracentric inversion
dicentric chromatid
chromatid with two centromeres
Allopolyploidy
chromosome sets are from two or more species. Arises from hybridization between two species; the resulting polyploid carries chromosome sets derived from two or more species.
Polyploidy
condition in which an organism has extra sets of chromosomes
Anueploidy
condition of a cell having an extra chromosome or missing a chromosome for its species.
Fragile sites
constrictions or gaps at a particular location in chromosomes of cells grown in culture. They are prone to breakage under certain conditions.
What are the four basic types of chromosome rearrangements?
duplications, deletions, inversions, and translocations.
Crossing over
exchange of genetic material between homologous chromosomes.
nonreciprocal translocation
genetic material moves form one chromosome to another without any reciprocal exchange.
15. A chromosome initially has the following segments: AB*CDEFG Draw the chromosome, identifying its segments, that would result from each of the following mutations. a. Tandem duplication of DEF b. Displaced duplication of DEF c. Deletion of FG d. The paracentric inversion that includes DEFG e. Pericentric inversion of BCDE
https://www.chegg.com/homework-help/questions-and-answers/21-chromosome-initially-following-segments-mu-abcdefg--draw-chromosome-identifying-segment-q38652178
reverse duplication
is when the duplication is inverted
acentric chromatid
lacking a centromere
chromosome deletion
loss of a chromosome segment
Translocation
movement of genetic material between nonhomologous chromosomes or within the same chromosome.
chromosome duplication
mutation in which part of the chromosome has been doubled
chromosome rearrangement
mutations that change the structure of individual chromosomes.
What are diseases or disorders that are associated with copy number variations?
osteoporosis, autism, schizophrenia, and a number of other diseases and disorders.
Unbalanced gametes
random segregation of the three homologous chromosomes create unbalanced gametes with various numbers of chromosomes.
tandem duplication
the duplicated region is immediately adjacent to the original segment
Robertsonian translocation
the long arms of two acrocentric chromosomes become joined to a common centromere through a translocation, generating a metacentric chromosome with two long arms and another chromosome with two very short arms. the smaller chromosome is often lost because very small chromosomes do not have enough mass to segregate properly during mitosis and meiosis. The result is an overall reduction in chromosome number. Robertsonian translocations are the cause of some cases of Down syndrome.
reciprocal translocation
two-way exchange of segments between the chromosomes
Chromosome mutations
variations in the number and structure of chromosomes