3 - Chromosome Variation
In the first half of Chapter 3, we will consider deficiencies in chromosome structure. Deficiencies in chromosome structure refer to altering the total amount of genetic information in the genome (deletions, duplications), rearranging the order of genes on a chromosome (inversions), or changing the location of a gene within the genome (translocations).
In the second half of Chapter 3, we will consider situations in which the number of chromosomes in an individual varies (variations in euploidy and aneuploidy).
A. Changes in Chromosome Structure
We have learned that each structural gene is present as a single copy on a particular chromosome. However, since we have two copies of each chromosome (one copy of the chromosome pair is inherited from the paternal parent; the other member of the pair is inherited from the maternal parent), each structural gene is actually present in two copies per genome. Changes to this general rule include the following (see figure 3.1):
- Deletions. A deletion occurs when a segment of a chromosome is missing. A deletion can be as small as a single base pair or can include the loss of several genes. The portion of the chromosome that is missing is called a deficiency.
- Duplications. A duplication occurs when a portion of the chromosome is repeated. In duplications, a single chromosome can have more than one copy of the same gene.
- Inversions. Inversions involve a change in the direction of the genes along a chromosome.
- Translocations. A translocation occurs when a portion of a chromosome becomes attached to a nonhomologous chromosome. For example, a portion of chromosome 1 could move to chromosome 5. There are three types of translocations:
- Simple (nonreciprocal) translocation. A simple translocation occurs when a segment of one chromosome becomes attached to a nonhomologous chromosome. The chromosome receiving the DNA segment remains intact.
- Reciprocal translocation. Reciprocal translocations involve nonhomologous chromosomes exchanging pieces.
- Robertsonian translocation. Robertsonian translocations involve the fusion of the long arms of two acrocentric chromosomes.
- What are the four major types of defects in chromosome structure?
- What are the differences between the three types of translocations?
One or more breaks in a chromosome can lead to the loss of a portion of the chromosome. This type of chromosomal aberration is called a deletion (see figure 3.2).
A single break in a chromosome can result in a fragment that contains a centromere (is retained by the cell) and a fragment that lacks a centromere (is lost during mitosis). This type of event is called a terminal deletion. Terminal deletions are usually generated by endonuclease damage or by treatments, such as ionizing radiation, that break the DNA backbone.
An interstitial deletion is a deletion that does not involved the telomere. Instead, the interstitial deletion involves interior portions of the chromosome. Interstitial deletions are generated by defects in synapsis and crossing over during meiosis I (see below).
- What is the difference between an interstitial and a terminal deletion?
In general, the larger the deletion (or the more genes involved), the more severe the phenotypic consequences. A detrimental phenotype can occur even though the individual may have a normal copy of the homologous chromosome, indicating that most deletions can be classified as dominant mutations.
Cri-du-chat (46, 5p-) is a genetic disease caused by a deletion in the p arm of chromosome 5 that occurs in 1 in 25,000–50,000 live births (see figure 3.3). Cri-du-chat is usually not inherited; instead, the disease is caused by the loss of part of chromosome 5 during meiosis. A cri-du-chat individual usually has one normal copy and a terminal deletion copy of chromosome 5. The deletion in chromosome 5 can be quite small or can encompass much of the p arm; however, it is thought that the absence of a specific gene causes cri-du-chat. This missing gene encodes the telomerase reverse transcriptase (TERT) portion of telomerase.
The cri-du-chat individual displays mental deficiencies, facial abnormalities, gastrointestinal, and cardiac complications. Those afflicted also tend to vocalize using a catlike cry, due to defects in the formation of the glottis and larynx.
- What change in chromosome structure produces cri-du-chat?
A gene duplication produces two copies of a structural gene on a single chromosome. Since the homologous chromosome contributes another copy of the same structural gene, a person with a duplication would now have three copies of the structural gene, instead of two copies.
Duplications and deletions can be produced simultaneously by the misalignment of synapsed homologous chromosomes during meiosis, followed by unequal crossing over (see figure 3.4). Unequal crossing over produces a chromosome product with an interstitial deletion, while simultaneously producing another chromosome product that contains a duplication.
As the region of the chromosome that is duplicated gets larger, the phenotypic effect on the individual becomes more severe. One example of a disease caused by a duplication is the neuropathic disease Charcot-Marie-Tooth disease type 1A (CMT type 1A), produced from a duplication on chromosome 17.
- How can a meiosis defect produce both a deletion and a duplication?
- What human disease is caused by a duplication?
Duplications Can Produce Gene Families
Small duplications can sometime be beneficial and are important in the formation of gene families, closely related genes that have similar but not identical functions. For example, the globin family of genes is thought to have been formed by multiple duplications from a single ancestral globin gene (see figure 3.5). To form the globin gene family, the ancestral globin gene was duplicated to produce two identical genes on the same chromosome. These two genes then accumulated mutations independently over the course of thousands of generations to become specialized in their respective functions. Later, these two genes duplicated additional times followed by divergence through the continued accumulation of mutations. The current globin gene family, consisting of fourteen member genes, includes genes that encode the protein subunits of hemoglobin, which is specialized to carry oxygen in the bloodstream, and the protein subunits of myoglobin, which carries oxygen within muscles. The globin family of genes is a good example of how gene duplication can produce the genetic variability to result in evolutionary change.
- Describe how a gene duplication can produce a gene family.
Types of Inversions
Inversions involve the rearrangement of genes along a chromosome. In essence, a portion of a chromosome has been flipped in the opposite direction. An inversion can be thought of as breaking the chromosome in two places, flipping the DNA between the breaks, and sealing the DNA breaks. The total amount of genetic material (number of genes) in the chromosome does not change. Inversions are quite common; about 2% of the human population carries a detectable inversion.
There are two types of inversions (see figure 3.6):
- Pericentric inversion. In a pericentric inversion, the breaks occur in both the p and the q arms of the chromosome, with the centromere lying within the inverted region. A pericentric inversion often changes the position of the centromere within the chromosome.
- Paracentric inversion. In a paracentric inversion, the breaks both occur within the same arm of the chromosome, with the centromere of the chromosome lying outside of the inverted region. A paracentric inversion does not change the position of the centromere within the chromosome.
Most inversions have no phenotypic consequences; however, if one of the chromosome breaks that form an inversion occurs within a gene, then a change in phenotype can occur. For example, in type A hemophilia, the breakpoint of an inversion in the X chromosome occurs within the factor VIII gene. The Factor VIII protein is required for proper blood clotting, so this inversion results in a nonfunctional protein, leading to a deficiency in clotting (hemophilia).
Sometimes the change in the position of a gene within the genome can alter the transcription of the gene or other genes nearby. This alteration of gene expression by an inversion is called a position effect. In some cases, the position effect can result in the overexpression of proteins that regulate the cell cycle, producing cancer.
- What is the difference between a pericentric and a paracentric inversion?
- What human disease is caused by an inversion?
Crossing Over in Inversion Heterozygotes
An inversion heterozygote is an individual who has a chromosome with a normal allele arrangement, while the homologous chromosome contains an inversion. An inversion heterozygote has a normal phenotype but produces gametes with abnormal chromosomes.
Defective gametes are produced because of events during prophase I of meiosis. Recall that homologous chromosomes synapse (align) during prophase I. For the normal chromosome and the inversion chromosome to synapse properly in an inversion heterozygote, one of the two chromosomes within the homologous chromosome pair forms an inversion loop. After the inversion loop is formed during synapsis, crossing over occurs between the two chromosomes. Abnormal chromosomes are produced after the crossover structures are resolved.
- Pericentric inversion. In a pericentric inversion, the centromere lies within the inverted region of the chromosome. When crossing over occurs between the homologous chromosomes and after meiosis is completed, the following gametes are produced (see figure 3.7):
- A gamete that contains a chromosome with the normal gene arrangement. This gamete will produce offspring with the normal gene arrangement.
- A gamete that contains an inversion chromosome. Thus, inversion chromosomes are passed from parents to offspring. The resulting offspring will suffer no negative phenotypic effects.
- Two gametes that contain abnormal chromosomes. Each abnormal chromosome contains a duplication of some genes and a deletion of other genes. The fertilized egg produced from either of these two gametes is not generally viable.
- Paracentric inversion. In a paracentric inversion, the centromere lies outside of the inverted region of the chromosome. After crossing over between the homologous chromosomes occurs and meiosis is completed, the following gametes are produced (see figure 3.8):
- A gamete that contains a chromosome with the normal gene arrangement. This gamete will produce offspring with the normal gene arrangement.
- A gamete that contains an inversion chromosome. The inversion chromosome is passed from parents to offspring. The resulting offspring will suffer no negative phenotypic effects.
- Two gametes that contain highly abnormal chromosomes. One gamete contains an abnormal chromosome containing duplications and deletions, but more importantly, the chromosome lacks a centromere and will be lost during meiosis. This chromosome that lacks a centromere is called an acentric fragment. The other gamete will contain an abnormal chromosome that has two centromeres (dicentric chromosome) along with duplications and deletions. Between the two centromeres is a region called a dicentric bridge. When a dicentric chromosome, attached to opposite spindle poles, tries to separate during anaphase of meiosis, it is torn apart. Breakage produces chromosome fragments that are missing genes.
50% of the gametes produced by pericentric and paracentric inversion heterozygotes fail to produce viable offspring. Thus, inversion heterozygotes have a 50% reduction in fertility.
- Describe the four gametes produced by an individual who carries a pericentric inversion.
- Describe the four gametes produced by an individual who carries a paracentric inversion.
- What is a dicentric bridge, and why does it produce lethal products?
A translocation occurs when a piece of a chromosome becomes attached to a nonhomologous chromosome. Translocations can be balanced, meaning that the total amount of genetic material in the individual does not change. Balanced translocations often do not have phenotypic consequences. Translocations can also be unbalanced; the nonhomologous chromosome pairs that have exchanged DNA also suffer duplications or deletions. Our discussion will focus mainly on balanced translocations.
As mentioned earlier, there are three types of translocations: simple translocations, reciprocal translocations, and Robertsonian translocations. Reciprocal translocations are formed by two general mechanisms (see figure 3.9):
- Chromosome breakage and defective DNA repair. Some chemicals or environmental agents can cause chromosomes to break at internal sites, forming reactive ends not protected by telomeres. Recall that telomeres are the structures found on the ends of linear eukaryotic chromosomes that are designed to prevent chromosome ends from sticking together (see Part 1). Cells contain repair enzymes to handle these situations, and in most cases, quickly repair these breaks. When nonhomologous chromosomes are broken simultaneously, the repair enzymes can inadvertently join nonhomologous chromosomes together, resulting in a reciprocal translocation.
- Nonhomologous chromosomes experience crossing over. If two nonhomologous chromosomes synapse and undergo crossing over during meiosis I, a reciprocal translocation occurs.
- What processes generate reciprocal translocations?
Meiosis in Cells with Reciprocal Translocations
How do nonhomologous chromosomes that have experienced a reciprocal translocation synapse and then segregate in meiosis I?
During synapsis, the two pairs of homologous chromosomes that include individual chromosomes that have suffered a reciprocal translocation attempt to synapse and form an unusual structure called a translocation cross.
For example, suppose a translocation cross is produced from a normal copy of chromosome 5, a normal copy of chromosome 13, and two translocation chromosomes in which chromosomes 5 and 13 have undergone a reciprocal translocation. Prior to meiosis, these four chromosomes are copied by DNA replication to produce eight sister chromatids. During synapsis, the normal copy of chromosomes 5 and 13 are diagonal from each other and the two translocation chromosomes are diagonal from each other within the translocation cross structure. The chromosomes in the translocation cross can then segregate during meiosis I in three possible ways (see figure 3.10):
- Alternate segregation (common). The chromosomes diagonal from each other segregate into the same cell at the conclusion of meiosis I. For example, the normal copy of chromosome 5 and 13 segregate with each other into the same daughter cell and the two translocation chromosomes segregate with each other into the same cell. After meiosis II, there are two normal gametes and two gametes that carry balanced translocations. All four gametes are capable of producing viable offspring.
- Adjacent-1 segregation (common). The two chromosomes on the bottom half of the cross (normal chromosome 13 and translocation chromosome 5) segregate with each other into the same daughter cell. The two chromosomes on the top half of the cross (normal chromosome 5 and one translocation chromosome 13) segregate with each other into the same daughter cell at the conclusion of meiosis I. After meiosis II, all four gametes carry duplications and deletions and therefore are not generally viable.
- Adjacent-2 segregation (rare). The two chromosomes on the right half of the cross (normal chromosome 5 and translocation chromosome 5) segregate with each other into the same daughter cell. The two chromosomes on the left half of the cross (normal chromosome 13 and translocation chromosome 13) segregate into the same daughter cell at the conclusion of meiosis I. One daughter cell receives, in essence, both copies of chromosome 5; the other daughter cell receives both copies of chromosome 13. Since both copies of chromosome 5 end up in the same cell and both copies of chromosome 13 end up in the same cell, adjacent-2 segregation can be considered a nondisjunction event (see below). After meiosis II, all four gametes carry duplications and deletions and are not generally viable.
- Which segregation pattern produces normal gametes?
- Why does adjacent-2 segregation occur so rarely?
A rare form of Down syndrome called familial Down syndrome is inherited (see figure 3.11). In familial Down syndrome, a phenotypically normal parent can contain a translocation. Such an individual would carry normal copies of chromosomes 14 and 21 and a chromosome that contains a fusion between the long arms of chromosome 14 and 21. In this person, the short arms of chromosome 14 and 21 have been lost, but since these regions do not carry vital genetic information, the individual can tolerate these losses. This type of translocation, involving the fusion of the long arms of two acrocentric chromosomes, is called a Robertsonian translocation. The Robertsonian translocation, which involves only chromosomes 13, 14, 15, 21, and 22, is the most common chromosome abnormality in humans.
A problem occurs during meiosis in this balanced carrier individual. The three chromosomes replicate, synapse, and attempt to segregate during anaphase of meiosis I and II. There are six possible types of gametes that can be produced by the carrier individual:
- Normal. When the normal gamete produced by the carrier parent fuses with a normal gamete from the other parent during fertilization, an offspring is produced that contains two copies of chromosomes 14 and 21. This offspring is phenotypically normal.
- Balanced carrier. When a gamete containing the Robertsonian translocation between chromosomes 14 and 21 fuses with a normal gamete from the other parent during fertilization, a carrier offspring that contains one copy of chromosome 14, one copy of chromosome 21, and a Robertsonian translocation chromosome is produced. The total chromosome number is 45 due to the loss of the short arms of chromosomes 14 and 21. This carrier is phenotypically normal but can produce familial Down syndrome offspring in the next generation.
- Familial Down syndrome. A gamete that contains the Robertsonian translocation between chromosomes 14 and 21 and a copy of chromosome 21 can fuse with a normal gamete from the other parent during fertilization. The offspring contains two copies of chromosome 21, one copy of chromosome 14, and the Robertsonian translocation chromosome. Since there are three copies of the long arm of chromosome 21 (trisomy-21), Down syndrome results. A familial Down syndrome patient has 46 total chromosomes.
- Unbalanced, lethal (three types). Fifty percent of the gametes produced by a balanced carrier individual will not produce viable offspring. The resulting offspring produced from these gametes are either missing an entire chromosome (monosomy-14 or monosomy-21) or have an extra copy of chromosome 14 (trisomy-14).
- What is a Robertsonian translocation?
- What chromosomes are involved in familial Down syndrome?
- How many total chromosomes are observed in a balanced carrier individual?
- How many total chromosomes are observed in a familial Down syndrome patient?
- What genes are located on the p arms of the five acrocentric chromosomes (see Part 1 reading). Why would the loss of a few copies of these genes not be lethal to the cell?
B. Changes in Chromosome Number
Euploidy and Aneuploidy
Sometimes the total number of chromosomes within an individual can vary. These variations in chromosome number are placed into two categories (see figure 3.12):
- Variations in euploidy (changes in the total number of chromosome sets). Variations in euploidy involves organisms or cells that are:
- Haploid (n; n = the number of chromosomes within a set). Haploid organisms or cells have one chromosome set (i.e., one copy of every chromosome). Haploid is the normal state for gamete cells.
- Diploid (2n). Diploid organisms or cells have two chromosome sets. In this case, a set of chromosomes is all chromosomes inherited from a single parent. One chromosome set is inherited from the paternal parent; one chromosome set is inherited from the maternal parent. Diploid is the normal state for many eukaryotic organisms and for most of the cells in the human body.
- Triploid (3n). Triploid organisms or cells have three chromosome sets. Triploid is an abnormal state for most organisms.
- Polyploid. Polyploid organisms or cells have more than two chromosome sets.
- Aneuploidy (changes in the number of chromosomes within a set). Aneuploidy occurs when an individual is missing or has an additional chromosome. Aneuploidy of the sex chromosomes is usually better tolerated than aneuploidy of the autosomes in many organisms.
- Trisomy (2n+1). A trisomic organism or cell has one more chromosome than normal. Trisomy is usually better tolerated than monosomy.
- Monosomy (2n-1). A monosomic organism or cell is missing a single chromosome.
- Disomy (2n). Disomy is the normal state in which an organism or cell has two copies of a particular chromosome.
- Nullisomy (2n-2). An organism or cell that is nullisomic is missing both copies of the chromosomes that constitute a homologous chromosome pair.
- What is meant by variations in euploidy?
- What does aneuploidy mean?
- How many chromosomes are found in a trisomic human cell?
- How many chromosomes are found in a monosomic human cell?
- How many chromosomes are found in a triploid human cell?
Aneuploidy and Gene Expression
A phenotypically normal individual has two copies of most structural genes. When the number of genes controlling a trait is out of balance (one copy of a gene; more than two copies of a gene), the phenotype is often affected in a negative way.
In trisomic individuals (three copies of a particular chromosome), the gene products produced from the three copies of the chromosome are 150% of the normal expression level. These individuals produce too much protein product, so the phenotype is negatively affected (see figure 3.12).
In the case of monosomy (one copy of a particular chromosome), the single copy of the chromosome can only produce gene products at 50% of the normal level. These individuals produce lower amounts of protein product, so the phenotype is negatively affected. Monosomic cells or individuals also have a second problem. In monosomic cells, recessive lethal alleles cannot be “masked” by the normal, dominant allele from the homologous chromosome.
In a trisomic or monosomic individual, the overproduction or underproduction of the gene products of hundreds of genes generally decreases the viability of these individuals. It is worth noting that there are many natural varieties of plants that have variations in euploidy or aneuploidy, indicating that plants can better tolerate variations in chromosome number than most animals.
- Why does trisomy have negative phenotypic consequences?
- What are the two reasons that monosomy produces negative phenotypic effects?
Aneuploidy in Humans
About 30% of all fertilization events in humans produce an embryo that is aneuploid. In most cases, the embryo does not survive. That being said, there are several aneuploid human conditions that result in live births. These aneuploid conditions in humans include:
- Trisomy 13 (47,13+). Trisomy 13 produces Patau syndrome, which occurs in 1 in 19,000 births. Patau syndrome causes mental and motor deficiencies, cleft palate, polydactyly (extra digits), microcephaly (a small head), defects in several organs, and an early death (usually by 3 months of age).
- Trisomy 18 (47, 18+). Trisomy 18 produces Edwards syndrome, which occurs in 1 in 8,000 births. Edwards syndrome causes skeletal abnormalities such as elongated skulls, deformed hips, and facial deformities. Most infants with this syndrome are females, and death usually occurs within 4 months after birth.
- Trisomy 21 (47, 21+). Trisomy 21 causes the conventional form of Down syndrome, which occurs in 1 in 800 births. Down syndrome results in mental deficiencies, almond-shaped eyes, flattened faces, round heads, and a short stature. There is another type of Down syndrome called familial (inherited) Down syndrome. Familial Down syndrome is caused by a Robertsonian translocation (see above).
- XXY (47, XXY). An individual with XXY sex chromosomes has Klinefelter syndrome, which occurs in 1 in 1000 male offspring. Klinefelter syndrome results in infertility (no sperm production) and the formation of breast tissue.
- XYY (47, XYY). An individual with XYY sex chromosomes has Jacobs syndrome, which occurs in 1 in 1000 male offspring. Jacobs syndrome produces mild phenotypic effects.
- XXX (47, XXX). An individual with XXX sex chromosomes has Triplo-X syndrome, which occurs in 1 in 1500 female offspring. Triplo-X syndrome produces mild phenotypic effects.
- XO (45, X). An individual with a single X chromosome has Turner syndrome, which occurs in 1 in 5000 female offspring. Turner syndrome females are short, have a webbed neck, and have reduced fertility.
The aneuploid conditions described above are the result of chromosome nondisjunction, a defect in chromosome segregation during meiosis (see below).
- The aneuploidies described above are essentially the only ones that result in live human births. Why do you think these aneuploidies are viable while aneuploidies of other chromosomes are not?
Some tissues in an animal can contain cells that have more than two chromosome sets, whereas the somatic cells in the rest of the animal are diploid. This situation is called endopolyploidy. For example, human liver cells can vary in euploidy (some cells are 4n, 8n, or 16n). Endopolyploidy allows liver cells to increase the production of proteins encoded by genes on these chromosomes to meet the metabolic demands of the body.
The fruit fly Drosophila is a diploid organism containing four pairs of homologous chromosomes. However, the salivary glands of the fruit fly contain higher variations in euploidy. To produce so many copies of the same chromosome, the homologous chromosomes pair with each other and then undergo several rounds of DNA replication without cell division. Replication in this way produces a polytene chromosome, a bundle of attached sister chromatids that lie parallel to each other.
- What is endopolyploidy?
- Provide two examples of endopolyploidy.
Nondisjunction occurs when chromosomes do not separate properly in anaphase of either meiosis or mitosis.
Meiotic nondisjunction produces aneuploid gamete cells that either have an extra chromosome or lack a chromosome (see figure 3.13). After fertilization, the offspring will be aneuploid. Meiotic nondisjunction can occur during anaphase of either meiosis I or meiosis II.
- Meiosis I nondisjunction. A pair of homologous chromosomes fails to separate from each other during anaphase I and instead segregates into the same daughter cell. All gamete cells produced from meiosis I nondisjunction are aneuploid. These aneuploid gametes will produce 50% trisomic offspring and 50% monosomic offspring.
- Meiosis II nondisjunction. In this case, meiosis I is normal; however, nondisjunction occurs in one of the two daughter cells of meiosis I. During meiosis II nondisjunction, the sister chromatids that constitute one chromosome fail to separate from each other. Two of the resulting haploid gametes are normal, while two of the gametes are aneuploid. Collectively, these gametes will produce 50% normal offspring, 25% trisomic offspring, and 25% monosomic offspring.
The aneuploid conditions discussed above (conventional Down syndrome, Klinefelter syndrome, Turner syndrome, etc.) are thought to be produced from either meiosis I or meiosis II nondisjunction.
On rare occasions, all chromosomes in a cell display nondisjunction and fail to separate properly during either meiosis I or II. This event is called complete nondisjunction and produces diploid gametes. If a diploid gamete fuses with a normal gamete, a triploid offspring is produced.
- What happens during meiosis I nondisjunction?
- Describe the four gametes produced by meiosis I nondisjunction.
- What happens during meiosis II nondisjunction?
- Describe the four gametes produced by meiosis II nondisjunction.
Nondisjunction can also occur during mitosis (mitotic nondisjunction). During mitotic nondisjunction, the sister chromatids that constitute one chromosome fail to separate from each other during anaphase. Mitotic nondisjunction produces one daughter cell with three copies of a chromosome (trisomic), while the other daughter cell has one copy of the chromosome (monosomic) (see figure 3.14). All future daughter cells produced from the trisomic cell will also be trisomic, whereas all daughter cells produced from the monosomic cell will also be monosomic. Because of this, mitotic nondisjunction produces a mosaic phenotype. When aneuploid somatic cells divide, a patch of monosomic tissue is produced, whereas another patch of tissue is trisomic.
Sometimes chromosomes lose their attachment to the mitotic spindle during anaphase. A detached chromosome fails to be retained in the nucleus, and is degraded by nucleases in the cytoplasm. This event produces one daughter cell with two copies of a chromosome (disomic), while the other daughter cell has one copy of the chromosome (monosomic). All future daughter cells produced from the monosomic cell will be monosomic. This failure of a chromosome to attach to the mitotic spindle also results in a mosaic phenotype.
- Describe the two processes that cause mitotic nondisjunction.
Fill in the Blanks:
- A _________________________ is a change in chromosome structure that produces an acentric fragment and a dicentric chromosome during meiosis.
- The disease ______________________ is caused by an inversion within the X chromosome.
- A(n) __________________ and a(n) _____________________ are two changes in chromosome structure that alter the amount of genetic information found on a chromosome or in the genome.
- ___________ % of the gametes produced by a meiosis II nondisjunction event will result in trisomic offspring.
- A terminal deletion in chromosome _________ causes ___________________, a disease that results in a malformation of the glottis and larynx.
- Nondisjunction in _____________ can produce trisomic and monosomic somatic cells.
- The term _______________ can describe an individual with 2n-2
- The globin gene family arose due to a __________________________.
- A ____________________ translocation involves only the acrocentric chromosomes.
- Reciprocal translocations of the __________________ segregation type result in two normal gametes and two balanced translocation gametes.
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