The following points highlight the four main types of structural changes in chromosomes. The types are: 1. Deletion 2. Duplication 3. Translocations 4. Inversions.
Structural Change: Type # 1. Deletion:
Deletion refers to loss of a portion of segment from a chromosome. It is also known as deficiency. Deletions have been observed in Drosophila, maize, tomato, wheat and several other crops. Depending upon the location, deletions are of two types, viz. terminal arid interstitial [Fig. 5.1(a)].
A. Terminal Deletion:
A chromosome has two ends or terminals. Loss of either terminal segment of a chromosome is known as terminal deletion. It may be of two types, viz. heterozygous and homozygous. When deletion occurs only in one chromosome of a homologous pair, it is known as heterozygous deletion.
When deletion occurs in both the chromosomes of a pair, it is termed as homozygous deletion. Usually, heterozygous deletions are observed, because the normal chromosome has the genes which are missing in the deficient chromosome. Homozygous deletions are rare and usually lethal. In case of terminal deletion, only one break occurs. After deletion the injured end is healed up.
B. Interstitial Deletion:
Sometimes, there is loss of a segment of chromosome from the intermediate portion or between telomere arid centromere. Such loss of intercalary portion of chromosome is known as interstitial or intercalary deletion. The interstitial deletion generally does not involve centromere. In such deletion, the break occurs at two places.
The intercalary portion comes out and the remaining portions are reunited. Intercalary deletions are usually more common than terminal ones. In both the cases, the deleted portion may contain one, two or several genes. The deleted segment is usually acentric which is eventually lost resulting in deficiency of genes in a chromosome which were contained in the deleted segment.
Sometimes, the broken portion of a chromosome reunites in the original sequence. Such reunion is referred to as restitution and is very rare. In other words, restitution refers to that reunion of broken segment of a chromosome which restores original sequence of genes. Deletions originate as a result of chromosomal breakage.
Deletions can be detected in two ways, viz. by cytological method and by generic method. In cytological method two criteria, viz. meiotic pairing and chromosome length are taken into consideration. In case of heterozygous deletion, one of the two homozygous chromosomes is shorter than other.
The pairing occurs between homologous segments. In case of terminal deletion, normal chromosome remains unpaired at one end and in case of interstitial deletion a loop is formed in the normal chromosome in the region of deletion [Fig. 5.2(a)].
This loop confirms the presence of deletion. The length of chromosome also helps in the detection of deletion. The deficient chromosome becomes shorter than normal one. Pseudo dominance, which refers to the expression of a recessive gene when present in a single dose, is a most reliable genetic method of detecting deletion.
When a deleted segment carries a dominant gene, a recessive gene present on the corresponding segment on the normal chromosome will express itself even in the single dose because the dominant gene which suppresses it is lacking.
Deletions have effects on fertility, viability, crossing over, phenotype and karyotype as described below:
The pollen fertility is reduced in the presence of deletion. The pollen produced by a deficient heterozygote is of two types, viz. functional and non-functional. The pollens with normal chromosomes are functional and those with deficient chromosomes are aborted or non-functional.
A very small deletion in the heterozygous state is viable, but is lethal in homozygous condition. However, a large deletion is lethal even in heterozygous condition. This reveals that all the genes (segments) of a chromosome are vital for normal viability and complete loss of any segment from the genome is harmful.
c. Crossing Over:
The crossing over is suppressed in the region of deficiency due to lack of corresponding segment in the area of deletion.
Deletion also affects the phenotype. In the absence of dominant gene in the deletion region the recessive gene expresses even in single dose. This results in the change in phenotype. Another example is in man a syndrome called cat cry (cri du chat). This results due to deletion in the short arm of chromosome. Babies with such defect cry continuously like a cat.
e. Change in Karyotype:
Generally the chromosomes with deletion can never revert to a normal condition. The gene number as well as karyotype of the individual is changed.
Deletions play an important role in species formation and releasing variability through chromosomal mutations. Deletions are important cytological tools for mapping genes. Deletion mapping has been widely used in Drosophila to locate various genes in polytene chromosomes.
Structural Change: Type # 2. Duplication:
Duplication refers to the occurrence of a segment twice in the same chromosome. It results in addition of one or more genes to a chromosome. Duplication is also known as repeat and was first reported in Drosophila by Bridges in 1919.
Now it has been reported in maize, wheat, barley, rice, Nicotiana, Tradescantia and several other species of crop plants by various workers. Duplications are of four types, viz. tandem, reverse tandem, displaced and reverse displaced.
In this case sequence of genes in the duplicated segment is similar to the sequence of genes in the original segment of a chromosome.
a b c [b c] d e f (a bc [bc] def)
B. Reverse Tandem:
Here the sequence of genes in the duplicated segment is reverse to the sequence of genes in the original segment of a chromosome. Tandem and reverse tandem duplications are known as adacent duplications, because they are adacent to the original segment.
a b c [c b] d e f (a bc [cb] def)
When the duplication is found away from the original segment but on the same arm of the chromosome, it is known as displaced duplication.
a b c d e f g h I j k Normal
a [d e] b c d e f g h I j k Displaced
D. Reverse Displaced:
Such duplication is also away from the original segment but on the other arm of a chromosome. These two types are known as non-adjacent duplications, because they are away from the segment which shows duplication.
It is believed that duplications originate due to unequal crossing over during meiosis. The homologous chromosomes usually pair in such a way that all the identical loci match with each other in their positions. This facilitates equal crossing over between non-sister chromatids.
Sometimes, homologous chromosomes pair in such a mis-aligned manner that the corresponding identical loci do not fall opposite to each other. Such situation leads to unequal crossing over between non-sister chromatids.
This gives rise to two types of chromatids, viz. one with duplication and other with deletion. When a gamete with duplication unites with normal ovum, it leads to formation of zygote with triplicate genes in a particular segment of a chromosome.
Duplications can be detected by cytological and genetic methods. Duplication loop can be observed during pachytene state when homologous chromosomes pair [Fig. 5.2(b)]. Moreover, chromosomes having duplicate segment are longer than normal chromosome.
If a duplicate segment includes centromere, it may be present as a small extra chromosome added to a normal chromosome complement. Duplications can also be detected by suppression of recessive characters. A single dominant gene in the duplicate region is enough to suppress the expression of two recessive alleles.
Duplications have been found to affect phenotype, crossing over, gene number and pollen fertility.
Duplication of certain genetic regions produces specific phenotypic effect. For example, gene for bar eye (16A segment) in Drosophila produces normal oval shaped eye when present in single dose, bar eye with two doses of 16A and ultra-bar eye with three doses of 16A segment.
b. Crossing Over:
Crossing over is suppressed in the duplicate region due to lack of corresponding duplicated segment in the normal chromosome.
c. The gene number is increased in the chromosome having duplication.
d. Presence of duplication leads to reduction in pollen fertility in plant species.
Duplications are less harmful than deletions. They do not reduce the viability of an individual. Duplications lead to addition of some genes in a population which after mutation play an important role in the evolution. Moreover, duplications sometimes cover the deleterious effects of deletion in an individual.
Structural Change: Type # 3. Translocation:
One way or reciprocal transfer of segments between non-homologous chromosomes is known as translocation. Translocations have been reported in Datura, maize, Oenothera, bailey, rye, wheat, Tradescantia, and several other crops by various researchers.
Translocation differs from crossing over in four main aspects as given below:
a. Translocations involve non-homologous chromosomes, whereas crossing over involves non-sister chromatids of homologous chromosomes.
b. Translocation homozygotes change the linkage map, whereas crossing over does not change the linkage map.
c. Translocations involve breakage and reunion, whereas crossing over involves chiasma foonation.
d. Crossing over does not lead to sterility, whereas translocations lead to pollen and ovule sterility.
Translocations are of three types, viz:
B. Shifts, and
C. Reciprocal (Fig. 5.1 c).
A.. Simple Translocation:
When a segment from one chromosome is transferred and attached to the end of a non-homologous chromosome, it is known as simple translocation. In such translocation only a simple break occurs in one chromosome. Since telomere does not easily allow integration of another segment, such translocations are very rare in nature.
Transfer of an intercalary segment from one chromosome to the intercalary position in a non-homologous chromosome is referred to as shift. Thus two breaks occur in a loser chromosome and one break in the gainer chromosome for transfer and integration of such segment.
C.. Reciprocal Translocations:
When there is mutual exchange of segments between non-homologous chromosomes, it is known as reciprocal translocation. In such translocations, one break occurs in each chromosome before exchange of segments. Such translocations are very common and have great evolutionary significance. Reciprocal translocations are of two types, viz. homozygotes and heterozygotes.
In case of reciprocal translocation homozygotes both the chromosomes of two pairs are involved. Translocations heterozygotes are more common than translocation homozygotes. Sometimes, more than two pairs of non-homologous chromosomes are involved in translocation. Such translocations are known as multiple translocations and have been reported in Drosophila and Oenothera.
Translocations originate through breakage and exchange of parts between non-homologous chromosomes. When only one chromosome from each pair of two homologues is involved, it gives rise to translocation heterozygotes and when both chromosomes from each pair are involved, it produces translocation homozygotes.
Translocations can be detected by cytological as well as genetic methods. Cytological method includes study of pachytene configurations and metaphase configurations. In translocation homozygotes, meiosis is normal with regular bivalent formation at pachytene. At anaphase, movement of the poles is normal and viable gametes are produced.
However, in translocation heterozygotes, instead of bivalent, a cross shaped configuration of quadrivalents is formed due to synapsis between homologous segments [Fig. 5.2(c)], At metaphase, the cross shaped complex opens up to form a chain or ring of four chromosomes, if chiasma does not form.
At anaphase, the chromosomes disjoin (segregate) in three different ways as given below:
a. Alternate Disjunction:
When two normal chromosomes [N1 and N2] move towards one pole and two trans-located chromosomes [T1 and T2] to another pole, is known as alternate segregation or disjunction. In such segregation, all gametes receive full complement of genes and will give rise to viable individuals.
b. Adjacent 1 Segregation:
The segregation of one normal chromosome with one trans-located is called adjacent 1 segregation. Such segregation occurs in open ring configuration. Here the chromosomes which go to one pole are non-homologous [T1 + N2 and T2 + N1].
c. Adjacent 2 Segregation:
Sometimes, in open ring configuration, two homologous chromosomes [T1N1], i.e., one normal and one trans-located move to one pole and other homologous chromosomes [T2N2] move to another pole. Such disjunction is known as adjacent 2 segregation.
This type of segregation is very rare. Both adjacent types of segregations will produce gametes with duplication and deficiencies which may cause semi sterility.
Translocations can also be detected by genetic methods, based on pollen sterility, gene segregation and linkage studies as described below.
i. Pollen Sterility:
Translocations can be detected by testing pollens. If there is 50% or more sterility in a normal diploid species, it indicates the presence of translocations.
ii. Gene Segregation:
Translocations can be detected by performing genetic crosses and observing segregation.
When progeny is of three types, viz. normal homozygotes, interchanged heterozygotes and interchanged homozygotes in 1 : 2 : 1 ratio, it indicates presence of translocation.
iii. Linkage Studies:
In translocation homozygotes linkage is changed. The genes located in the translocated segment are delinked from the original chromosome and are linked with the genes of chromosome to which such segment is integrated. Study of such change in linkage can be used to detect a translocation and identify the involved chromosome.
The main effects of translocation are on sterility, crossing over, karyotype and phenotype as described below:
Translocations lead to duplication and deletion of genes. Gametes with duplication and deficiencies are inviable. Thus translocations result in pollen and ovule sterility. If there is a ring of four chromosomes, 50% sterility is observed and if there is a ring of six chromosomes, 75% sterility is found. Thus presence of translocations reduces the yield in crop plants due to formation of imbalanced zygotes.
b. Crossing Over:
The crossing over is generally suppressed in translocated chromosomes probably due to competition in pairing.
Translocations bring changes in chromosome number and karyotype. They may alter the size of chromosome as well as position of centromere.
Translocations also change phenotype. For example, in human, Down Syndrome (Mangolism) can arise in the progeny of an individual heterozygous for a translocation involving chromosome number 21. Such individuals look normal phenotypically, but produce gametes with duplication and deficiencies. Union of such gametes with normal one leads to development of individual with Down Syndrome.
Translocations alter the chromosome size, chromosome number, and karyotype and thus play an important role in the formation of species. Translocation homozygotes lead to establishment of new linkage relationship.
They are useful in locating the position of genes, centromere and other genetic markers on the chromosomes. They are useful tools in breeding programmes for transfer of desirable characters from one species to another.
Structural Hybridity in Oenothera:
In the genus Oenothera, somatic chromosome number is 14. There are several species of Oenothera which exhibit various degrees of ring formation. Some species have permanent translocation heterozygosity involving almost all chromosomes.
Such species include Oenothera lamarckiana, O. muricata, and O. biennis. O. lamarekiana has a ring of 12 chromosomes and one bivalent pair. In O. muricata, all the 14 chromosomes are united to form a giant ring. O. biennis shows one ring of 8 chromosomes and another of six chromosomes.
In some species of Oenothera, permanent structural hybridity is maintained due to their superiority in adaptive value. In such species, a balanced lethal system operates by way of gametic and zygotic lethality. Such system favours only heterozygotes.
The balanced lethal system in Oenothera was first discovered by Renner which is also known as Renner complex. Some examples of balanced lethal system in Oenothera are given below.
It is composed of gaudens—valens (G/V) genes and three types of offspring viz. G/G. G/V, and V/V should arise after selfing of G/V individuals in the ratio of 1 : 2 : 1. However, only heterozygotes (G/V) survive and homozygotes (G/G and V/V) die.
It can be explained as follows. One complex contains one recessive lethal gene (l1) and the other contains another lethal (l2)and each has the dominant allele of others lethal. The heterozygous types (L1l1/L2l2) thus survive, but the homozygotes (l1l1/L2L2 and L1L1/l2l2) die.
This species consists of the gene rigens-curvens (R/C). In this case also selfing of heterozygote (R/C) gives rise only to heterozygote (R/C). The homozygotes (R/R and C/C) die because of lethality.
This species consists of gene albicans-rubens (A/R). Selfing of heterozygote (A/R) produces only heterozygote, i.e., A/R (L1l1/L2l2). The homozygotes A/A (l1l/L2L2) and R/R (L1L1/l2l2) die because of lethality.
Structural Change: Type # 4. Inversion:
Inversion refers to structural change in a chromosome in which a segment is oriented in a reverse order. Thus the inverted segment is rotated to a full 180°. Inversion was first discovered by Sturtevant in 1926 in Drosophila. Now it has been reported in maize, Nicotiana and several other plants. Depending upon whether centromere is involved or not inversions are of two types, viz., paracentric and pericentric [Fig. 5.1(d)].
A. Paracentric Inversion:
The inversion in which centromere is not involved is called paracentric inversion. In this type of inversion both breaks occur in one arm of the chromosome.
When only one chromosome of a homologous pair has inversion, it is called inversion heterozygote. When both the members of a homologous pair have similar type of inversion, it is called inversion homozygote. Meiosis is normal in inversion homozygotes.
Crossing over within the inversion loop in a paracentric inversion heterozygote results in the formation of dicentric bridge and an acentric fragment after exchange (Fig. 5.3a). The other two chromosomes remain normal.
The dicentric chromosome leads to formation of bridge at anaphase. The bridge is later on broken due to pull from both the poles. The acentric segment is lost due to lack of movement. Thus out of four gametes, two are normal and two deficient for some genes. In plants, gametes with deficiencies are inviable.
B. Pericentric Inversion:
When centromere is involved in the inversion, it is known as pericentric inversion. When a break occurs in each of the two arms of a chromosome, the centromere is included in the detached segment resulting in a pericentric inversion.
Crossing over within the inversion loop results in the formation of chromatids with duplication and deficiency. Out of four chromatids two are crossover products and two are normal. One of the non-crossovers has original gene sequence and the other has inverted gene sequence.
The crossover chromosomes produce inviable gametes or zygote due to imbalance of genes. The viable progeny is produced by non-crossover chromosomes only [Fig. 5.3(b)].
Inversions result when there are two breaks in a chromosome and the detached segmer reunited to the same chromosome in the reverse order.
Both cytological and genetical methods are used to detect the presence of inversion. Three cytological criteria, viz. pachytene configuration, anaphase configuration and position of centromere are used for detection of inversion. Inversions can be detected in the meiotic nuclei by the presence of an inversion loop in the paired homologues during pachytene.
At anaphase first, the pericentric inversions can be detected by the presence of dicentric bridge. Pericentric inversions shift the position of centromere which helps in the detection of inversions. Inversions can also be detected genetically by pollen sterility. Both types of inversions with formation of chiasmata lead to 50% sterility in the gametes.
Inversions have effects on fertility, crossing over, gene order and karyotype as discussed below:
The crossing over in the inversion loop leads to formation of chromosomes with duplications and deficiencies. Gametes with such chromosomes are inviable and lead to 50% sterility.
b. Crossing Over:
Inversion heterozygotes often have pairing problems in the area of inversion. Thus competition for pairing reduces crossing over in the area of inversion.
c. Gene Order:
The gene order is changed in the inverted segment of a chromosome. Inversion heterozygotes exhibit a linkage map with different gene order. In inverted chromosome there is no loss of genetic material provided crossing does not occur in the inversion loop.
Pericentric inversions sometimes results in change of karyotype by shifting the position of centromere. Crossing over in the inversion loop may lead to shift in the position of centromere.
Inversions are very much useful in production of duplications and deficiencies. A large cross over in the inversion loop produces chromatids with duplication and deficiency. Inversions also play an important role in the evolution of new species by changing the karyotype of an individual.
Inversions suppress crossing over and tend to retain the original combination of genes. This may help in preserving original gene combinations. Moreover, inversions provide strong evidence that out of four only two chromatids are involved in the crossing over.