Mutations

Charles Darwin (1859) in his book 'Origin of Species' postulated that evolution or formation of newer species of organisms from the pre-existing ones occurs through accumulation of variations in the organisms. Without variations all the individuals of a species shall be alike. None of them would have better characteristics for being favoured in the struggle for existence and natural selection. A change in the environment could kill all of them. Variations increase the adaptability of the individuals and make them better fitted in the struggle for existence and hence natural selection. There are two sources of variations in the populations - recombinations and mutations. (i) Recombinations are also called continuous variations. They are not new traits but are formed by new combination of genes or traits already existing in the population.

Recombinations are produced due to three reasons: 

(a) Crossing over of nonsister chromatid segments during meiosis. 

(b) Random segregation of chromosomes at the time of meiosis or gametogenesis. 

(c) Random coming together of chromosomes at the time of fertilization. 

(i) Recombinations may bring recessive genes together. They can also bring together various useful variations found in the population so that some individuals become better fitted in the struggle for existence as compared to other. However, recombinations or continuous variations alone cannot produce new species. Bateson (1884) was the first to propose that evolution is caused by large variations or discontinuous variations. 

(ii) Mutations are new discontinuous variations which are brought about by the change in the sequence of a DNA segment corresponding to a gene, change in number of genes and chromosomes. Depending upon their utility, mutations are of three types- useful, harmful and neutral. Some of the harmful mutations get eliminated through their deleterious effect on the capability of the individuals in the struggle for existence. Other mutations persist and accumulate. Thus mutations are the fundamental source of all variations in the population.

I. CHROMOSOME ALTERATIONS/MUTATIONS/ABERRATIONS

Chromosome alterations are changes that occur in the number and sequence of genes in the chromosomes without altering their ploidy. Chromosome alterations are also called chromosomal aberrations/mutations because they change the morphology and architecture of the chromosomes. They were first analysed by Muller (1928) in Drosophila with the help of polytene chromosomes (salivary chromosomes) and Ms Mc Clintock (1930) in Maize with the help of pachytene changes. 

Chromosome alterations involving the change in the number of genes are of two types : deficiency or deletion (loss of genetic material) and (ii) duplication (gain of genetic material). Changes involving the sequence of genes are also of two types : (a) inversion (rotation of chromosome segment) and (b) translocation (transfer of chromosome segment). Deletions and inversions are intrachromosomal alterations/aberrations while duplications and translocations are interchromosomal alterations/aberrations.

Characteristics

1. Chromosome alterations/aberrations involve breaking of chromosome segments.

 2. The broken segments may unite with the same or different chromosomes. They may also be lost.

3. Chromosome alterations/aberrations are of two types, spontaneous and induced. Induced alterations/aberrations are chromosomal modifications which are produced artificially by providing cells with high energy radiations, lasers, temperature extremes and other radiomimectic agents like chemicals and pollutants. Spontaneous aberrations are natural restructuring of the chromosomes. The causal mechanism is not definitely known. They may be due to cosmic radiations, changes in temperature and hydration, nutritional deficiencies or imbalances, or irregularity in replication and separation of chromosomes.

4. The rate of spontaneous chromosome alterations/aberrations is higher in older and organisms as compared to the younger ones. This can be observed by sowing old seeds. The roots of their seedlings would have a greater likelihood of chromosome aberrations. 

5. Changes in temperature, oxygen availability, ions and metabolic state of cells influence the rate of chromosome alterations/aberrations. 

6. Chromosome alterations/aberrations or mutations occur in less than 1% of the cells of a given tissue.

7. Chromosome alterations/aberrations can be induced with the help of ionising radiations and chemical agents.

8. Broken ends of chromosomes are sticky and have a tendency to get attached with other chromosomes or chromosome segments having such sticky ends. 

9. A broken acentric chromosome segment must get attached to some chromosome or it gets lost after some divisions due to nonattachment to spindle apparatus. Parts having centromeres may persist as independent chromosomes.

10. Chromosomal alterations/aberrations or may be homozygous (similar in both the homologous chromosomes) or heterozygous (present in one chromosome of a homozygous pair or two different types of aberrations in the two chromosomes). Heterozygous alterations/aberrations are more common. 

11. There is irregularity of synapsis in the region of chromosome aberration/alteration. The same may appear in the form of loop, unpaired regions cross, circle , chain etc.

12. Occasionally altered chromosomes develop pole to pole bridge.

13. Unlike gene mutations, chromosome aberrations/alterations are usually irreversible.

Breakage-Fusion Bridge Cycle

All types of chromosome aberrations/alterations are caused by one or more breaks in the chromosomes or their chromatids. The broken ends are sticky and tend to get attached to similar sticky ends. If the broken parts rejoin in the original position, the gene sequence and number are not disturbed. The process is called restitution union. The union may, however, occur with a different end of the broken segment or some other segment. The phenomenon is known as nonrestitution union. Loss of chromosome segment or its nonrestitution brings about chromosome aberration/alteration. 

Mc Clintock (1941) studied the behaviour of sticky ends of chromosomes in the gametophyte and endosperm of Maize. She observed that when a chromosome with a broken end does not get a suitable companion, its two sister chromatids may fuse at the point of breakage. It gives rise to a dicentric chromatid. The dicentric chromatid becomes stretched between the two poles of the spindle like a bridge because its two centromeres find attachment to the two different poles. The bridge ultimately breaks but often at a point different from the fusion region. This produces two chromosomes, one with duplication (e.g., a b c dd) and the other with deficiency (abc...). Both the chromosomes may again show fusion of their chromatids, bridge formation and breakage. This repetitive breakage, fusion of chromatids and bridge formation is known as breakage-fusion bridge cycle.

(1) DEFICIENCY OR DELETION

Loss of a chromosome segment is known as deficiency or deletion. The lost segment may be terminal (deficiency) or interstitial (deletion). Accordingly, deficiency or deletion may be terminal or intercalary. Terminal deficiency is produced by a single break near the end of chromosome. It is, however, quite rare. Interstitial or intercalary deletion is produced by a double break in the chromosome, loss of the interstitial segment followed by union in the regions of the two breaks (Fig. 4.1). The separated fragment is usually acentric, that is, without a centromere. It is, therefore, unstable and unable to show normal division and distribution to the two spindle poles. The fragment may fuse with some other chromosome or persist in one of the daughter cells in its nucleus or cytoplasm to be ultimately eliminated. Chromosome fragments persisting in the daughter cells can often be recognised as more or less rounded bodies in the cytoplasm of stained preparations. Such bodies are called "micronuclei". Fragment having a centromere may persist in the daughter cells as a small the chechromosome.

Loss of segment generally occurs in a single chromosome. The phenomenon is called heterozygous deficiency. If it takes place in both the homologous chromosomes at the same point, the process is called homozygous deficiency or deletion. Homozygous deficiency cannot be detected cytologically except in case of a larger loss which causes appreciable shortening of the size, loss of band or interband area in a chromosome. On the other hand, heterozygous deficiency is easier to detect cytologically during meiotic prophase. In case of terminal deficiency, the normal chromosome will appear longer than the other homologue in which deficiency has occurred. Intercalary or interstitial heterozygous is recognisable by the appearance of a loop in the normal chromosome opposite the area of deletion (Fig. 4.2). It is because the identical parts of the two homologous chromosomes come to lie exactly opposite during synapsis.

The lost chromosomal segment may have several genes, a single gene or a part of the single gene. Homozygous deficiency due to the loss of a few genes is usually lethal or sublethal to the organism or tissue because of genetic imbalance and loss of some vital gene. Deficiency of a single gene may be lethal, sublethal or without a harmful effect depending upon the physiological or morphological effect of the missing gene. In Drosophila a sex-linked 'notch wing' deletion is lethal in homozygous state in female and hemizygous state in the male.

Heterozygous deletion or deficiency may or may not show its effect immediately. Notchwing X-chromosome heterozygous deletion of Drosophila female produces small notches in the tips of wings. It acts like a 'dominant mutation'. A second deletion in this chromosome has no effect on the fly in heterozygous state but brings about yellow body colour in the homozygous state (in females and hemizygous state in males). It acts like a 'recessive mutation'. Another effect connected with heterozygous deletion is pseudodominance. It is the phenomenon of phenotypic expression of a recessive gene due to the loss of dominant gene in heterozygous deletion. An organism heterozygous for a pair of alleles A and a, shows the effect of dominant gene A. However, when portion of chromosome having the dominant gene A is lost, the recessive allele a presenton the other chromosome becomespl phenotypically expressed. If deficiency or deletion is limited to certain cells, a mosaic of two effects can be observed, variegated aleurone in Maize.

Gates (1921) crossed a waltzing mouse (v v, moves erratically until exhausted) with a normal mouse (VV). The progeny is normal because of the heterozygous condition in the offspring (Vv). However, in one cross, six mice were normal (Vv) while the seventh was found to be waltzing. Apparently, a deletion removed the dominant gene and allowed the recessive gene to express itself (-v).

Importance

1. Homozygous deficiency or deletion is usually lethal as some genes are completely lost from genotype.

2. Gametes bearing deficient chromosome may become functionless or sterile if some vital gene is lost. 

3. Genetic imbalance creeps in case of heterozygous deficiency or deletion. Consequently deformities malfunctions may appear, e.g., notch wing margin in Drosophila (Bridges, 1917; Mohr, 1923). 

4. In human beings deficiencies or deletions are known to produce a number of malfunctions, e.g., chromosomes 18 (enlarged ears fingers), 5 (cri-du-chat or cat cry with small head and low mental ability), 4 (similar to cri-du-chat but without cat cry).

5. It may cause the phenotypic expression of even recessive genes when a portion having the dominant genes is deleted (pseudodominance), e.g., waltzing mouse (Gates, 1921).

6. Deficiencies or deletions, especially the overlapping ones, have been employed in locating the position of different genes on the chromosomes for constructing and verifying linkage or chromosome maps ( = cytological maps). For example, in the laboratory some Drosophila flies developed achaete and scute characters while the rest of the genetic stock did not have the same. Examination of paired X-chromosomes showed deficiency of the terminal end in one. In another lot only the achaete character appeared. Examination of the X- chromosomes show a loss of a smaller terminal segment in one. Clearly the genes for achaete and scute traits are present in terminal and subterminal portions close together.

(II) DUPLICATION (Addition)

It is the phenomenon of having a similar extra chromosome segment in addition to the normal chromosome complement so that one or more genes are present in more than the  normal dose ( of two for diploid organisms). The extra chromosome segment may get attached to a homologous or nonhomologous chromosome. In case the extra segment has a centromere, it may behave like an independent chromosome and become part of the chromosome complement of the cell. In all the cases the cell comes to have some genes in more than the two or normal dose. Duplication can be of five types- extrachromosomal, tandem, reverse tandem, displaced and transposed (Fig. 4.3).

1. Extrachromosomal Duplication. In the presence of a centromere, the extra chromosome segment or duplicating part may behave as an independent chromosome. Extrachromosomal duplication differs from trisomic condition (having three chromosomes of one type) in that the new chromosome is much smaller than the normal chromosome and represents only a part of it.

2. Tandem Duplication (Repeat). Here the duplicating segment is incorported next to me normal corresponding section of the chromosome so that a block of genes is present twice carrying the same genes in the same sequence. Let the sequence of genes in a chromosome be ABC.DEFG (dot (.) representing the position of centromere). If the section containing the =nes DE is duplicated, the sequence of genes in tandem duplication will be ABC.DE DE FG.

3. Reverse Tandem Duplication. The duplicating segment is incorporated next to the normal corresponding section of the chromosome but the order of genes is just the reverse. Reverse tandem duplication of DE section of the chromosome ABC.DEFG would be ABC.DEEDFG.

4. Displaced Duplication. The duplicating segment gets attached away from the normal section either on the same arm (homobrachial) or other arm (heterobrachial) of the same chromosome. DE displaced duplication on the chromosome ABC.DEFGH can be 

Homobrachial Displaced Duplication - ABC.DEFDEG

Heterobrachial Displaced Duplication - ADE BC.DEFG

5. Transposed Duplication. The duplicating segment gets attached to a nonhomologous chromosome in intercalary or terminal position. Say two nonhomologous chromosomes are ABC.DEFG and NOPQ. RST. Segment DE is duplicated and transposed to the other chromosome

forming N DE OPQ.RST (intercalary) or NOPQ.RSTDE (terminal).

Duplications are generally caused by unequal crossing over in which breaks occur at different levels in the two chromosomes. When one chromosome develops duplication, the other comes to have deficiency (Fig. 4.4). Duplication may be present in only one of two homologous chromosomes when it is called heterozygous duplication. If it is found in both the homologous chromosomes, it is known as homozygous duplication. In homozygous duplication, the chromosomes show normal synapsis. A loop or buckle shall appear in the region of interstitial duplication when synapsis occurs between duplicated and normal chromosomes. In case of terminal duplication, the restructured chromosome appears longer than the normal one during synapsis. Synaptic irregularities appear in case of transposed duplication.

Importance

1. Duplication increases the number of genes in a genotype.

2. It has been observed that more evolved organisms have a large number of repeats or duplications of chromosome segments. Even in Drosophila, salivary gland chromosomes show a large number of repeats in different chromosome segments.

3. It is believed by certain workers that different gene pairs which affect the same character (e.g., multiple factors, complementary genes) developed initially as duplications.

4. Duplications have a high degree of survival because they are seldom lethal.

5. Duplications are useful in evolution because they add new genetic material. The added genetic material mutates to form new genes without disturbing existing adaptability.

6. It increases genetic redundancy. The latter protects the organism against harmful recessive genes and lethal deletions.

7. Duplication of a gene may sometimes produce abnormalities or deleterious effect on one or more characters, e.g., hairy wing, eyeless dominant, thickened veins, bar character of eye in Drosophila. The gene for eye shape and development, B, is located on the X-chromosome of Drosophila. It occupies a five-band segment called 16A (Bridges, 1936). Presence of a single B gene on each of the X-chromosome in female produces the normal eye with 780facets. Occurrence of an extra B gene on X-chromosome (due to duplication) forms a bar eyewhich is smaller and narrower than the normal. It has only 325-358 facets. A homozygous duplication further reduces the size of bar eye with about 68 facets. The bar-effect increases with the increase in the dose of B-genes. An extermely narrow eye called ultrabar or double bar appears if out of four, three B-genes occur on one chromosome and one on its homologue. It is due to position effect. The number of facets in ultrabar eye is 45. This is further reduced in case of homozygous ultrabar. Here the number of eye facets is only 23.

8. Duplications provided the first conclusive proof of position effect of genes. Position effect is the altered phenotypic expression when genetic material is relocated without altering its quantity. Zelang studied the inheritance of homozygous bar eye (narrow reduced eye due to presence of homozygous duplication of B gene for eye BB/BB) in Drosophila. Zelang found that in one out of every 1600 offspring flies, wild type normal eye was restored (B/B) while a very small bar eye, called double bar eye, appeared in the progeny in equal frequency (Fig. 4.5). Study of salivary gland chromosomes indicated that double bar condition is heterozygous with one chromosome having B-gene in triplicate and its homologue possessing a single B-gene. Both bar eye and double bar eye have same gene complement (4 B genes). The double bar eye has been formed due to position effect through the shifting of one B-gene from one chromosome to its homologue (Fig. 4.6).