In this essay we will learn about Mutation. After reading this essay you will learn about: 1. Meaning of Mutation 2. Gene Mutation 3. Chromosomal Aberrations.
Contents:
- Essay on the Meaning of Mutation
- Essay on Gene Mutation
- Essay on Chromosomal Aberrations
Essay # 1. Meaning of Mutation:
Mutation of a plant or an animal means a sudden change in its hereditary make-up. Suddenly a mutated organism arises and the changed or mutated appearance is usually found to be hereditary, i.e., it breeds true. Since heredity is controlled by genes, it follows that the genes somehow change their behaviour.
While mutations are always taking place is nature very few of them are of a major nature to be readily apparent. In many cases, the changes are so minor that they are not readily apparent while in most cases they are recessive and remain latent.
The de Vriesian concept of mutation has changed now as many changes which were considered as fluctuating variations by Darwin and de Vries have been found to be true mutations of a minor nature.
The actual change in the genes may be of two types:
(I) Intragenic or
(II) Intergenic.
I. Intragenic change means something has happened within the gene itself or the gene has mutated, i.e., it has undergone some chemical or structural change. Since the gene itself has mutated this may be called gene mutation which is sometimes considered as the true type of mutation. Gene mutation is also called point mutation since the mutation has taken place at a point or locus on the chromosome.
II. Intergenic changes involve merely the rearrangement of the genes already existing or the loss of some of them. So, this type of mutation does not involve the formation of mutated genes.
They are chromosomal mutations or chromosomal aberrations which are caused either (A) by a change in the number of chromosomes which may involve either individual chromosomes (aneuploidy) or whole sets of genomes (euploidy), or (B) by a change in the structure of chromosolies.
Essay # 2. Gene Mutation:
As we come across a really mutated gene only in this type of mutation, sometimes the word mutation is applied to gene mutation or point mutation alone. In nature, genes are constantly duplicating themselves. Every cell division means the formation of new genes—daughter genes from mother genes. In this duplication the original gene is generating an exact replica of itself.
But it is possible that while this process is going on for millions of times, once a while something may go wrong and the new gene may not be an exact replica of the original.
The gene, formed of nucleoproteins, must have a complex molecular structure and there may be a slight change somewhere. This change, however small it may be, gives rise to a mutated gene whose behaviour is different from its predecessors in some degree. Such spontaneous gene mutations are always occurring in nature.
Among some 10 million specimens of Drosophila melanogaster examined, a few hundreds of gene mutants were observed. These mutants give rise to true-breeding stable genotypes varying in some characters (eye colour, wing type, etc.) from the original wild type. Mutations like these are always observed in all types of organisms from human beings to bacteria. They arise all on a sudden.
Most mutations bring about such a minute change that they evade common observation. Moreover, most mutations are recessive as well as lethal. They produce no effect before appearing in a homozygous form in some future generation. They are, therefore, undetectable. However, there are some special cytogenetical methods (e.g., the CIR method) for detecting lethal mutations in X chromosomes of Drosophila.
CIB Method:
In this method of detecting lethal mutations in the X chromosomes of Drosophila the irradiated male fly is first crossed with a female of a special stock called CIB.
In this, of the two X’s of the female, one is normal (shown by the thick bar in the figure) while the other contains C (an inverted segment of chromosome), 1 (a lethal gene which cannot act being dominated by the normal X homologue which has no lethal) and B (bar-eye character) one fourth of the offspring would be female with one treated X (shown by a thick bar in the figure) and one CIB X, one fourth would be male with one CIB X which would die and half would be normal.
Only the female from the first quarter is crossed with a normal male in a second cross. Among the off-springs all males would die if a lethal gene has been induced. If no lethal gene has been induced there would be 2: 1 female to male (Fig. 864).
Mutations may affect every imaginable character in every possible way. Most mutations are harmful, possibly because they disturb the genie balance.
The rate of mutation of genes has been studied. This is different for different genes. While some genes are highly stable and do not mutate easily, there are others which are remarkably unstable.
Among such unstable genes mention may be made of the gene c in chromosome IX of maize causing a colourless aleurone in the grain which was found by Jones to easily mutate into the dominant C developing colour so that such mutations in the somatic aleurone cells give rise to spots on the grain.
Similarly the rose-a and lavender-a genes in Delphinium (Larkspur) were found by Demerec to mutate into purple giving rise to purple patches on the otherwise rose or lavender petals. This mutation may also take place on the germ cell giving rise to a fully purple-flowered mutation.
The stability of genes seem to be different in different species. In Drosophila the possibility of mutation of a gene has been calculated as 1 in 105, in higher organisms 1 in 106. In maize the rate of mutation of the R gene is of the order 492 in 1.5 X 105 while the same for the I gene is 106, for the S gene 1 and for the Wx gene none. Haldane had calculated the rate of mutation in human beings as 1 to 5 in 106.
If this be the rate of mutation of a single gene, the possibility of a mutation in an individual is great considering that there are thousands of genes in a cell. It has been suggested by Muller that there is some mutation in one out of every 20 gametes in Drosophila and in human beings there is a possibility of having one mutated gamete out of every 10.
The rate of mutation has been found to be affected by environmental and other factors viz., it increases with temperature and with age. The effect of X-rays and other ionising radiations is discussed later. The rate of mutation may even be affected by other genes in the same cell.
Rhoades has shown that the Dt gene in chromosome IX of maize causes the gene in chromosome III to mutate into A1 so that colourless plants show purple streaks on leaf and purple dots in aleurone.
Mutations may take place in somatic cells as well as in germinal cells. While the latter gives rise to mutate seeds and mutated progenies, the former, as seen above, gives rise to patches of tissues with mutated cells. Some such tissue may develop buds and shoots and bud sports or bud mutations arise in this way.
When vegetatively propagated, these buds may give rise to clones of mutated plants. Many agricultural and horticultural varieties have arisen in this way. Washington Navel orange plants are known to give rise to single mutated fruits as a result of somatic mutation just before the development of the flower bud.
Although most of the mutations are useless and even harmful, sometimes a mutant may be of some biological value and they certainly play a great part in the evolution of new species. Many new breeds of domesticated animals and strains of cultivated crops must have arisen by gene mutation.
Different breeds of sheep (e.g., the short-footed Ancon sheep), swine, rats, horses, dogs, pigeons, etc., are known to have arisen in this way.
The Shirley poppy, the dwarf ‘Cupid‘ sweet pea and many varieties of showy flowers also arose by gene mutation. The cultivated Cicer gigas arose as a mutation of the common gram, Cicer arietinum (Fig. 865). Among cereals, an ageotropic mutant called ‘lazy’ (Fig. 866) is very common.
Different types of lethals, part lethals, chlorophyll deficiencies and albinos (human as well as in many animals like elephants, tigers, snakes etc.) are common and well-known instances of gene mutation.
Induction of Gene Mutation:
Since environmental factors are known to affect the rate of gene mutation. Naturally, they have been employed for the artificial induction of mutations.
Although attempts at induction of such mutation had been made before and Morgan had obtained some Drosophila wing mutants by radium treatment, the first concentrated work was by Muller who published in 1927 his work on X-ray induced mutations in Drosophila, a work for which he was awarded a Nobel Prize in 1946. This was followed by Stadler’s work on barley and then spread to other fields.
The most important mutagenic agents have been found to be ionising radiations, viz., α- (projected nuclei of helium atoms), β- and y- radiations of radioactive substances; X-rays; neutrons (projected electrically neutral particles, mainly from hydrogen nuclei) and protons.
All these act by ionising the atoms of the matter through which they pass so that they eject electrons. The ejected electrons, in their passage through matter, cause further ionisation losing energy with each collision until they finally halt. The actual cause of mutation is still a matter of speculation. One theory is that mutation is caused by the direct ‘hit‘ of the ‘target‘ by electrons.
The other view holds the cause of mutation as indirect being resultant of the chemical and physical changes brought about in the molecules surrounding the genes. The a-, β -, y-and X-rays are measured in roentgen (r) units which are products of the strength of the rays and the duration through which they work.
It has been found that the rate of mutation is directly proportional to the amount of total radiation received (in r-units) by the cells and not to the strength or the duration (Fig. 867) alone and not even to the type of radiation employed.
Induction of mutation by radiation has been tried on all types of organisms. The resultant mutations are sometimes visible, sometimes latent, being apparent only in future generations as they mainly occur as recessive genes.
In Drosophila 80% of the mutated genes have been found to be of the lethal type. The same may be true for other organisms. Mutagenic treatments may ensure one of getting mutants but the type of resultant mutations cannot be predicted.
Induction of mutation has been tried on plant viruses and on bacteria with variable degrees of success. Among fungi, mutations have been induced in Neurospora.
There are numerous instances in higher plants—barley (Stadler and Gustafsson found hundreds of chlorophyll deficiencies and a few other types like the erectoid form), maize, tobacco, Antirrhinum, Oenothera, Datura, etc., and they have all been found to be equally responsive.
However, most spectacular results have been obtained in Drosophila.
Besides the, numerous recessive lethal genes (the X chromosome shows 1.5 lethal mutations in every thousand chromosomes per generation and this rate increases 70 times when the flies are subjected to a dosage of 4800 r-units) which are detectable only by adapting special cytogenetic techniques (c.f. CIB method), a large number of visible mutants have been obtained.
The most interesting of these are the eye colour mutants. White eye (w+) mutates to eosin (we) and apricot (wa) while coral (wco), buff (wbf), cherry (wch), apricot (wa), eosin (we) and tinge (wt) have been found to mutate to white. So the recessive white may also mutate to its dominants like eosin. These mutants are similar to the spontaneous mutations at the same locus.
Besides ionising radiations, non-ionising ultraviolet rays, chemical mutagens like those of the mustard gas group, and temperature have been found to be effective in inducing mutations but not to the same extent as the first.
Mustard gas (tried on Drosophila, Neurospora and bacteria) has sometimes been found to be as effective as X-rays. Mutation rate generally increases with temperature but the rate of increase is different in different cases and there are some genes (specially the unstable ones) in which the mutation rate is lower in higher temperatures.
Multiple Allelomorphs:
Since the same gene may mutate again and again, and all such genes will remain at the same locus of the same homologous chromosomes, it is apparent that there may be a series of genes-occupying the same position. Then they will form a series of multiple allelomorphs, any two of which may act as an ordinary allelomorphic pair.
The presence of multiple allelomorphs helps to understand the nature of alleles and disproves the presence and absence hypothesis. Common examples of multiple allelomorphs are found in the anthocyanin genes of cotton, lice, maize, jute and in the eye colour genes of Drosophila.
The eye colour of Drosophila is controlled by an allelomorphic series of at least twelve genes—Red wild type (W or +), white (w+), ivory (wi), pearl wP), tinged (wt), buff (wbf), honey (wh), apricot (wa), cherry (wch), eosin (we), blood (wb1) and coral (wco). In cotton, the cultivated Old World cottons Gossypium arboreum and G. herbaceum show an allelic series of some 20 genes at the R2 locus causing pigmentation of different plant organs and petals.
The New World cottons G. hirsutum and G. barbadense contain another series of alleles also controlling anthocyanin colouration at the R1 locus in addition to R2. This proves the hypothesis that the New World cotton originated after hybridisation with the Old World cotton. A third similar allelic series of genes are known at the R3 locus in G. anomalum.
In maize, the allelic series P in chromosome I, A1 in chromosome III and R in chromosome X control anthocyanin colouration. In rice, at least two different allelic series C and Sp, located in two different chromosomes, seem to control anthocyanin colouration. In Jute, 5 alleles at the A locus, also controlling anthocyanin colouration, have become known.
Essay # 3. Chromosomal Aberrations:
These result from chromosomal aberration disturbing the normal number or structure of the chromosomes. Thus, they may also be called chromosomal mutations. In these, the gene disturbance is intergenic not involving their internal structures.
According as the aberration involves the number or the structure of the chromosomes, these may be of two types:
A. Changes Involving Change in the Number of Chromosomes:
The number of chromosomes for every species of organism is fixed. In higher plants and animals the ordinary cells contain In number of chromosomes (i.e., two sets of the same genome) and the organisms are, therefore, called diploid. It is found that this fixed number may change causing a mutation into a new type. If such a change be sufficiently great and hereditary, then the mutant forms a new species.
Change in the number of chromosomes sometimes involves multiplication of the whole chromosome sets (genomes), i.e., the number, instead of being 2n, becomes 3n, 4n, 6n, etc. These are generally called Polyploids or true polyploids (Euploids).
Haploids (n) caused by a decrease in genomes, is usually considered along with the Euploids. In other cases, the change involves increase or decrease in the number of single chromosomes and not of whole sets. Thus numbers like 2n-1 or 2n + 1 may be obtained. These are irregular polyploids or Heteroploids or Aneuploids.
Besides the above, some changes in the chromosome number may be caused by fusion of chromosomes, by breaking down of chromosomes or by formation of chromo- some-like bodies by inert material which may not contain any gene (supernumerary chromosomes, e.g., B-chromosomes of maize and m-chromosomes of mosses).
(a) Euploids or True Polyploids:
These again are of two types. Some involve the multiplication of exactly the same genome (Allopolyploid) while in others different types of genomes are involved (Allopolyploid). These polyploids have played an important role in the evolution of new species.
Many cultivated crops have developed from wild plants by means of allopolyploidy. Sometimes a polyploid may be sterile but it may still be a very important agricultural or horticultural crop by vegetative propagation. Among animals, polyploids are rare.
Allopolyploids:
Cytological investigations have shown the presence of polyploid series among some cultivated plants Thus, Fig. 868 shows the chromosomes of diploid (2n), triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid (6n) and octoploid (8n) rose. Fig. 869 shows the appearance of haploid (n), diploid, triploid and tetraploid Datura flowers.
Haploid (n):
Although not really polyploids, haploids are considered in connection with them because they are members of the same polyploid series. Haploids usually occur as a result of parthenogenesis—whether spontaneous or induced.
They have been found in rice containing double embryos of which one is normal and the other is parthenogenetic. The stimulus of parthenogenesis may come from ineffective pollination when foreign pollen or X-rayed pollen is used in pollination.
Haploid plants have been found in a number of species like Datura stramonium, Oryzasativa, Oenotherasp., Triticum sp., Zea mays, etc. Haploid plants are all-round weak and small but there is a report of a haploid plant in pepper as healthy as a normal diploid. As there is only one set of chromosomes, there should be no pairing during meiosis so that normal pollens or eggs are not formed.
A normal gamete is possible only if by chance all the chromosomes remain at the equator and form a single nucleus during irregular meiosis. Haploids are, therefore, usually sterile. In some cases, however, bivalents are noticed during meiosis. This indicates that in the original genome duplication of chromosomes was already present and the basic number of the species is actually less than the haploid number.
This throws a light on the origin of the species. Haploids may be utilised in cytogenetics in another way. By applying colchicine the chromosome set in the haploid may be doubled and normal diploid may be obtained. The diploid will be perfectly homozygous—a condition rarely obtained in nature. Such a diploid plant is of great use in cytogenetics and plant breeding.
In the animal world, the males of bees, wasps and other Hymenoptera are normally haploid. They have adjusted their lives to this nature. There is no reduction during meiosis. There is no synapsis and either during the first division all the chromosomes normally pass to one pole or the first division is completely eliminated. The meiosis, therefore, is equational and normal sperms are formed.
Triploid (3n):
Autotriploids have been observed or obtained like the haploids in a number of plants like Oenothera, Datura, rose, rice and many others. Drosophila triploids are well known. Triploids are usually formed as a result of the fusion of a diploid gamete (formed abnormally) with a normal haploid one. Diploid pollens or eggs may come from tetraploid plants or sometimes occur abnormally under the influence of X-rays, etc.
As in the case of haploids, the three sets of genomes are unable to pair during meiosis so that functional gametes are rarely formed. As a result, triploid plants are usually sterile like the haploids. But, triploids may be useful in horticulture as, when propagated vegetatively, they have been found to develop seedless fruits in many cases. Triploid plants may or may not be bigger than normal diploids.
Tetraploids (4n):
Tetraploids are the most important of the polyploids. They are found widely in nature and may also be induced artificially. Numerous cultivated varieties have autotetraploid varieties along with normal diploids. Since the four sets of chromosomes may easily pair between themselves normal diploid gametes are formed which form normal tetraploid seeds.
During diakinesis one often sees tetravalents of chromosomes instead of the bivalents seen in normal diploids. Nevertheless, varying degrees of sterility are observed among tetraploids. Tetraploid plants are usually larger, more succulent and have bigger pollens, stomata, fruits, etc.
The succulence may render the tetraploid more susceptible to disease. Their relative sterility often precludes their use as commercial crops but the large size of fruits etc. may be useful in horticulture. Tetraploids sometimes arise as somatic bud mutations. They have been obtained by the decapitation of tomato plants when the new shoots become tetraploid.
High temperature induces tetraploidy. But, the most popular method is treatment with chemicals, the most important of which is colchicine which has become extremely popular since its practical application by Blakeslee and by Nebel simultaneously in 1937.
Since then it has been applied to thousands of plants. Its application in liquid form or as jelly to seeds and growing tips is not very difficult although the results are not always certain. In the simplest cases, seeds soaked in dilute solutions germinate into tetraploid plants.
Colchicine prevents the: separation of daughter chromosomes after they have divided by arresting the formation of the spindle and forms nuclei with double the number of chromosomes (4n) during mitosis. As a result, a 4n tissue arises forming a diploid shoot.
Higher-ploids:
Pentaploids (5n), Hexaploids (6n), Octoploids (8n), etc., have also been found among wild and cultivated plants (e.g., rose in Fig. 868). All these show how the formation of new species is facilitated by polyploidy.
Allopolyploids:
While the autopolyploids are formed by the multiplication of the same set of chromosomes (genome) all the allopolyploids are formed by the union of different genomes from different plants.
A very interesting example of an allopolyploid obtained artificially is the Raphano- brassica (Fig. 870) obtained by Karpechenko (1927). He crossed Raphanus sativus (radish, 2n = 18) with Brassica oleracea (cabbage, 2n = 18).
The hybrid was intermediate between radish and cabbage and had 9 +9 (9R of Raphanus and 9B of Brassica) chromosomes which did not pair during meiosis as they were from different plants. The hybrid was sterile. Accidentally, polyploidy occurred.
The 9R+9B chromosomed hybrid doubled its chromosomes (18R + 18B). Now, the chromosomes could pair between themselves and perfect seeds were formed. This pairing of chromosomes is autosyndesis as actually, chromosomes of similar genomes (R or B) are pairing with their respective homologues.
The 36 chromosomed final plant is a tetraploid but it is an allotetraploid as the 36 chromosomes are coming from two different genomes. This sudden doubling of chromosomes which formed the allotetraploid is called amphidiploidy.
Subsequently, it has been found that such amphidiploidy may be induced artificially by colchicine. As a result, some hybrids which were sterile have been transformed into fertile allotetraploids by the induction of amphidiploidy.
An examination of the cultivated plants shows that allopolyploidy has played a very important role in the evolution of these species. Cytogenetical analysis of cultivated wheat plants has yielded very valuable results. Investigations by Kihara and others show that the wheat species fall under three groups—diploid, allotetraploid and allohexaploid. These were formed from three different genomes.
The genome C is present in the grass Aegilops squarrosa (2n = 14) which grows wild in the region from Armenia to Afghanistan. Thus, in the evolution of the wheat species, one may suppose that Triticum monococcum (genome A) hybridised with another plant having genome B and also having 2n =14. There was amphidiploidy.
T. durum (2A + 2B, 2n = 28) resulted. Again, a member of the durum group hybridised with Aegilops squarrosa (genome C, 2n = 14) and an amphidiploidy gave T. aestivum (=vulgare) (2A + 2B + 2C, 2n = 42).
Actually two such hybridisations have been successfully done. Mcfaden and Sears obtained a wheat-like Triticum spelta (a member of the 42 chrvulgare group) by inducing amphidiploidy on a T. dicoccoides (a wild member of emmer group from the region Armenia to Palestine) X Aegilops speltoides hybrid.
Kihara has actually obtained a bread wheat similar to the cultivated T. aestivum (=vulgare) var. trythospermum hyamphidiploidising a T: persicum (a cultivated emmer wheat) x Aegilops squarrosa hybrid. It has now been established that the B genome has been derived from Aegilops speltoides. It is reasonable to suppose that the bread wheat originated in some place between Armenia, Persia and Afghanistan.
If the genome constitution of a natural species shows that it is an amphidiploid or allopolyploid, it may be possible to synthesise the species artificially. A very interesting example is that of Brassica juncea. Among cultivated mustards there are three species —Brassica campestris (rape, 2n=20), Brassica nigra (black mustard, 2n = 16) and Brassica juncea (rai, 2n=36).
Genome analysis showed that the third species was an amphidiploid of the first two. Subsequently, a plant has been obtained synthetically by inducing amphidiploidy in the sterile Brassica campestris X Brassica nigra hybrid (Fig. 872). This synthetic plant differs only slightly from natural Brassica juncea.
Segmental allopolyploids:
In the allopolyploids, it has been presumed that the chromosomes of the two parents have no affinities between themselves and so, there is absolutely no pairing between themselves. But, Stebbins is of opinion that in nature, in most allopolyploids there is some affinity between the chromosomes of the two different genomes resulting in allosyndesis for some chromosomes.
During meiosis of these allopolyploids some tetravalents as well as some bivalents are seen and this results in some sterility. These have been named segmental allopolyploids and examples are found in Primula kewtruis (derived from P. floribunda x P. verticillata), Tradescantia canaliculata-himulis, Delphinum gyposophilum (derived from D. hesperium x D. recurvatum), etc.
(b) Aneuploids or Heteroploids or Irregular Polyploids:
In the aneuploids, one is concerned not with the multiplication of whole sets of chromosomes but with the increase or decrease of the number of homologues. Ordinarily, two homologues of each chromosome are present in the diploid so that the chromosome number is 2n.
But, in exceptional cases there may be disturbances in the division of chromosomes so that the number of homologues instead of being two is changed to three (trisomic), four (tetrasomic) one (monosomic) or none (nullisomic).
Such disturbances may be spontaneous due to crossing between polyploids and heteroploids, different types of non-disjunction (aberration), incompatibility due to hybridisation between distant plants, etc., or may be induced by X-rays, etc. Aneuploids are usually sterile.
When aneuploids show an increase in chromosome number (e.g., 2n + 1) it may be called a hyperploid. When there is a decrease (e.g., 2n—1) it is a hypoploid.
Trisomies (2n + 1):
These are formed by the addition of an extra homologue of one chromosome. Many of the Oenothera ‘mutations’ found by de Vries are trisomics. Trisomies are widespread in nature and have been extensively studied in Datura and also in maize, tomato, wheat, Nicotiana and Drosophila. They are readily obtained by selfing a triploid or by crossing diploids with triploids.
Blakeslee and his students studied the Datura trisomies very closely and obtained 12 trisomies (Fig. 873) for the triplication of every one of the 12 chromosomes in Datura stramonium.
As can be seen from the figure, every one of the trisomies is distinct from the other. Besides these normal ones they also obtained other trisomies in which the three chromosomes were exactly alike, there being some segmental interchanges or translocation. In a secondary trisomic the translocations are from the same chromosome.
Thus, while three 1.2 chromosomes give the rolled normal trisomic (Fig. 873), two l.2 and one 1.1 gives sugarloaf and two 1 .2 with one 2.2 gives polycarpic.
In a secondary trisomic a closed ring (c.f.., Fig. 877) of the three chromosomes is possible. Ina tertiary trisomic, one of the three chromosomes contains a bit from a different chromosome. Thus, the trisomic hedge has one 1.9 chromosome (produced by translocation between 1.2 and 9.10) in addition to two l.2 chromosomes.
Sears obtained trisomies in bread wheat (Triticum aestivum), but these do not differ much from the normal plants, possibly because the species is an allohexaploid.
A trisomic (47 instead of the 2n=46 chromosomes) mutation in human beings is known to cause the disease ‘Down’s syndrome’ (mongoloid features combined with mental slowness).
Double trisomic (2n + 1 +1) shows three homologues of each of two different chromosomes.
Tetrasomic (2n +2) has four homologues of the same chromosome. Sears obtained these as well in Triticum aestivum but these also do not differ from normal plants.
Monosomic (2n— 1):
There are two homologues of each chromosome excepting one of which there is one only. Apparently, the homologue of this chromosome is somehow lost. Monosomies arise in the same way as the trisomies but they are not usually viable. Datura monosomies are not viable. But, while the monosomic are not viable in true diploids, they have been successfully obtained in some allopolyploids.
In the allopolyploid Nicotiana tabacum (2n=48) Clausen and his colleagues obtained all the 24 possible monosomies and these are different from one another. Similarly Sears obtained all the 21 possible monosomies in Triticum aestivum (=vulgare) but these do not differ much from the normal. Monosomic analysis greatly facilitates assignment of genes to the linkage groups.
Nullisomic (2n—2):
In these plants, both the homologues of a particular chromosome somehow get lost so that the chromosome is completely missing from the plant. Such a plant should clearly show what genes were contained in the missing chromosome.
Nullisomic analysis, combined with monosomic, has greatly helped in determining linkage groups. Nullisomics are obtained by selfing monosomies. They are usually in-j viable like the monosomies. They are, however, known in maize and in Triticum aestivum (=vulgare) Sears obtained 17 of the 21 possible nullisomics.
B. Changes Involving Change in the Structure of Chromosomes:
Some accidents sometimes occur which end in the breaking-down of chromosomes. The broken bits may get healed up or get re-attached in a wrong way or may even get lost. These accidents are not to be confused with the normal crossing-overs.
Such incidents cause structural modifications of chromosomes involving a re-arrangement or loss of genes which may influence heredity by causing aberrations and certainly influence linkage and crossing-over.
In some cases, it has been found that a gene located at one position on a chromosome behaves differently when placed on a different position. This is known as the position effect. A very striking example of this is the ‘bar-eyed’ character of Drosophila.
While structural modifications of chromosome occur in nature it has been possible to get a great number of them by subjecting dividing cells to harsh treatment, chiefly by X-rays and other ionising radiations in the same way as explained for gene mutation. Structural modifications may be of several types (Fig. 874).
Deficiency:
A deficiency has a bit of a chromosome lost altogether. Some genes are, therefore lost. A deficiency may be terminal when it involves the end of a chromosome, or intercalary when it is an intermediate part that is deficient. Intercalary deficiency is also called deletion. Both terminal and intercalary deficiencies are known in maize. They may arise spontaneously or as a result of artificial radiation.
Deletions have been extensively studied in Drosophila where the salivary glands have enabled another method of location of genes.
It has been explained that in a salivary gland chromosome the two homologues remain in a stage of synapsis. If a mutation arises by deletion and if this mutated individual be crossed with a normal, the hybrid will be a heterozygote and in its salivary gland cells there will be the pair of chromosomes of which one homologue is normal and the other deleted.
Close examination of the band of this chromosome shows exactly which bands are missing and, since the two homologues pair band by band, there will be a short curvature at the point of deletion since one homologue is shorter by a few bands there (Fig. 875).
By this observation, it is possible to locate the gene on the particular chromosome and by studying a number of such deleted mutants in the same way it is possible to get a sort of chromosome map as obtained by studying crossover percentages.
It is found that the two types of chromosome maps tally with each other keeping in mind that the crossover values at certain regions of the chromosomes are known not to be exactly proportional to the distance. Deletion causes a disturbance in the genie balance and gametes with deleted chromosomes are often inviable. In Drosophila, deletion of a bit containing more than 50 bands has lethal effect.
Duplication:
A broken bit of a chromosome may remain free in the nucleus as a fragment in addition to two complete homologues. However, no such fragment can survive if it does not contain a centromere.
Thus, some alleles will be represented thrice. The broken bit, instead of remaining free may also remain attached to some other broken chromosome (which may or may not be its homologue) at an intercalary position. It should be remembered that there can be no attachment to the unbroken telomere end.
Translocation:
A broken bit of a chromosome may get attached to some other chromosome. Translocations are usually reciprocal—somewhat resembling crossing-over but very different from the latter as whole chromosomes are involved here. Such reciprocal translocation may involve homologous or non-homologous chromosomes.
Simple translocation of only one bit of a chromosome to another is extremely rare. If that rare event happens, the broken bit may even get re-attached in a different position on the mother chromosome.
Inversion:
A segment of a chromosome gets inverted during reattachment. Thus, a chromosome having the genes abcdef in linear order may get the segment cd inverted.
Then the new arrangement will be abdcef.
Fig. 874 shows that structural modifications cause typical appearances during pachytene pairing as the allelic genes tend to come side by side.
Inversions also are widely found. An inversion not containing a centromere (called a paracentric inversion) and followed by a crossover may cause an anaphasic inversion chromatid bridge and an acentric fragment as explained in Fig. 876.
Translocation is widely present in plants and animals—in Datura, maize, pea, wheat, Tradescantia, Rhoeo, etc. Segmental interchanges between chromosomes (translocation) lead to ring formation (Fig. 877).