Euploidy

Euploidy –Gr eu =even,ploid =unit ,designates genomes containing chromosomes that are multiple of some basic number .The euploids are those organisms which contain balanced set or sets of chromosomes in any number..The number of chromosomes in a basic set is called monoploid number –x . Those euploid types whose number of sets is greater than two are called polyploids.Thus 1 x-monoploid,2x diploid and Polyploid 3x (triploid),4x (tetrap and so on olid),5x (pentaploid)

Monoploidy �

• Monoploids have a single basic set of chromosomes eg 7 in barley and 10 in corn Monoploids are common in plants and rare in Animals
• Origin and production of Monoploids Monoploids in some cases are found normally due to parthenogensis-Male(drones)-bees ,wasps ,ants.In angiosperms monoploids may also originate spontaneously due to parthenogenetic development of egg. –Tomato,Cotton.
• Rarely monoploid plant may originate from the pollen tube ,synergids and antipodals of the embryo sac.

​

• Monoploids can be produced by artificially by following methods (1) X- ray treatments (2)delayed pollinations (3) Temperature shock –Cold treatment (4) Colchicine treatment (5) Distance hybridization (6)anther or pollen culture. Amongst these techniques - Distance hybridization and anther or pollen culture are important
• Distance hybridization – Interspecific crosses in genera of Solanaceae have been used for the production of both parthenogenetic and androgenic monoploids .By this technique monoploids have been obtained in large number in potato. Kasha and Kao have used this technique for producing monoploids in large number in barley. They discovered that when diploid barley,Hordeum vulgare is pollinated using a diploid wild relative Hordeum bulbosum, fertilization occurs ,but during the somatic cell divisions ,the chromosomes of H bulbosa are preferentially eliminated from the zygote , resulting in a haploid embryo( such a haplodization process appears to be caused by a genetic incompatibility between the chromosomes of the different species) The resulting haploids can be doubled with colchicine treatment .This technique has resulted in the rapid production of new varieties of Barley and other plants

• Anther or Pollen culture -This technique was developed by Maheshwari and Guha .In this technique the cell of anther is induced by cold treatment to grow into an embryoid ,a small mass of cells .The embryoid may be grown on agar to form a monoploid plantlet.

​

• Morphology of monoploids- Monoploid plants have reduced size of all vegetative and floral parts .In monoploid Nicotiana Kostoff reported that the leaves ,flowers and overall plant size were smaller. The size of seed and stomata as well as diameter of pollen were found smaller in monoploids than in diploids .Even the size of nucleus of a monoploid often was found to be just half of the nucleus of the diploid cell .

• Use of monoploids –Since there is only one copy of each chromosome and only one allele of each gene ,every gene whether dominant or recessive will express . This facilitates genetic experiments . Monoploid strains of Nicotiana , Datura have been successfully developed ,from which pure breeding stains resistant to insecticides and toxic compounds produced by parasites

Polyploid

Auto polyploids

• The autopolyploid consists of same basic set of chromosomes multiplied .e.g. if a diploid species has two similar sets of chromosomes or genomes (AA)an auto triploid will have three similar and an genomes (AAA) and an autotetraploid will have four such genomes (AAAA)
• Origin and production of autopolyploids –The autopolyploids may occur in nature or may produce artificially.When they are found nature ,their autopolyploidy nature is deduced by their meiotic behaviour.eg Doob grass - Cynadon dactylon. Polyploids arise naturally as a result of (i)As a result of interference with cytokinesis ,once chromosome replication has occurred (ii) It may either occur in somatic tissues which give rise to tetraploid branches or during meiosis which produces unreduced gametes –due to chilling .

• Induced polyploidy –Auto polyploidy has been induced in many plant and animal cells by artificial means such as chemicals –Colchicine ,mercury chloride , sulphanil amide .Radioactive substances –radium and X rays, And Temperature shock .These inducers usually disturb the mitotic or meiotic spindle and cause non segregation of already duplicated chromosomes ,during cell division .
• Aqueous solution of colchicine is found to prevent the formation and organization of spindle fibers so the metaphase chromosomes of the affected cells (Called –C-metaphase) do not move to a metaphase plate and remain scattered in the cytoplasm, even the cytokinesis is prevented by colchicine and with duplication of chromosomes the number goes on increasing. As colchicine interferes with spindle formation ,its effects are limited to dividing and meristematic cells

• Effects of Auto polyploids ---Leaves ,Flowers and fruits of auto polyploids are larger in size than a diploid plant. With increase in cell size ,the water content increases ,which leads to a decrease in osmotic pressure .This results in loss of resistance against frost .Due to slower rate of cell division ,the plants growth rate decreases , which leads to delay in flowering. At higher ploidy level ,such as auto-octoploids ,the adverse effects become highly pronounced and lead to the death of the plant.
• Uses –Certain autotriploids are of outstanding attraction because of their larger flower ,longer period of flowering ,greater uniformity and increased vigor .Autotriploids are highly sterile and rarely produce seeds .This is advantageous in ornamental plants such as roses ,Dahlia and in production of seedless fruits as in apple ,pears oranges etc. Tetraploids are able to adopt even in less favorable habitats .Some autotetraploids are larger and more deeply coloured than their normal diploids .

Allopolyploids

• When the polyploidy results due to doubling of chromosomes number in a F1 hybrid which is derived from two distinctly different species then it is called allopolyploid and such an organism is called a allopolyploid . Raphanobrassica is a classical example of allopolyploidy. Which is a cross between radish and Brassica
• Cotton Gossypium hirsutum or the new world cotton is an interesting example of allopolyploid .Old world cotton Gossypium herbaceum ,has 13 pairs of chromosomes while American cotton also contains 13 chromosomes. J.O Beasly crossed the old world cotton and American cotton and doubled the chromosome number in the F1 hybrids.The allopolyploids thus produced resembled the cultivated New world cotton and when crossed with it gave fertile F1 hybrids .These results suggested that tetraploid Gossypium hirsutum originated from two diploid species –Gossypium herbaceum (2n=26)and G.raimondii(2n=26)

​

​

Gossypium herbaceum x Gossypium raimondi

(Old world cotton) (American cotton)

(2n =26: 13 bivalents) (2n =26: 13 bivalent

​

F1 Hybrid

(2n =26:26 univalents)

​

Colchicine

New world cotton

(Gossypium hirsutum)

(2n=52:26 bivalents)

​

• Triticum spelta is a Hexaploid wheat which was artificially synthesized in 1946 by E.S.McFadden and E.R.Sears and also by H.Kihara.They crossed Triticum dicoccoides (tetraploid:2n=28) with goat grass ,Aegilops squarrosa (diploid 2n=14) and doubled the chromosome number in the F1 hybrid.
• P1 Triticum dicoccoides x Aegilops squarrosa
• (Tetraploid wheat) (Diploid goat grass)
• AABB DD
• (2n =28;14 bivalents) (2n=14;(bivalents)
• ABD
• Triploid hybrid
• (2n=21;21 univalents)
• Colchicine
• AABBDD
• Synthesized hexaploid wheat
• Triticum spelta
• (2n=42;21 bivalents)

• The artificially synthesized hexaploid wheat was found to be similar to the primitive wheat T.spelta.When the synthesized hexaploid wheat was crossed with naturally occuring T.spelta ,the F1hybrid was completely fertile .This suggested that Hexaploid wheat must have originated in the past due to natural hybridization between tetraploid wheat and Goat grass followed by subsequent chromosome doubling

Trisomy in Humans

Downs Syndrome –This syndrome is associated with 21 st chromosome .It was first described by Langdon Down and is popularly known as Mongolian idiocy because the facial features of the person suffering from this syndrome resembles Mongolians.

Downs syndrome is a congenital syndrome originating from the non –disjunction of chromosomes of 21 st pair during meiosis in one of the parents .People suffering from downs syndrome therefore have 47 chromosomes instead of 46.

This genetic disorder is characterized by following features .The Mouth is constantly open and the tongue is protruded.The neck is short and broad. The nose is flat and oblique. The ears are malformed .The epicanthal folds ,brushfield spots around the iris, short fingers and a gap between the first and second toe are main characteristics .

They are mentally retarded .congenital heart disease and leukemia occur in many cases . A male affected is usually sterile and females have a diminished fertility and rarely produce offspring The dermatoglyphic pattern shows a line called Simian crease and distal axial tardius and all ten fingers show ulnar loops .

Patau's syndrome

• Patua and his colleagues described a clinical disorder originating from trisomy of ‘D’ group of Chromosomes (13-15).The d group of chromosomes are all much alike in size and shape .Thus any abnormality resulting from trisomy of any one of these chromosomes is called D syndrome .Most affected persons affected of pataus syndrome are characterized by small head with abnormalities of the face ,eyes and forebrain , cleft lip and palate, low set deformed ears ,small chin and the hands are with extra fingers or toes , deafness. Heart defects are common.

Edwards Syndrome

• Edward and his associates described a syndrome due to trisomy of 18 th chromosome (E group) This syndrome occurs more in females than in males.The victims are characterized by mottled skin with lanugo like hair on the back , extremities and forehead .They are not able to cry loudly. Their mouth and mandibles are small ( micrognathia ).The nails are often hypo plastic .The ears are deformed and lowest small sternum,the pelvis is narrow and the hips are frequently dislocated. Ventricular septal and patent ductus arteriosus are the most usual cardiac abnormalities hence their physical and mental growth is retarded and death usually occurs in early chilhood.

Klienfelter syndrome

• H.F.Klienfelter reported this abnormality and Jacobs and Strong demonstrated these individuals have 47chromosomes i.e Trisomy of (XXY)chromosomes number in their somatic cells .They possess two X-Chromosomes and a Y Chromosome .It is caused by non disjunction of XX chromosomes .The affected individuals appear normal in childhood but the abnormalities become visible only in adult males .This syndrome is characterized by enlargement of breast, small testes and absence of spermatogenesis , amount of Male hormones is low .Genitalia are poorly developed and sterile. They are tall ,show gynecomastia , rounded body ,neuro physiological impairments .