Introduction to Chromosomal Aberration

This presentation is aimed at undergraduate and graduate students and covers details of the structural and numerical aberration of chromosomes. The presentation will help easy understanding of the beginners.

2. BASIC TERMS TO BE
REMEMBERED

Karyotyping: refers to a full set of
chromosomes from an individual,
or a photographic arrangement of a
set of chromosomes of an
individual.

Chromatid: either of the two
daughter strands of replicated
chromosome that are joined by
centromere and separate during cell
division.
© Dr. Riddhi Datta
Karyotype

3. BASIC TERMS TO BE REMEMBERED

Constituent of chromosome: DNA, RNA , HISTONES (and
some non-histone proteins)

No. of chromosomes in human: 22 pairs of autosomes & a
pair of sex chromosomes.

Diploid: 46 (44+XX or 44+XY)
Haploid : 23 (22+X or 22+Y)
© Dr. Riddhi Datta

4. © Dr. Riddhi Datta

5. © Dr. Riddhi Datta

6. ABERRATION
Any departure or deviation from normal.
CHROMOSOMAL ABERRATION

•Any aberration in the shape, size or structure of a chromosome
is called chromosomal aberration.

•It reflects an atypical number or a structure in one or more
chromosomes.
© Dr. Riddhi Datta

7. Numerical
1.Trisomy
2.Monosomy
3.Nullisomy
4.Polyploidy

Structural
1.Deletion
2.Duplication
3.Fragile site
4.Inversions
5.Ring chromosome
6.Translocation
© Dr. Riddhi Datta

8. 1.Deletion: Part of a chromosome segment is lost, maybe small or large portion.


1.Duplication: Section of a chromosome is in duplicate, usually less harmful.
Extra genetic material causes birth defects.



2.Inversion: Two breaks in chromosome & then broken fragment reinserted in
inverted fashion.
STRUCTURAL ABNORMALITIES
© Dr. Riddhi Datta

9. 4.Translocation: Transfer of full or a part of a chromosome to another
chromosome.





5.Fragile site: Constriction at sites other than centromere. There is more
tendency to break. Ex: X linked mental retardation, fragile X syndrome.

6.Ring chromosome: Two breaks in the chromosome and these broken ends
stick back together.
© Dr. Riddhi Datta
STRUCTURAL ABNORMALITIES

10. DELETION
•A missing chromosome
segment is referred to either as
a deletion or as a deficiency.

•Large deletions can be detected
cytologically by studying the
banding patterns in stained
chromosomes, but small ones
cannot.
© Dr. Riddhi Datta

11. DELETION
•In diploid organisms, the deletion of a
chromosome segment makes part of the genome
hypoploid.

•May be associated with a phenotypic effect,
especially if the deletion is large.

•In case of a deletion heterozygote, where the
dominant allele of a gene has been deleted, the
recessive alleles of the deleted gene is generally
expressed. This is called pseudodominance.
© Dr. Riddhi Datta

12. •Example: cri-du-chat syndrome in human
•Caused by a deletion in the short arm of chromosome 5
•Individuals heterozygous for the deletion have the
karyotype 46 del(5)(p14)
•Bands in region 14 of the short arm (p) of one of the
chromosomes 5 is missing.
•These individuals may be severely impaired, mentally as
well as physically
© Dr. Riddhi Datta
DELETION

13. DUPLICATION
•An extra chromosome segment is referred to as a duplication.

•The extra segment can be attached to one of the chromosomes, or it
can exist as a new and separate chromosome, that is, as a ―free
duplication‖.
© Dr. Riddhi Datta

14. DUPLICATION
•The organism becomes hyperploid for part of its genome.
•May be associated with a phenotypic effect.
•Example: Effect of duplications for region 16A of the X chromosome on
the size of the eyes in Drosophila.

© Dr. Riddhi Datta

15. •Tandem Duplications are
adjacent to each other.

•Reverse Tandem Duplications
result in genes arranged in opposite
order of the original.

•Tandem duplication at the end of
chromosome is a Terminal tandem
duplication.



DUPLICATION
© Dr. Riddhi Datta

16. INVERSION
•An inversion occurs when a chromosome
segment is detached, flipped around
180º, and reattached to the rest of the
chromosome.
•The order of the segment’s genes is
reversed.
•Pericentric inversions include the
centromere, whereas paracentric
inversions do not.
© Dr. Riddhi Datta

17. •Pericentric inversion:
•Inverted segment includes the centromere
•May change the relative lengths of the two arms of the
chromosome
•If an acrocentric chromosome acquires a pericentric
inversion, it can be transformed into a metacentric
chromosome and vice versa.
© Dr. Riddhi Datta
INVERSION

18. •Paracentric inversion:
•Inverted segment does not include the centromere
•Does not change the relative lengths of the two arms
of the chromosome
•If an acrocentric chromosome acquires a paracentric
inversion, the morphology of the chromosome will not
be changed.
© Dr. Riddhi Datta
INVERSION

19. Suppression of recombination in an inversion heterozygote
•An individual in which one chromosome is inverted but its homologue is
not is said to be an inversion heterozygote.
•During meiosis, the inverted and non-inverted chromosomes pair point-
for-point along their length. Because of the inversion, the chromosomes
must form a loop to allow for pairing in the inverted region.
•Any one of the chromosomes is looped, and the other conforms around it.
© Dr. Riddhi Datta

20. Suppression of recombination in an inversion heterozygote
Pericentric inversion:
•As a result of crossing over inside
the loop, daughter chromosomes
with deletions/duplications for the
inverted region are produced which
are non-viable.
•Only the gametes with parental
chromosomes (one normal and one
inverted) are recovered.
© Dr. Riddhi Datta

21. Suppression of recombination
in an inversion heterozygote
Paracentric inversion:
•Here, as a result of crossing over,
one acentric and one dicentric
chromosomes are formed.
•In addition, the recombinant
products contain
deletions/duplications for the
inverted region and are non-
viable.
•Only the gametes with parental
chromosomes are recovered.
© Dr. Riddhi Datta

22. TRANSLOCATION
•A translocation occurs when a segment from one chromosome is detached
and reattached to a different (that is, non-homologous) chromosome.
•When pieces of two non-homologous chromosomes are interchanged without
any net loss of genetic material, the event is referred to as a reciprocal
translocation.

© Dr. Riddhi Datta

23. •During meiosis, the translocated
chromosomes pair with their
untranslocated homologues in a
cruciform, or crosslike, pattern.
•This pairing configuration is
diagnostic of a translocation
heterozygote.
•Cells in which the translocated
chromosomes are homozygous do
not form a cruciform pattern but
pair normally with its structurally
identical partner.

TRANSLOCATION
© Dr. Riddhi Datta

24. Altogether there are three possible disjunctional events:
1.Adjacent disjunction I: If centromeres 2 and 4 (i.e. centromeres from non-
homologous chromosomes that are next to each other) move to the same pole,
forcing 1 and 3 to the opposite pole, all the resulting gametes will be
aneuploid—because some chromosome segments will be deficient for genes,
and others will be duplicated.
TRANSLOCATION
© Dr. Riddhi Datta

25. 2.Adjacent disjunction II: If centromeres 1 and 2 (i.e. centromeres from the
homologous chromosome that are next to each other) move to one pole and 3
and 4 to the other, only aneuploid gametes will be produced. Each of these
cases is referred to as adjacent disjunction because centromeres that were next
to each other in the cruciform pattern moved to the same pole.
TRANSLOCATION
© Dr. Riddhi Datta

26. 3.Alternate disjunction: If centromeres 1 and 4 (i.e. centromeres from non-
homologous chromosomes that are alternate to each other) move to the
same pole, forcing 2 and 3 to the opposite pole, only euploid gametes will
be produced, and half of them will carry only translocated chromosomes.
•Translocation heterozygotes are therefore characterized by lowfertility.
TRANSLOCATION
© Dr. Riddhi Datta

27. ROBERTSONIAN TRANSLOCATION
•Non-homologous chromosomes can fuse at their centromeres,
creating a structure called a Robertsonian translocation.
•For example, if two acrocentric chromosomes fuse, they will
produce a metacentric chromosome; the tiny short arms of the
participating chromosomes are simply lost in this process.
© Dr. Riddhi Datta

28. ROBERTSONIAN TRANSLOCATION
•Human chromosome 2, which is metacentric, has arms that
correspond to two different acrocentric chromosomes in the
genomes of the great apes.
•Chromosomes can also fuse end-to-end to form a structure with two
centromeres. If one of the centromeres is inactivated, the
chromosome fusion will be stable.
© Dr. Riddhi Datta

29. COMPOUND CHROMOSOMES
•Sometimes one chromosome fuses with its homologue, or two sister chromatids become
attached to each other, forming a single genetic unit.
•A compound chromosome can exist stably in a cell as long as it has a single functional
centromere.
•If there are two centromeres, each may move to a different pole during division, pulling
the compound chromosome apart.
•A compound chromosome may also be formed by the union of homologous chromosome
segments. For example, the right arms of the two second chromosomes in Drosophila might
detach from their left arms and fuse at the centromere, creating a compound half-
chromosome.
•This structure is called an isochromosome, because its two arms are equivalent.
•Compound chromosomes differ from translocations in that they involve fusions of
homologous chromosome segments. Translocations, by contrast, always involve fusions
between non-homologous chromosomes.
© Dr. Riddhi Datta

30. POSITION EFFECT VARIEGATION
•Gene action can be blocked by proximity to the heterochromatin regions of
chromosome and this can result from translocation or inversion events.
•The locus for white eye color in Drosophila is near the tip of the X
chromosome.
•The allele w+ gives red color while the recessive w allele gives white color.
•If a translocation occurs in which the tip of an X chromosome carrying w+ is
relocated next to the heterochromatic region of chromosome 4, w+ locus is
inactivated.
© Dr. Riddhi Datta

31. POSITION EFFECT VARIEGATION
•The w+ allele is not always
expressed because the
heterochromatin boundary is
somewhat variable: in some
cells it engulfs and inactivates
the w+ gene, thereby allowing
the expression of w.
•If the position of the w+ and
w alleles is exchanged by a
crossover, then position-effect
variegation is not detected.
© Dr. Riddhi Datta
•Position-effect variegation is observed in flies that are heterozygotes for such a
translocation.

32. NUMERICAL CHANGES OF CHROMOSOME
•The changes in the number of chromosomes are usually described as
variations in the ploidy of the organism.
•Organisms with one or more complete sets of chromosomes are said to be
euploid.
•Organisms that carry more than two sets of chromosomes are said to be
polyploid and the level of polyploidy is described by referring to a basic
chromosome number, usually denoted n.
•Haploids: carry a single set of chromosomes (n)
•Diploids: carry two chromosome sets (2n)
•Triploids: carry three chromosome sets (3n)
•Tetraploids: carry four chromosome sets (4n)
•An individual of a normally diploid species that has only one chromosome set
is called a monoploid to distinguish it from an individual of a normally
haploid species (n).
© Dr. Riddhi Datta

33. © Dr. Riddhi Datta

34. POLYPOIDS AND ANEUPLOIDS
•Organisms in which a particular chromosome, or chromosome
segment, is under- or overrepresented are said to be aneuploid.
These organisms usually suffer from genetic imbalance.

•Aneuploidy refers to a numerical change in part of the genome,
usually just a single chromosome, whereas polyploidy refers to a
numerical change in a whole set of chromosomes.
•Aneuploidy implies a genetic imbalance, but polyploidy does
not.

© Dr. Riddhi Datta

35. © Dr. Riddhi Datta
POLYPOIDS AND ANEUPLOIDS

36. ANEUPLOIDY
•Aneuploidy describes a numerical change in part of the genome,
usually a change in the dosage of a single chromosome. This includes
individuals that:
•have an extra chromosome
•are missing a chromosome
•have a combination of these anomalies
•have a chromosome whose one arm has been deleted
•The under- or overrepresentation of a chromosome or a chromosome
segment can affect a phenotype.

© Dr. Riddhi Datta

37. •An organism in which a chromosome, or a piece of a
chromosome, is underrepresented is referred to as a
hypoploid.

•An organism in which a chromosome or chromosome
segment is overrepresented is referred to as a hyperploid.
© Dr. Riddhi Datta

38. CAUSE OF ANEUPLOIDY

•Nondisjunction during meiosis or
mitosis:
•Disjunction is normal
segregation of homologous
chromosomes or chromatids to
opposite poles at meiotic or
mitotic divisions.
•Nondisjunction is a failure of
this process, in which two
chromosomes or chromatids
incorrectly go to one pole and
none to the other.


© Dr. Riddhi Datta

39. CAUSE OF ANEUPLOIDY

•Loss of a chromosome that has a
centromeric deletion.

•Loss of the small chromosome
produced by Robertsonian
translocation.


© Dr. Riddhi Datta

40. ANEUPLOIDY TYPES
Nullisomy 2n - 2 Missing both copies of a homolog in a diploid
Monosomy 2n - 1 Missing one copy of a homolog in a diploid
Trisomy 2n + 1 Having an extra copy of one homolog in a diploid
Tetrasomy 2n + 2 Having two extra copies of one homolog in a diploid
Disomy n + 1 Having an extra copy of a homolog in a haploid
n = haploid number of chromosomes
2n = diploid number of chromosomes

41. NULLISOMY
•Nullisomy occurs when both the homologs in the otherwise diploid
genome is missing.
•This is represented as 2n-2.
•The number of possible nullisomics in an organism will be equal to
the haploid chromosome number.
© Dr. Riddhi Datta

42. NULLISOMY
•Although nullisomy is a lethal condition in diploids, an organism
such as bread wheat, which behaves meiotically like a diploid
although it is a hexaploid, can tolerate nullisomy.
•The four homoeologous chromosomes apparently compensate for a
missing pair of homologs.
•In fact, all the possible 21 bread wheat nullisomics have been
produced. Their appearances differ from the normal hexaploids;
furthermore, most of the nullisomics grow less vigorously.
© Dr. Riddhi Datta

43. MONOSOMY
•Monosomy occurs when one chromosome is missing in an
otherwise diploid individual.
•It is represented as 2n – 1.
•The number of possible monosomies in an organism is equal to the
haploid chromosome number.
•When one copy of each of two non-homologous chromosomes are
lost, it is called double monosomy (2n – 1 – 1).

© Dr. Riddhi Datta

44. Suppose, gene A/a is on chromosome 2. Crosses of a/a with monosomic
for chromosome 1 will give all progeny with A/a genotype.
© Dr. Riddhi Datta
MONOSOMY
But, crosses of a/a with
monosomic for chromosome 2
will give the following
progenies:
50% A/a
50% a/0
The appearance of recessive
phenotype in the
heterozygotes indicates a
missing chromosome.
A: Dominant
a: recessive (mutant)

45. MONOSOMY
•TURNER SYNDROME:
•In humans, there is only one viable
monosomic, the 45, X karyotype.

•These individuals have a single X
chromosome as well as a diploid
complement of autosomes.

•Phenotypically, they are female, but
they are almost always sterile.

•They are short in stature; have webbed
necks, hearing deficiencies, and
significant cardiovascular abnormalities.
© Dr. Riddhi Datta

46. MONOSOMY
© Dr. Riddhi Datta
•They originate from eggs or sperm that lack a sex chromosome or from
the loss of a sex chromosome in mitosis sometime after fertilization.
•This latter possibility is supported by the finding that many Turner
individuals are somatic mosaics.
•People with the 45, X karyotype have no Barr bodies in their cells,
indicating that the single X chromosome that is present is not inactivated.

47. TRISOMY
•Trisomics are those organisms which have an extra chromosome.
•It is represented as 2n + 1.
•The number of possible trisomies in an organism is equal to the
haploid chromosome number.
© Dr. Riddhi Datta

48. TRISOMY
© Dr. Riddhi Datta
Suppose, gene A/a is on chromosome 2. A trisomic (A/a/a) for
chromosome 2 will give the following meiotic segregation ratio:
A: Dominant
a: recessive (mutant)

49. TRISOMY TYPES
•Primary trisomies: extra chromosome is identical to both the homologues
•Secondary trisomies: extra chromosome is an iso-chromosome with two
genetically identical arms
•Tertiary trisomies: these are the products of translocation
© Dr. Riddhi Datta

50. TRISOMY
© Dr. Riddhi Datta
Trisomies show
irregular meiosis.
Since the trisomies
have an extra
chromosome which
is homologous to
one of the
chromosomes of the
complement, they
form a trivalent.

51. TRISOMY IN PLANTS
•Datura stramonium is a diploid species and has
12 pairs (=24) of chromosomes in the somatic
cells.
•Albert Blakeslee and John Belling analyzed
chromosome anomalies in Datura plants with
irregular phenotypes.
•By examining the chromosomes of the mutant
plants, they found that in every case an extra
chromosome was present.
•Altogether there were 12 different mutants,
each corresponding to a triplication of one of
the Datura chromosomes.
•Such triplications are called trisomies.
© Dr. Riddhi Datta

52. TRISOMY IN HUMAN
•DOWN SYNDROME: a condition associated
with an extra chromosome 21 (Trisomy 21).
•First described in 1866 by a British physician,
Langdon Down
•People with Down syndrome are typically
short in stature and loose-jointed,
particularly in the ankles; they have broad
skulls, wide nostrils, large tongues with a
distinctive furrowing, stubby hands with a
crease on the palm and impaired mental
abilities.
•Their life span is much shorter and develop
Alzheimer’s disease. © Dr. Riddhi Datta

53. TRISOMY IN HUMAN
•Trisomy 21 can be caused by chromosome nondisjunction in one of the meiotic cell
divisions, more likely in females.
•The frequency of nondisjunction increases with maternal age. Thus, among mothers
younger than 25 years old, the risk of having a child with Down syndrome is
about 1 in 1500, whereas among mothers 40 years old, it is 1 in 100.
© Dr. Riddhi Datta

54. TRISOMY IN HUMAN
•Trisomies 13 and 18 have also been reported. However, these
are rare, and the affected individuals show serious phenotypic
abnormalities and are dying within the first few weeks after birth.

•Another viable trisomy is the triplo-X karyotype, 47, XXX.
•These individuals survive because two of the three X chromosomes
are inactivated, reducing the dosage of the X chromosome .
•Triplo-X individuals are female and are phenotypically normal, or
nearly so; sometimes they exhibit a slight mental impairment and
reduced fertility.
© Dr. Riddhi Datta

55. TRISOMY IN HUMAN
•KLINEFELTER SYNDROME: The 47, XXY
karyotype is a viable trisomy in humans.
•These individuals have three sex
chromosomes, two X’s and one Y.
•Phenotypically, they are male, but they
can show some female secondary sexual
characteristics and are usually sterile.
•Abnormalities associated with this
condition, now called Klinefelter
syndrome; these include small testes,
enlarged breasts, long limbs, knock-
knees, and underdeveloped body hair.
© Dr. Riddhi Datta

56. TRISOMY IN HUMAN
•The 47, XXY karyotype can originate by:
•fertilization of an exceptional XX egg with a Y-bearing sperm
•fertilization of an X-bearing egg with an exceptional XY sperm.
•All individuals with Klinefelter syndrome have one or more Barr
bodies in their cells, and those with more than two X chromosomes
usually have some degree of mental impairment.
© Dr. Riddhi Datta

57. TRISOMY IN HUMAN
•The 47, XYY karyotype is another viable trisomy in humans.
•These individuals are male, and except for a tendency to be
taller than 46, XY men, they do not show a consistent
syndrome of characteristics.

•All the other trisomies in humans are embryonic lethals,
demonstrating the importance of correct gene dosage.
© Dr. Riddhi Datta

58. DISOMY

•A disomic (n+1) is an aberration of a haploid organism.
•In fungi, they can result from meiotic nondisjunction.
•In the fungus Neurospora (a haploid), an n-1 meiotic product aborts
and does not darken like a normal ascospore
•So we may detect MI and MII nondisjunctions by observing asci with
4:4 and 6:2 ratios of normal to aborted spores, respectively.
•In these organisms, the disomic (n+1) meiotic product becomes a
disomic strain directly.
© Dr. Riddhi Datta

59. © Dr. Riddhi Datta

60. NUMERICAL CHANGES OF CHROMOSOME
•The changes in the number of chromosomes are usually described as
variations in the ploidy of the organism.
•Organisms with one or more complete sets of chromosomes are said to be
euploid.
•Organisms that carry more than two sets of chromosomes are said to be
polyploid and the level of polyploidy is described by referring to a basic
chromosome number, usually denoted n.
•Haploids: carry a single set of chromosomes (n)
•Diploids: carry two chromosome sets (2n)
•Triploids: carry three chromosome sets (3n)
•Tetraploids: carry four chromosome sets (4n)
•An individual of a normally diploid species that has only one chromosome set
is called a monoploid to distinguish it from an individual of a normally
haploid species (n).
© Dr. Riddhi Datta

61. MONOPLOIDS
•An individual of a normally diploid
species that has only one
chromosome set is called a
monoploid.
•Male bees, wasps, and ants are
monoploid.
•They develop by parthenogenesis
(the development of unfertilized
egg into an embryo).
© Dr. Riddhi Datta

62. MONOPLOIDS
•In most other species monoploid zygotes fail to develop.
•All diploid individuals carry a number of deleterious recessive
mutations, together called a “genetic load.”
•The deleterious recessive alleles are masked by wild-type alleles in
the diploid condition, but are automatically expressed in a
monoploid derived from a diploid.
•Monoploids, if surviving to adulthood, are sterile because their
chromosomes have no pairing partners during meiosis.
© Dr. Riddhi Datta

63. POLYPLOIDS
•Organisms that carry more than two sets of chromosomes
are polyploids.
•Polyploid plants are often larger and have larger component
parts than their diploid relatives.
•Polyploids with odd numbers of chromosome sets, such as
triploids, are sterile or highly infertile because their gametes
and offspring are aneuploid.

© Dr. Riddhi Datta

64. AUTOPOLYPLOIDS AND ALLOPOLYPLOIDS
•Autopolyploids: They have multiple chromosome sets originating
from within one species
•Allopolyploids: They have sets of chromosomes from two or more
different species.
•Allopolyploids form only between closely related species; however,
the different chromosome sets are only homeologous (partially
homologous), not fully homologous as they are in autopolyploids.
•Generally plants seem to be much more tolerant of polyploidy
than animals.
© Dr. Riddhi Datta

65. AUTOPOLYPLOIDS: TRIPLOIDS
•Triploids are usually autopolyploids.

•They may arise spontaneously in nature.

•They can also be constructed by geneticists from the cross of
a 4n (tetraploid) and a 2n (diploid). The 2n and the n
gametes produced by the tetraploid and the diploid,
respectively, unite to form a 3n triploid.
© Dr. Riddhi Datta

66. AUTOPOLYPLOIDS: TRIPLOIDS
•Triploids are characteristically sterile
because of their problem in synapsis
during meiosis.
•The three homologous chromosomes of a
triploid may pair in two ways at meiosis:
a trivalent
a bivalent plus a univalent.
•It is unlikely that a gamete will receive two
for every chromosomal type, or that it will
receive one for every chromosomal type.
Hence, they will be aneuploids.
© Dr. Riddhi Datta

67. AUTOPOLYPLOIDS: TETRAPLOIDS
•Autotetraploids arise by the doubling of a 2n complement to 4n.
•This doubling can occur spontaneously, but it can also be induced
artificially by applying chemical agents that disrupt microtubule
polymerization (ex: colchicine).
•In colchicine-treated cells, the S phase of the cell cycle occurs, but
chromosome segregation or cell division does not. As the treated cell
enters telophase, a nuclear membrane forms around the entire
doubled set of chromosomes.
•Therefore, if a diploid cell of genotype A/a ; B/b is doubled, the
resulting autotetraploid will be of genotype A/A/a/a ; B/B/b/b.

© Dr. Riddhi Datta

68. AUTOPOLYPLOIDS: TETRAPLOIDS
•Because 4 is an even number,
autotetraploids can mostly have a
regular meiosis.
•The chromosomes may pair as:
•two bivalents (segregate normally
forming diploid gametes)
•a quadrivalent (segregate
normally forming diploid
gametes)
•a trivalent plus an univalent
(segregate to produce non-viable
aneuploids)
© Dr. Riddhi Datta

69. ALLOPOLYPLOIDS
•An allopolyploid is a plant that is a hybrid of two or more species,
containing two or more copies of each of the input genomes.
•Allopolyploid plants can be synthesized by crossing related species and
doubling the chromosomes of the hybrid or by fusing diploid cells.
Raphanobrassica:
•An allotetraploid synthesized by G. Karpechenko in 1928
•He wanted to make a fertile hybrid that would have the leaves of the
cabbage (Brassica) and the roots of the radish (Raphanus)
•Each of these two species has 18 chromosomes.
2n
1 = 2n
2 = 18
•The species are related closely enough to allow intercrossing.
© Dr. Riddhi Datta

70. ALLOPOLYPLOIDS
Raphanobrassica:
•Fusion of an n
1 and an n
2 gamete produced
a viable hybrid progeny but it was sterile.
•Eventually, one part of the hybrid plant
produced some seeds due to accidental
chromosome doubling which enabled
synapsis during meiosis.
•This resulted in a allotetraploid individual
(2n
1 + 2n
2) which would produce n
1 + n
2
gametes. These gametes fuse to form 2n
1
+ 2n
2 progeny again.
© Dr. Riddhi Datta

71. ALLOPOLYPLOIDS
Raphanobrassica:
•This kind of allopolyploid is sometimes
called an amphidiploid, or doubled
diploid.
•Unfortunately, the hybrid exhibited leaves
like radish and roots like cabbage!
© Dr. Riddhi Datta

72. ALLOPOLYPLOIDS
Bread wheat:
•A particularly interesting natural allopolyploid is bread wheat,
Triticum aestivum (6n = 42).
•Bread wheat is composed of two sets each of three ancestral
genomes.
•At meiosis, pairing is always between homologs from the same
ancestral genome.
•Hence, in bread wheat meiosis, there are always 21 bivalents.
© Dr. Riddhi Datta

73. ALLOPOLYPLOIDS
© Dr. Riddhi Datta

74. APPLICATION OF POLYPLOIDY IN AGRICULTURE
•Monoploids: Monoploids help breeders to select recessive characters
in hybrids.
•Autotriploids: Commercially available seedless bananas are sterile
triploids (3n = 33). Seedless watermelons are another example.
•Autotetraploids: Many autotetraploid plants have been developed
as commercial crops to take advantage of their increased size. Large
fruits and flowers are particularly favored.
•Allopolyploids: Best example is bread wheat. New World cotton is
a natural allopolyploid that occurred spontaneously. A synthetic
amphidiploid, Triticale, has been widely used commercially. This is an
amphidiploid between wheat (Triticum, 6n=42) and rye (Secale,
2n=14). Hence, for Triticale, 2n=2x(21+7) 56. This novel plant
combines the high yields of wheat with the ruggedness of rye.
© Dr. Riddhi Datta

75. Thank you
© Dr. Riddhi Datta