B_T_2015_02_

1. octaploid Triticale


2. hexaploid Triticale

(c) Wheat

• Common wheat (Triticum aestivum) is an important
example of natural allopolyploid.

•    Common wheat is hexaploid, i.e., 2n = 6x = 42 with

three genomes, A, B and D, i.e., AABBDD.

•    These three genomes are derived from three different

diploid species, such as :

• Triticum aegilopoides – AA (2n = 14).


• Aegilops speltoides – BB (2n = 14).


• Aegilops squarossa – DD (2n = 14).
  • By crossing first two diploid species, Triticum
  dicoccoides, 2 n = 4x = 28, i.e., tetraploid – AABB,
  is produced, which on crossing with third species
  gives rise to Triticum aestivum, 2 n = 6x = 42, i.e.,
  hexaploid – AABBDD.

(iii) autoallopolyploidy or segmental poly-

ploidy

•    This is intermediate between the first two types of

polyploidy.

•    Wheat is nowadays considered to be an example of

segmental polyploidy, rather than allopolyploidy, as

earlier thought because three genomes in wheat A,

B and D, are not fully distinct from each other, and

have some portions common in them (i.e., only some

segments are homologous).

•    Hence A, B and D show homoeologous relationship

(not homologous), so pairing can occur upto some

extent.

•    Such polyploids where different genomes show

homoeologous relationship are called autoallopolyploids

or segmental polyploids.

Intext Practice Questions

4. Explain the different types of polyploidy.


5. Fill in the blanks with appropriate words/phrases :
   (a) Organisms having genetic constitution, 2n – 2, are called __.
   (b) Aneuploidy occurs due to __ during meiosis.
   (c) Organisms with genetic constitution, 2n – 1 – 1, are called __.

role in evolution

•    One of the most frequent cause of evolution

is a change in the order of genes as a result of

chromosomal aberrations.

•    The majority of plant species originate from an abrupt

and rapid change in nature, and aneuploidy and

polyploidy are the prime sources of variation.

•    In the animal kingdom polyploidy is not so important.

Among vertebrates, different species of fishes have

a different number of chromosomes. Amphibians

are generally characterized by a specific number for

each family.