This document discusses balanced tertiary trisomics (BTT), which are a type of tertiary trisomic plant that can be used for hybrid seed production. Tertiary trisomics have an extra chromosome that is the result of a translocation. BTTs are constructed so that a dominant marker gene linked to the translocation breakpoint is on the extra chromosome. The two normal chromosomes carry recessive alleles. This allows BTTs to be distinguished from diploids for use in hybrid seed production schemes, such as one developed for barley where BTTs are male fertile and function as pollen parents, while diploids are male sterile and function as seed parents. The progeny of BTT selfing can then

1. BALANCED TERTIARY
TRISOMICS
Guided by:
Dr. Koushik Kumar Panigrahi
Asst. professor
PBG
Submitted by:
Soumya Ranjan Das
54c/15

2. Trisomy:
•Trisomics are those organisms which have one extra chromosome (2n+1). Since
the extra chromosome may belong to any one of the different chromosome pairs,
the number of possible trisomics in an organism will be equal to the haploid
chromosome number.
•In a trisomic ,one of the pairs of chromosomes has an extra member and forms a
trivalent during anaphase I of meiosis.
•Two chromosomes will go to one pole and one chromosome will go to other pole
.As a result , two types of gametes are formed i.e. n and n+1.
•In plants the first case of trisomy was investigated in Jimsom weed i.e. Datura
stramonium by A.F. Blakeslee and J. Belling in1924.
•Datura (2n = 24) normally has 12 pairs of chromosomes in somatic cells. But in
an individual, they discovered 25 chromosomes.

3. •The size, shape and spine characteristics of seed capsule of this trisomic plant
were different from seed capsule of wild type species.
•Through experimental breeding, Blakeslee and his associates succeeded in
producing all 12 possible trisomics. When these were grown, each was found
to have a distinguishable phenotype that was attributed to extra set of genes
present on the extra chromosome contained in each of the 12 pairs of
chromosomes.
•An individual having two extra chromosomes each belonging to a different
chromosome pair is called double trisomic (2n + 1 + 1).
•Depending on the nature of extra chromosome, simple trisomics are of three
types.
a)Primary trisomics: The additional chromosome is normal one in primary
trisomics.
b)Secondary trisomics: Trisomics having isochromosome as additional
chromosome.
c)Tertiary trisomics: When additional chromosome in a trisomic is
translocated one, it is known as tertiary trisomic.

5. Tertiary Trisomics:-
•When additional chromosome in a trisomic is translocated one, it is known as
tertiary trisomic.
•A tertiary trisomic individual consists of an interchanged non homologous
chromosome in addition to the normal somatic chromosome complement.
•Tertiary trisomics have been utilized effectively in tomato for determining
centromere positions on the linkage maps and for associating a gene with a
particular arm of a chromosome (Khush and Rick, 1967c).
•They have been used in barley for the construction of ―balanced tertiary
trisomics (BTTs) for hybrid seed production (Ramage, 1965).
•The first human trisomic syndrome discovered was the one involving ‘G‘ group
of chromosomes called Mongolism or Down‘s syndrome.

6. Sources of Tertiary Trisomics:
•In general, interchange heterozygotes that throw tertiary trisomics in their
progenies have been described in maize (Burnham, 1930, 1934), pea (Sutton,
1939), Oenothera lamarckiana (Emerson, 1936), Oenothera blandina
(Catcheside, 1954), Datura stramonium (Avery, Satina, and Rietsema, 1959),
barley (Ramage, 1960), tomato (Khush and Rick, 1967c; Gill, 1978), Phaseolus
vulgaris (Ashraf and Bassett, 1987), Pennisetum glaucum (Singh et al., 1982),
Secale cereale (Janse, 1985, 1987), and lentil (Ladizinsky, Weeden, and
Muehlbauer, 1990).
•Among the published results on tertiary trisomics, the investigation of Khush and
Rick (1967c) in tomato is the most thorough . They reported seven tertiaries; two
were isolated from tertiary monosomics and five from the interchanged
heterozygotes.
•In Datura, the tertiaries appeared spontaneously in a low frequency.

7. •Tertiary trisomics originate from an interchange heterozygote in the following
way. An interchange heterozygote occasionally forms a noncooriented
quadrivalent configuration at diakinesis and metaphase-I, and a 3:1 random
disjunction of chromosomes at anaphase-I will generate n + 1 gametes. Thus,
eight possible types of 2x + 1 individuals are expected in the progeny of a selfed
interchange heterozygote. Of the four tertiaries, two will be in homozygous
background and the other two in translocation heterozygous background.
•For isolating tertiaries in tomatoes, Khush and Rick (1967c) hybridized
interchanged heterozygotes with normal diploids. According to this procedure,
the progeny should segregate in a proportion of two tertiaries (homozygous) to
two primaries (interchange heterozygous) in the 2x + 1 fraction, and one normal
to one interchange heterzygote in the 2x fraction.
•A plant is designated as a primary trisomic if an extra chromosome is normal;
the trisomic may be a primary trisomic, a primary trisomic interchange
heterozygote, or a primary trisomic interchange homozygote. On the other hand,
if the extra chromosome is a translocated chromosome, a plant is designated as a
tertiary trisomic (Ramage, 1960).

8. BalancedTertiaryTrisomics:
•Ramage (1965) proposed a scheme utilizing balanced tertiary trisomics (BTT) for
production of hybrid barley seed.
•Balanced tertiary trisomics are tertiary trisomics constituted in such a way that the
dominant allele of a marker gene closely linked with the interchange breakpoint is
carried on the tertiary chromosome.
•The recessive allele is carried on each of the two normal chromosomes. The
dominant marker allele for a mature plant character, such as red plant color (R),
may be carried on either the centromere portion or on the interchanged segment of
the extra chromosome and should be linked with a male fertile gene (Ms).
•The two normal chromosomes should carry the corresponding recessive male
sterile allele (ms). All balanced tertiary trisomics would be male fertile and red,
and all diploids would be male sterile and green.

9. Utilization of balanced tertiary trisomics (BTT) to produce hybrid barley

10. •All functioning pollen produced by the balanced tertiary trisomics would carry the
male sterile (ms) and the green plant color alleles (r).
•The self-progeny of such a trisomic would be planted in an isolation block.
•Diploid plants would be green and male sterile. All seed set on them would
produce male sterile diploids in the next generation.
•The diploid plants would be harvested separately, and seed produced on them
would be used to plant the female rows in the hybrid seed production field.
•Balanced tertiary trisomics planted in isolation would be dominant and would be
harvested separately.
•Seed produced on balanced tertiary trisomic plants would produce approximately
70% male sterile diploids and 30% balanced tertiary trisomics.
•This seed would be used to plant an isolation block the following year.

11. Prerequisites for utilizing BTT system in hybrid seed production
•The prerequisites for utilizing the BTT system in hybrid seed production are
that balanced tertiary trisomics produce abundant pollen and that during
anthesis, sufficient wind and insects are available for pollen dissemination.
Several other alternatives to distinguish tertiary trisomics from diploids have
been suggested, such as resistance or susceptibility to phytocides, seed size or
shape, and differential plant heights (Ramage, 1965).

12. Thank you….