Backcross Method for Dominant and Recessive Gene Transfer.

Submitted to- Dr. J. N. Patel,

Associate Research Scientist,

Main Vegetable Research Station,

AAU, Anand.

Submitted by- Gediya lalji N,

M.Sc in Genetics and Plant Breeding,

AAU, Anand.

Content…

Introduction Requirement of backcross programme Application of backcross method Genetic consequences of repeated backcrossing Transfer of a dominant gene Transfer of a recessive gene Merits of backcross method Demerits of backcross method Achievement

•What is Backcross ?

• A cross between a hybrid (F1 or a segregating generation) and one of its parents is known as backcross.

• In BACKCROSS METHOD, the hybrid and the progenies in the subsequent generation are repeatedly backcrossed to one of the parents of the F1.

•Why backcross method is used?

•Since there are many varieties which are deficient in some characters, therefore that deficient character present in recurrent parent is crossed with donor parent in which that character is present, so we use backcross for the transfer of the character•The objective of backcross method is to imprrovw one or two specific defect of a high yielding variety, which is well adapted to the area and has a other desirable characteristics.

•What is Recurrent Parent and Non-recurrent Parent ?

In backcross breeding, the parent from which few or more genes are transferred to the recurrent parent is called the Donor parent.In backcross breeding, the parent to which few or more genes are transferred from the donor parent is called the Recurrent parent.

Requirement for a Backcross Programme-

1. A suitable recurrent parent, which lacks in one or

two characters.

2. A suitable Donor parent that has an intense form of

character which is deficient in recurrent parent.

3. The character to be transferred must have high

heritability and governed by one or few genes.

4. Sufficient number of backcrosses should be made so

that genotype of the recurrent parent is recovered in

full.

Applications of Backcross Method

This method is commonly used to transfer disease resistance from one variety to another. But it is also useful for transfer of other characteristics.

1. Intervarietal transfer of simply inherited characters E.g. Disease resistance, seed coat colour

2. Intervarietal transfer of quantitative characters. E.g. Plant height, Seed size, Seed shape.

3. Interspecific transfer of simply inherited characters E.g. Transfer of disease resistance from related species to cultivated species. E.g. Resistance to black arm disease in cotton from wild tetraploid species into G.hirsutum .

4. Transfer of cytoplasm This is employed to transfer male sterility. The female parent will be having the sterile cytoplasm and recurrent parent will be used as male parent. E.g. Sesamum malabariucum x S.indicum Female parent Recurrent parent.

5. Transgressive segregation Back cross method may be modified to produce transgressive segregants. The F1 is backcrossed to recurrent parent for 2 to 3 times for getting transgressive segregants.

6. Production of isogenic lines

7. Germplasm conversion E.g. Production of photo insensitive line from photo Sensitive germplasm through back crossing. This was done in the case of sorghum. Popularly known as conversion programme.

Genetic Consequences of Repeated Backcrossing

1. Reduction in heterozygosity

2. Increased similarity with the recurrent parent

3. Selection for gene under transfer

4. Extended opportunity for breaking undesirable linkages

5. Selection for Recurrent Parent type

Generation Average proportion (in %) of genes from the recurrent parent

F1 50BC1 75BC2 87.5BC3 93.75BC4 96.875BC5 98.438BC6 99.218BC7 99.609BC8 99.805BC9 99.902BC10 99.951

SELECTION OF PARENTS Backcross method of breeding change the genotype

of recurrent parent only for the gene(s) under a transfer to correct the specific defect of the recurrent parent.

But some unexpected changes in one or more character may also occur due to gene tightly linked with the gene being transferred.

Therefore , the recurrent parent must be the most popular variety of the area, which has high yielding ability, desirable quality and high adaptability.

In each crop, one or two varieties dominant, and they are vary popular with the farmers. Such a variety may have one or two defects, e.g.,susceptibility to disease or undesirable seed size or colour, which may be removed by this method.

The nonrecurrnt parent is selected for high intensity of the character that is to be improved in the recurrent parent, and yielding ability and other feature of the parent are not important.

The intensity of the character should preferably be more than that desired in the recurrent parent because the intensity may decline during the transfer and in the new genetic background of the recurrent parent.

Transfer of a Dominant Gene –

Suppose that a high yielding variety and widely

adapted wheat variety A is susceptible to stem rust.

Another variety B is resistant to stem rust and this

resistance is dominant to susceptibility.

Therefore, variety A is taken as Recurrent parent and

variety B is used as Non-recurrent parent.

Procedure is simple for dominant gene transfer than

recessive gene transfer.

Transfer of Recessive Gene – When rust resistance is due to recessive gene, all the

backcross cannot be made one after the other. After the

first backcross, and after every two subsequent

backcrosses, F2 generations must be grown to identify

rust resistant plants.

The F1 and the backcross progenies are not inoculated

with rust because they will be uniformly susceptible so,

only the F2 populations are tested for rust resistance.

Here, Variety B is rust resistance(rr) and it is Non-

recurrent parent, whereas Variety A is agronomically

desirable and it is as Recurrent parent.

Merits of Backcross Method – The genotype of new variety is nearly identical with

that of the recurrent parent, except for the gene transfer. Thus the outcome of a backcross programme is khown before hand and it can be reproduced any time in future.

Much smaller population is needed in this method than in pedigree method.

It is not necessary to test the variety developed by the backcross method in extensive yield tests because the performance of recurrent parent is already known. This may save up to 3-4 years time and considerable expense.

The backcross programme is not dependent on the environment, except for the needed fpr selection of the character under transfer.

This is the only method for inter-specific gene transfers, and for the transfer of cytoplasm.

Defect like susceptibility to the disease of a well adapted variety can be removed without affecting its performance and adaptability.

Demerits of the Backcross Method

The new variety generally cannot be superior to the recurrent parent, except for the character that is transferred.

Linkage drag (close linkage between a desirable and undesirable gene.)

Hybridization required for each backcross; which is often difficult, time taking and costly.

By the time backcross programme improves it, recurrent parent may have been replaced by other varieties superior in yielding ability and other characteristics.

Achievements.. Rust resistance has been transferred to Kalyan Sona

from several

diverse sources, eg. Robin, HS19, Bluebird etc.

Tift23A was used in backcross programme with

resistant lines from India and Africa to develop

downy mildew resistant male sterile lines, such as

MS521, MS541, etc

G. herbaceum varieties Vijapla, Vijay, Digvijay and

Kalyan are some of the cotton varieties developed by

this method.

REFERENCE

• Plant breeding; Principles and method, B.D.Singh,Kalyani publishers

• An introduction to plant breeding, Jack Brown and Peter Caligari, Blackwell publishers

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