In this article we will discuss about:- 1. Meaning and Features of Backcross Method 2. Genetic Basis of Backcrossing Method 3. Breeding Procedure 4. Achievements 5. Merits and Demerits.
Meaning and Features of Backcross Method:
Backcross refers to crossing of F1 with either of its parents. When the F1 is crossed with homozygous recessive parent, it is known as test cross.
A system of breeding in which repeated backcrosses are made to transfer a specific character to a well-adapted variety for which the variety is deficient is referred to as backcross breeding. The backcross method of breeding is commonly used in self and cross pollinated species. In vegetatively propagated crops like sugarcane and potato this method is rarely used and that too with some modifications.
Main features of this method are briefly presented below:
1. Application:
The backcross method is generally used to improve specific character of a well-adapted variety for which it is deficient such as resistance to a specific disease. This method is more commonly used for transfer of monogenic or oligogenic characters than polygenic characters.
In other words, it is more successful when the character has high heritability. Oligogenic characters have high heritability than polygenic traits. Backcross method is applicable in all three groups of crop plants, viz. self-pollinated, cross pollinated and asexually propagated.
2. Parental Material:
Backcross method involves two types of parents, viz. recipient parent and donor parent. The parent which receives a desirable character is known as recipient parent. The recipient parent is repeatedly used in the backcross method; hence it is also called as recurrent parent.
The recipient parent is generally a well-adapted high yielding variety of an area which is deficient in one or few characters. The parent which donates the desirable character is known as donor parent. Since donor parent is used only once in the crossing, it is also known as non-recurrent parent. The donor parent is generally poor in agronomic characters. Thus backcross method is used when one of the parents is un-adapted type.
3. Genetic Constitution:
Backcross method retains the genotype of original variety except for the character which is improved by backcrossing. In other words, the new variety resembles the parent variety in all the characters except for the character under transfer.
4. Number of Backcross:
Generally 5 to 6 backcross are sufficient to retain the genotype of original variety with new character.
5. Basic Requirements:
The basic requirements to start a backcross programme are:
(i) Recurrent parent,
(ii) Donor parent, and
(iii) High heritability of the character under transfer.
Genetic Basis of Backcrossing Method:
Backcross increases the frequency of desirable individuals in a population. For example, from a cross involving single locus (AA x aa), we will get only 1/4 desirable individuals (AA) in F2 through selfing (1AA: 2Aa: 1 aa). In case of backcrossing (AA x Aa), we get 1/2 desirable individuals in the BC F1 (1 AA: 1 Aa).
The same is expected for each gene pair. The population gradually becomes identical to the recurrent parent. The population is not divided into 2n homozygous genotypes as happens in case of selfing.
However, in backcrossing homozygosity is attained at the same rate as with selfing which is given below:
Proportion of homozygous individuals = [(2m – 1)/2m]n
where,
m = number of backcrossing or selfing and
n = number of gene pairs.
Moreover, the chances of breaking linkage between desirable and undesirable genes are more with backcrossing than with selfing. Suppose, gene A is desirable and it is linked with undesirable gene b. The desirable gene A has to be transferred from a donor to a well-adapted variety.
The cross between adapted and donor parents will produce AaBb hybrid. The genes A and ‘a’ have the tendency to, inherit together to make it difficult to obtain AB combination. Since gene B is reintroduced with each backcross, there will be several chances for the crossover to take place.
Thus the probability of elimination of b gene is as given below:
Probability of eliminating of b gene = 1 – (1 – p)m+1
where,
p = recombination fraction and
m = number of backcrosses.
Breeding Procedure of Backcross Method:
Some characters are governed by dominant gene and others by recessive gene. The breeding procedure of backcross method depends on whether the character under transfer is controlled by dominant or recessive gene.
The breeding procedure for both the situations is briefly presented below:
1. Transfer of Dominant Gene:
Suppose wilt resistance in cotton is controlled by a dominant gene (RR). The donor parent is a strain (B) from the germplasm. The resistance has to be transferred to an adapted variety (A) which is susceptible to wilt. The adapted variety (A) will be used as recurrent parent and strain (B) as donor parent.
The F1 will be wilt resistant but heterozygous (Rr). Backcrossing of F1 (Rr) with susceptible variety (rr) will produce resistant and susceptible plants in equal number in BC1 F1 (1Rr: 1rr). The resistant cotton plant (Rr) can be identified by growing the material in wilt sick plot. The resistant plants (Rr) are then backcrossed to the adapted variety.
Generally, 6-8 backcrosses are sufficient to obtain plants identical to adapted variety except for the added genes for wilt resistance. The wilt resistant plants are heterozygous (Rr). They are selfed for one generation to obtain homozygous (RR) resistant plants. All the resistant true breeding plants are bulked and new variety is released. The variety developed in this way is identical to the adapted variety (A) expect for wilt resistance (Table 20.1).
2. Transfer of Recessive Gene:
Suppose wilt resistance in cotton is governed by a recessive gene (rr). In such case, the progeny of each backcross will segregate into two genotypes (RR and Rr) which cannot be identified. Therefore, it is necessary to self the population after each backcross to obtain resistant homozygous recessive plants (rr).
The resistant plants are identified by growing the F2 material in wilt sick plot. The resistant plants are backcrossed with adapted variety. Here each backcross is followed by one selfing, whereas with dominant gene continuous backcrosses are made.
3. Transfer of Quantitative Traits:
Backcross method is generally used for transfer of monogenic or oilgogenic characters. It can also be used for transfer of polygenic traits. However, transfer of polygenic characters is somewhat difficult due to low heritability of such characters and more influence of environment in the expression of polygenic characters. For successful transfer of polygenic character, the non-recurrent parent with extreme phenotype for the polygenic character under transfer should be chosen.
For example, if we want to improve protein percentage from 20 to 25%, we should select non-recurrent parent with 30% protein. This will make identification of the character easy. Moreover, after each backcross one or two generations of selfing are required, to get the desirable segregants. Furthermore, large populations have to be raised to achieve the desired combination. In other words, the observations should be based on large samples.
Sometimes, several characters have to be transferred into an adapted cultivar through backcrossing.
This can be achieved in two ways:
(i) Transfer of genes in separate backcross programme and then combining them into single genotype, and
(ii) Simultaneous transfer of genes into single genotype in one backcross programme.
For simultaneous transfer of multiple characters, backcross seeds have to be produced in more quantity to be sure to get a genotype with all desirable genes.
Achievements of Backcross Method:
Backcross method has been widely used for the development of disease resistant varieties in both self and cross pollinated species. It has also been used for interspecific gene transfer and development of multiline varieties in self-pollinated species. Several varieties resistant to various diseases have been developed, by this method in wheat, cotton and several other crops.
In cotton varieties V797, Digvijay, Vijalpa and Kalyan which belong to Gossypium herbaceum have been developed by backcross method. A brief comparison of pedigree, bulk and backcross breeding methods is presented in Table 20.2.
Merits and Demerits of Backcross Method:
Merits:
1. Backcross method retains all desirable charters of a popular adapted variety and replaces undesirable allele at a particular locus.
2. This is a useful method for transfer of oilgogenic character like disease resistance. It is also useful in the incorporation of genes for quality such as protein content.
3. This method is extensively used in the development of varieties with multiple disease resistance. Multiline varieties carrying resistant genes for different races of pathogen are also developed by backcross method. This is used for development of isogenic lines and multiline variety, a mixture of several isogenic lines.
4. The male sterility and fertility restorer genes are transferred to various agronomic bases by this method.
5. This is the only breeding method which is used for interspecific gene transfer.
6. The variety developed by this method does not require multi-locational testing, because it is identical to parent variety except for the character under transfer.
Demerits:
1. This method is used to rectify the defect of an adapted variety. The new variety differs from the old one only in respect of defect which has been rectified.
2. It involves lot of crossing work. The backcrosses have to be made for 6-8 generations. In pedigree and bulk methods hybridization is done only once.
3. Sometimes, undesirable character is tightly linked with desirable one, which is also transferred to the new variety.
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