Thus in both cases linked genes tend to remain together during hereditary transmission. Haldane used the terms cis and trans for coupling and repulsion, respectively. Types of Linkage of Genes : Linkage is generally classified on the basis of three criteria, viz:. Linkage in which crossing over does not occur is known as complete linkage or absolute linkage.
In other words, when only parental types are obtained from the test cross progeny, it refers to complete linkage. Good example of complete linkage is Drosophila male and female silk moth.
If some frequency of crossing over also occurs between linked genes, it is known as incomplete linkage. Incomplete linkage has been observed in maize, pea, Drosophila female and several other organisms. It refers to linkage either between dominant genes or between recessive genes. Such linkage has been reported in pea, maize and several other crops. It refers to linkage of some dominant genes with some recessive genes.
This type of linkage has also been observed in pea, maize and several other crops. Linkage Groups : Linkage group refers to a group of genes which are present in one chromosome.
In other words, all those genes which are located in one chromosome constitute one linkage group. The number of linkage groups is limited in each individual. The maximum number of linkage groups is equal to the haploid chromosome number of an organism. However, in case of species having dissimilar sex chromosomes, linkage groups are one more than the haploid chromosome number.
For example, there are ten linkage groups in corn, 7 in garden pea, 7 in barley, 4 in Drosophila melanogaster and 23 in man. Genes are assigned to various chromosomes of a species with the help of deletion, monosomic and nullisomic analyses. Linkage groups are assigned to different chromosomes in a linear fashion and same sequence as they normally list.
Generally, a relative length of various linkage groups in a species exhibits a close agreement with the relative length of chromosome in which they exist.
Detection of Linkage of Genes : Test cross is the most common method of detecting the linkage. In this method, the F 1 heterozygous at two loci say AaBb is crossed to a double recessive parent aabb and the phenotypic ratio of test cross progeny is examined.
If the phenotypic ratio of test crosses progeny shows ratio of parental and recombinant genotypes, it indicates absence of linkage.
If the frequency of parental types and recombinant types deviate significantly from the normal test cross ratio of 1 : 1 : 1 : 1, it reveals presence of linkage between two genes under study.
There is another way to detect the presence or absence of linkage. The individual heterozygous at two loci AaBb is self-pollinated. If there is complete dominance at each locus and no epistasis, the segregation ratio of the progeny will be 9 : 3 : 3 : 1. Presence of linkage either in coupling or repulsion phase will lead to significant deviation from ratio. Linkage and Pleiotropy : A close association between two or more characters may result either due to linkage or pleiotropy or both.
Pleiotropy refers to the control of two or more characters by a single gene. A tight linkage between two loci can be confused with pleiotropy. The only way to distinguish between linkage and pleiotropy is to find out a crossover product between linked characters.
Inter-mating in segregating populations may break a tight linkage, but a huge population has to be raised to find out the crossover product. If a crossover product is not found in-spite of repeated inter-mating, there seems to be the case of pleiotropy rather than linkage. Linkage Significance in Plant Breeding : In plant breeding, linkage has impact in three principal ways.
Effects of Linkage on the progress of selection, genetic variances and genetic correlation are discussed below:. Linkage between two or more loci controlling different desirable character is advantageous for a plant breeder. Because desirable alleles will occur together more frequently in segregating population than would be expected with independent assortment. Thus the breeder can easily obtain individuals with desirable alleles for two characters.
Linkage is undesirable when desirable and undesirable genes are linked together. To obtain sergeant with desirable alleles, breeder has to grow larger populations than with unlinked loci.
The estimates of genetic variances for quantitative characters are greatly influenced by the presence of linkage. Linkage disequilibrium influences the gene action by way of causing bias in the estimates of genetic variances. Linkage in repulsion phase in segregating population causes an upward bias in the estimates of dominance variance and downward bias in the estimates of additive genetic variance.
Linkage has great impact on the genetic correlation. The linked characters generally show high values of genetic correlation and also of co-heritability.
A linkage between genes controlling two different desirable characters will help in simultaneous improvement of both characters. If there is no linkage between two desirable characters, the breeder has to combine such characters from two different sources.
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