Linkage and crossing over

 The tendency of one cross over to inhibit another cross over in its adjacent region is called interference. The term interference was used by Muller. Sometimes, the occurrence of crossing over in one region enhances crossing over in adjacent regions, this phenomenon is called negative interference. The degree of interference may vary in different regions. The lesser the distance between genes, the greater is the interference and vice versa. Interference is calculated as Co-efficient of interference (%) = 1-coefficient of coincidence X 100. The term coincidence was used by Muller to explain the degree or strength of interference. The coefficient of coincidence is calculated by If the observed double cross-over is equal to expected, the coefficient of coincidence will be 1 and zero interference. If the observed double cross over is less than expected, the coefficient of coincidence will be less than 1 and interference will be positive If the observed double cross-over is more than e...

 As the distance between genes increases, the possibility of double cross-over increases. When the two loci are farther apart on a chromosome, double crossovers occur between them and will tend to mask the recombinants. In a two-point test cross, some parental combinations result from double crossovers. These crossovers could not be identified as they do not produce recombinant gamete and progenies, all progenies will be of a parental type and the recombination frequency will become 0%. Hence it underestimates the actual distance between the genes. This problem can be solved by including another marker (gene) between distantly located genes so that the recombinants can be identified. Hence three-point test cross is preferred for chromosome mapping. An important feature of all linkage maps is their linearity i.e., all genes in a given linkage group can be shown to map in a linear array. Let us presume that there are three genes A, B, and C present on the same chromosome (i.e., they ...

 A map is a line diagram consists of three things; the components, their relative position, and relative distance between them. Similarly, a chromosome map is a line diagram consists of the genes, their relative position, and the distance between these genes. The process of assigning genes on a chromosome is called chromosome mapping. Scientists use several methods to map genes to the appropriate locations. These methods include family studies, somatic cell genetic methods, cytogenetic techniques, and gene dosage studies. Family studies are used to determine whether two different genes are linked close together on a chromosome. If these genes are linked, it means they are close together on the same chromosome. Additionally, the frequency with which the genes are linked is determined by recombination events (crossing over of the chromosomes during meiosis) between known locations or markers and determines the linear order or genetic distance. Because the frequency of crossing over b...

 Crossing over is useful in three principal ways, viz: • Creation of variability, • Locating genes on the chromosomes, and • Preparing linkage maps Creation of variability Crossing over leads to recombination or new combination and thus is a potential genetic mechanism for creating variability which is essential for the improvement of genotypes through selection. Locating genes Crossing over is a useful tool for locating genes in the chromosomes. Linkage or chromosome maps Crossing over plays an important role in the preparation of chromosome maps or linkage maps. It provides information about the frequency of recombinations and sequence of genes that are required for the preparation of linkage maps.

 The frequency of crossing over is influenced by several factors which are briefly discussed below i. Distance- The distance between genes affects the frequency of crossing over. The greater the distance between genes is the higher is the chance of crossing over and vice versa. ii. Age- Generally crossing over decreases with advancement in the age in the female Drosophila. iii. Temperature- The rate of crossing over in Drosophila increases above and below the temperature of 22°C. iv. Sex- The rate of crossing over also differs according to sex. There is a lack of crossing over in Drosophila male and female silk moths. v. Nutrition- The presence of metallic ions like calcium and magnesium in the food caused a reduction in recombination in Drosophila. However, the removal of such chemicals from the diet increased the rate of crossing over. vi. Chemicals- Treatment with mutagenic chemicals like alkylating agents was found to increase the frequency of crossing over in Drosophila female...

 Crossing over is a process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage, each chromosome has replicated into two strands called sister chromatids. The two homologous chromosomes of a pair synapse or come together.\ While the chromosomes synapse, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome. Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes, i.e., alleles. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell. Crossing over results in the recombination of genes found on the same chromosome, called linked genes, that would otherwise always be transmitted together. 1. Synapsis Du...

 The first cytological evidence in support of genetic crossing over was provided by Curt Stern in 1931 on the basis of his experiments conducted with Drosophila. He used cytological markers in his studies. He selected a female fly in which one X-chromosome was broken into two segments. Out of these two segments, one behaved as X-chromosome. The other X-chromosome had a small portion of Y-chromosome attached to its one end. Thus, both the X chromosomes in the female had distinct morphology and could be easily identified under a microscope. In female fly, the broken X-chromosome had one mutant allele (carnation) for eye color and another dominant allele (B) for bar eye shape. The other X-chromosome with an attached portion of the Y chromosome had alleles for normal eye color (red eye) and normal eye shape (oval eye). Thus, the phenotype of females was barred. A cross of such females was made with carnation male (car+). As a result of crossing over female flies produce four types of g...

 Depending upon the number of chiasmata involved, crossing over may be of three types, viz., single, double and multiple i. Single Crossing Over- It refers to the formation of a single chiasma between non-sister chromatids of homologous chromosomes. Such cross-over involves only two chromatids out of four. ii. Double-crossing over It refers to the formation of two chiasmata between non-sister chromatids of homologous chromosomes. Double crossovers may involve either two strands or three or all four strands. But only two strands involve in the crossing-over process at each cross-over point. Again, there are two types in this; complementary and reciprocal. In reciprocal crossing over the same two strands involve in both the cross overs and in complementary cross over different pairs of chromatids are involved in both cross overs. iii. Multiple crossing over - The presence of more than two crossovers between non-sister chromatids of homologous chromosomes is referred to as multiple cr...

 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. Significance of linkage in plant breeding In plant breeding, the linkage has an impact in three principal ways. (i) Effect on selection- Linkage between two or more loci controlling different desirable characters is advantageous for a plant breeder. Because desirable alleles will occur together more frequently in segregating populations than would ...