In Neurospora species of ascomycetes during spores formations, the meiosis takes places in the tight confinement known as ascus. The arrangement of ascospore of this the arrangement of each chromatid of homologous chromosomes during meiosis I and Meiosis II is fixed. By this the distance between the gene and centromere can be predicted. Each ascus has 8 haploid ascospores.

12.1.    Mapping through Tetrad
12.1.1.    First division segregation :

If there is no crossing over between the gene and centromere occurs then the resulting eight ascospore are arranged in four white spores and four black spores order. These black and white spores are observed on either side of imaginary line separating the 4th and 5th ascospores. This is known as first division segregation.
There are two types of possible arrangements in first division segregation.

12.1.2.    Second division segregation : 
If a crossing over occurs between the centromere and a gene then a second division segregation pattern is observed. Now both white and black types of spores are found on either side the imaginary line between the 4th and 5th ascospores. There are four types of possible arrangements in second division segregation. 

    Un Order Tetrad:- Produces three types of spaces
1)    Parental Di type – When the complete octad has both types of parental spores equal in number.
2)    Non-parental different type – An octad containing four recombinant haploid spores. 
3)    Tetratype – A octad containing four different types of spore. In tetratype 50% of the gametes are parental and 50% of the gametes are non-parental i.e. recombinant.

The distance between two genes is calculated d by

12.2.    The drawback in linkage mapping
1    The linkage mapping is based upon crossing over frequency and the frequency of crossing over between two genes is not fixed, it's highly variable. There are many factors that affect the crossing over frequency between two genes.
2     The linkage map is a genetic map. It is not a physical map. Distances on genetic maps are measured in map units (abbreviated map unit); one map unit equals 1% recombination. Map units are also called centiMorgans (cM), in honour of Thomas Hunt Morgan. 100 centiMorgans equals one Morgan. Genetic distances measured with recombination rates are approximately additive: if the distance from gene A to gene B is 5 map unit and the distance from gene B to gene C is 10 map unit, then the distance from gene A to gene C is 15 map unit. Thus gene B must be located between genes A and C. On the basis of the map distances given, we can draw a simple genetic map for genes A, B, and C, as shown here:

3    The distance between genes in the linkage map is given in Morgan or centiMorgan. There is no relation between Morgan and metres. 
4    Recombination frequencies less than 20 percent estimate map distance very accurately; however, recombination frequencies greater than 20 percent underestimate map distance because multiple crossover events do not always produce recombinant chromosomes. Two crosses over between genes produce parental. And as distance increase between genes frequency does not increase in that proportion.

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