Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)

Content
Linkage and Crossing Over in Diploid Organisms (Higher Eukaryotes)
Coupling and repulsion hypothesis
A testcross in maize
Crossing over and meiosis 
Crossing over and chiasma formation
Mechanism of genetic recombination
Crossing over and linkage maps 
Recombination frequencies from a test-cross
Recombination frequencies from F2 data
Interference and coincidence
Linkage maps
Mapping function and poisson distribution
Linkage groups
Chi-square test 
Cytological basis of crossing over
Creighton and McClintock's experiment in corn
Meselson and Weigle's experiment using lambda (λ) phage
Crossing over at four strand stage

Law of independent assortment suggests that when two or more than two factors are considered together, these factors would show independent and random assortment during distribution into gametes. For instance, in an individual with genotype AaBb, if two factors undergo independent assortment during meiosis, four types of gametes viz. AB, Ab, aB and ab will be produced in equal proportions (1:1:1:1). Dihybrid F2 ratio of 9:3:3:1 and testcross ratio of 1 : 1 : 1 : 1 in diploid organisms are actually based on this general principle of independent assortment. A detailed study of meiosis will suggest that this independent assortment of characters is based on independent assortment of non-homologous chromosomes. It is, therefore, necessary that if two characters have to assort independently, these should be located on separate non-homologous chromosomes. However, there is no reason to believe that not more than one gene can be located on the same chromosome. When we are faced with such a situation, where two or more than two genes are located on same chromosome, chromosome theory of inheritance would expect that independent assortment will not take place (except when genes are separated by lbng distances on the same chromosome). This kind of situation in diploid organisms will be discussedin this section. Related phenomena in haploid organisms will be discussed in the next main topic.