this cross...PLS TELL ME HW THIS CROSS COMES???? I DNT UNDERSTAND THIS CROSS AT ALL...ALL THE OTHER CROSSES R OK. THERE IS NO EXPLAINATION IN TB..WHICH IS DOMINATE..ETC...Y IS THERE A YELLOW AD VIOLET IN THE SAME CHROM. ?? pls explain in SIMPLE LANGUAGE. in detail hw each comes...meritnation video(study m) doest give this one too..... :(

The detailed figure (figure-5.11) of two crosses carried out by Morgan, given in NCERT, chapter -5, page -84, explains about linkage.
 
F1 progenies were obtained in each cross, which were inter-crossed.
•F2 progenies were obtained and F2 ratio was observed.
•F2 ratio was observed to be significantly different from 9:3:3:1 as observed in Mendelian dihybrid cross, due to physical association of genes on a chromosome ( linkage)

 
 The chromosomes  with particular distance between  genes placed on them  are shown in  each of the crosses A and B.  Yellow with dot chromosomes are X chromosomes and blue chromosome with dot  are X chromosomes in males , dark blue chromosomes are Y chromosomes in males.  Crosses A and B are done to show  how difference in distance between genes can make a difference in  ratio of parental and recombinants types.
 
If crossing over does not occur, the products are parental gametes. If crossing over occurs, the products are recombinant gametes( non parental-wild gene with non wild gene).
Alleles tightly linked and present on same chromosome have higher chances of staying together as they have no chances of separating by crossing over and are  transmitted together.


 
Due to independent assortment yellow and purple chromosomes have come up together.
 
 
 Cross A (cross between  chromosome with genes yellow white and other chromosome with wild type- dominant alleles) shows less recombination frequency because two genes(y and w- white and yellow) are tightly linked ( with small distance between them) and located on the same chromosome. The proportion of parental gene combinations  is much higher than those of non-parental ( yellow with wild type and other is white with wild type gene due to crossing over) in F2 generation  and showed only 1.3 per cent recombination.

Cross B ((cross between  chromosome with genes white miniature and  other chromosome with wild type  - dominant alleles) shows high recombination frequency of two genes (w and m- white and miniature wings) because they  are loosely linked. This cross showed  37.2 per cent recombination (miniature with wild type and other is white with wild type gene due to crossing over)in F2 generation  .


Thus, the strength of linkage between genes y and w is higher than w and m.

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