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BIO dihybrid crosses linkage help?

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For Bio, I realize my main issue revolves around the usage of dihybrid crosses and punnet squares, mainly involving linkage. Does anyone know any links to good explanations or examples of dihybrid linkage crosses?

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Do you not understand about the whole concept of dihybrid linkage crosses?  I will try to find links and post it here as soon as possible. 

Edited by inriya

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When the two gene loci are on the same chromosome then we must consider the event of crossing over between them. We will examine the gametes formed by an individual that has the genotype AaBb. Writing the genotype will allow the person and you to know that this is dihybrid genes with two or more characteristics controlling the phenotype. 

Illustration of how to draw a punnet square for linked gene:  For example: Let's say we cross two cats with two different characteristics such as fur colour and fur length. For this example we will be using the alleles as follows:

  • S = allele for long fur
  • s = allele for short fur
  • B = allele for black fur
  • b = allele for brown fur

Then in the exam, write as many combination of the allele as possible, (look at the question if it said linked gene with crossing over, if it said linked gene, I will assume there is crossing over) 


In linked gene, we must also notice if there is crossing over or not. If there is no crossing over, it will produce two types of gamete will result: ½ AB and ½ ab 

If there is crossing over, like sex chromosomes (at times) [A with D and a with d] , we will produce 1/4 AD,  1/4 ad, 1/4 Ad  and 1/4 aD (two recombinants because of crossing over in prophase 1 in meiosis). 


In summary:

1.  If the two genes A and B are on different chromosomes and independent assortment occurs (SL syllabus of meiosis in Chapter 3):  ¼AB, ¼ Ab, ¼ aB, ¼ ab

2. If the two genes A and B are linked (A with B and a with b) and no crossing over occurs: ½ AB, ½ ab

3. If the two genes A and B are linked (A with B and a with b) and crossing over occurs: Ab, AB, ab, aB  (bold as original, unbold as recombinant genes - the reason why I did not put any ratio in front is because it all depends on the frequency of crossing over, which is determined by.... SPACE or known as "loci widely spread". If it is widely spread, then we will have higher chances of recombinant frequency. I wish I have a graph for it (I can't find it anywhere) but I am sure you can imagine it. ) 

Sources (Copyright Issue, duh, hope I won't break any rules here):

Recommended reading source + hearing plus watching source, bring a pen and paper to try to do it: 

 Hope this will help you. If you have any question, send me a message.



Edited by inriya

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