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What really happens in a cross

Our previous example shows a dihybrid cross, where we observe the pairings of genes that underlie two different traits.  What would the predicted outcome of a cross look like with three genes (Q/q; S/s; T/t)?  I wrote a computer program to determine the result:

 


  QST QSt qsT qst QsT Qst qST qSt
QST QQSSTT QQSSTt QqSsTT QqSsTt QQSsTT QQSsTt QqSSTT QqSSTt
QSt QQSSTt QQSStt QqSsTt QqSstt QQSsTt QQSstt QqSSTt QqSStt
qsT QqSsTT QqSsTt qqssTT qqssTt QqssTT QqssTt qqSsTT qqSsTt
qst QqSsTt QqSstt qqssTt qqsstt QqssTt Qqsstt qqSsTt qqSstt
QsT QQSsTT QQSsTt QqssTT QqssTt QQssTT QQssTt QqSsTT QqSsTt
Qst QQSsTt QQSstt QqssTt Qqsstt QQssTt QQsstt QqSsTt QqSstt
qST QqSSTT QqSSTt qqSsTT qqSsTt QqSsTT QqSsTt qqSSTT qqSSTt
qSt QqSSTt QqSStt qqSsTt qqSstt QqSsTt QqSstt qqSSTt qqSStt

 

OK, big deal, you say.  A few minutes’ work with a pencil and paper could produce this chart.  Well, then let’s try four different genes (Q/q; S/s; T/t; V/v):

 

Image 


A bit more difficult to do on  paper, wouldn't you agree?

I would show ten genes, but the program would need to run for many hours to produce the gigabyte-sized result.  Plus, it might crash your computer to download the massive thing. 

 

Now, let’s get back to nature: what really happens during a cross?  Plants like orchids can have many thousands of genes.  Humans have around 30,000 genes.  So in a human cross, you’d have 230,000 allele combinations in each parent, making the total number of potential combinations in a cross 230,000 x 230,000.  Try writing all those genotypes on a piece of paper!

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