The importance of genetic background in the study of mutant, characters.<p>

Butler, L.<p>
     
Several research workers have suggested that the tomato mutant genes be placed
on a uniform genetic background so that phenotypes would become more
standardized and genetic and environmental interactions could be more easily
studied. The effect of genetic background was brought home to me very clearly
in the results obtained by my genetics class this fall.  The students planted
seed from a Xa Xa^+ plant and were told to expect a 2 : 1 ratio but to watch
for the XaXa plant which would germinate and then the in the cotyledon stage
illustrating lethal effects.  Much to my surprise all the expected lethals
germinated giving a 1 : 2 : 1 ratio in constrast to the results of other
years.  The contrasting segregations of the last two years are given bellow:<p>

<pre>
        Xa Xa          Xa Xa^+          Xa+ Xa+
1954     22             520              268
1955    217             387              215
</pre>
The seed used in 1954 was from the standard mutant stock while that used in
1955 was frcm an F2 selection.  Phenotypically there was no difference between
the two parent plants.  But evidently Xa Xa plants with one genotypic
background could survive and germinate more readily than those with the other
genotypic background.<p>


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