Testcrosses
Assume the R marker for red aleurone color is located
on the shortest maize chromosome which is chromosome
10. We have ten different trisomic genetic stocks. Each
stock is trsomic for a different chromosome.
Let us testcross the genetic stock which is 2n+1 and
trisomic for chromosome 9 to a diploid (2n) tester which
is homozygous recessive for the R-factor.
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R |
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male |
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9 |
r |
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10 |
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x |
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r |
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female |
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r |
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The
resulting segregation
ratio is 1:1 This 1:1 ratio
will be true for trisomic
stocks 1-9.
Now let us testcross the genetic stock which is trisomic
for chromosome 10 and heterozygous for the R locus to
an rr diploid plant used as a female seed parent.
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R |
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R
male |
| 1 |
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8 |
9 |
r |
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10 |
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r |
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female |
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r |
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RR |
Rr |
Rr |
R |
R |
r |
| r |
non-viable |
non-viable |
non-viable |
Rr |
Rr |
rr |
For the genetic stock which is trisomic for chromosome
10, the segregation ratio will be 2:1. This distorted
segregation ratio is evidence that the R locus is located
on chromosome 10.
