DNA sample
is derived from a homozygous wild-type, heterozygous, or homozygous mutant
individual. One way of calling alleles is by comparing Ct values. In a
sample containing the wild-type alleles only, the TET signal should be
relatively strong and should appear in earlier amplification cycles (Ct
= 26), whereas the FAM signal should be at background level (Ct = 40;
Figure 3 ). In the case of a heterozygote, both FAM and TET signals should
be seen, but the Ct values should be slightly higher than that for the
homozygote wild type or mutant because only one allele each (i.e., the
wild-type and the mutant allele) is detected. The Ct values for the heterozygote
were 27 (TET) and 27 (FAM) (
Figure
3).
Another method for calling alleles is to examine the fluorescence at
the last annealing stepin this case, at cycle 40. The final fluorescence
provides a qualitative readout because components in the reactions might
be limiting at the end of PCR. However, when the proper dilutions of target
DNA are included in the reaction, the final fluorescence is often related
to the amount of the wild-type or mutant allele in the sample (cycle 40;
Figure 3) and can be easily extrapolated by the software of the real-time
reader (Table
3). In this analysis, the wild-type sample had a TET signal of 2.2
(normalized endpoint fluorescence), the mutant sample had a FAM signal
of 1.34, and the heterozygote showed approximately one-half of the TET
or the FAM signal.
Table 3
Allelic Discrimination by Endpoint Fluorescence Values
FAM (mutant)
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Page: All 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Related biology technology :1.
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