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Last Modified on 2/13/05
wmitchel@ramapo.edu

Linear Regression

A streight line can be drawn between any two points. If you have more than two data points, and the points aproximate a line, than a line can be drawn even though the data is not perfectly linear.

Example:

On a horizontal agraose gel for separating DNA you run two DNA samples. One sample contains DNA size markers of 2000, 1500, 1000, 500, 200 and 100 bp, and the other sample contains one kind of DNA of unknown size. When the gel is electotroporesed, stained and viewed we see that the marker bands migrated 0.4, 0.6, 1.1, 2.0, 2.4 and 2.8 cm. The unknown band migrated 1.6 cm. How big is the unknown DNA?

Answer:

We know from molecular biology that the migration of DNA molecule is inversely proportional to the log of the size. This can be restated as log [size] = slope [migration] + y-intercept, where the slope is negative.

Fill out a table for just the DNA marker data.

Use a linear regression program on a calculator or computer to enter the data points of the molecular markers to obtain the slope and y-intercept of the standard curve.

log[size] = -0.5072[migration] + 3.532

Insert the migration of the unknown DNA and solve for log[bp].

log[size] = -0.5072[1.6] + 3.532

log[size] = 2.720

Solve for size in bp.

size = antilog 2.720

size of unknown DNA = 525 bp

Dr. Petro has a web page showing how to use Excell for a linear regression
http://orion.ramapo.edu/%7Espetro/labs/excel2000.htm