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# Calculating the chi squared value for a monohybrid cross

Gregor Mendel discovered that tall was dominant and short recessive in pea plants. When he crossed two plants that were both heterozygous tall (Tt) he found a 3:1 ratio of tall to short (75% tall, 25% short) in the offspring leading him to conclude that in a monohybrid cross, the offspring would show a 3:1 ratio of dominant to recessive phenotypes. A student in a biology lab wants to know if plant flower color is also determined by simple Mendelian genetics. First she crosses a true breeding white flowering plant with a true breeding red flowering plant and observes that the offspring all produce red flowers. She then crosses two of these red flowering plants and records the phenotypes of the offspring. Analyze her data below using Chi square to determine if her results are similar to MendelĂ˘??s 3:1 ratio. Calculate the Chi square, Critical Chi square value at P=0.05.

Red Offspring White Offspring
81 29

#### Solution Preview

Chi squares is a mathematical formula that is used to determine is the validity of our hypothesis. In this case, we can use it to determine if the trait is Mendelian.

Here are the equations we will need:
x^2= Sum of (Observed-Expected)^2/expected for each phenotype

Df (degrees of freedom) = N-1 where N = the number of phenotypes expected

Let's start by assuming that plant flower color is a simple Mendelian trait caused by a single gene with two alleles (R = red, r = white) in a dominant/recessive ...

#### Solution Summary

Details explanation on how to calculate chi squared to determine whether or not a monohybrid cross contains Mendelian traits.

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