## Quantitative genetics - What statistical tools are used by quantitative geneticists?

Covariance

Imagine the individuals have been measured for two characters, X and Y . The covariance between the two is defined as

cov(x,y) = (1/(n-1)) ∑ x_{i}y_{i}

Covariance measures whether, if an individual has a large value of X it also has a large value for Y . If the x_{i} and y_{i} of an individual are both large, the product x_{i} y_{i} will also be large; if y_{i} is not large when x_{i} is, the product will be smaller. Generally, if X and Y covary, the product (and so the covariance) is large; and if they do not, the sum of the products will come to zero.

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