Biology Reference
In-Depth Information
proportion of the total variance observed in a sample of phenotypes
that can be explained by dif erences in genotype. Variance is a measure
of the breadth of a distribution of phenotypes around the average. For
example the set of numbers {4, 4, 5, 6, 6} has a mean (average) value of
5. So does the set {3, 4, 5, 6, 7}. But which one has more variance? h e
second set, of course.
Imagine the population of Europe. When you travel from country to
country, you notice that people look somewhat dif erent. h is is still
true today even with modern globalization. When I took my i rst trip
to the Netherlands, I was struck by how tall people were. Walking
through the train station at 1.75 meters, I felt short. In fact, men and
women in the Netherlands constitute the tallest population on earth.
Men on average are 1.84 meters tall (Wikipedia); women on average are
1.70 meters tall. If you randomly sampled the heights of men across the
Netherlands, you would have a distribution with a broad range. Some
of that range would be due to developmental events inl uenced by diet,
disease, or other factors, but some of it would be due to the genes they
inherited from their parents. h ere are statistical methods that allow
you to estimate the proportion of the variance in height that is due to
the height of the parents, the heritable portion, if you are able to sample
the parents as well as their of spring. h e heritable portion can be further
subdivided into additive and nonadditive parts. h e additive portion
is called heritability in the narrow sense and is the additive ef ect of
each allele at each quantitative gene; the nonadditive ef ect includes
all the interactions of the genes and the interactions of the genes with
the environment.
You could ask what proportion of the variance in height is due to be-
ing a male or female. You would i nd that gender is a signii cant con-
tributor to the distribution of heights in the total, combined popula-
tion. Or you could take a random sample of men from New York City
who came from immigrant French and Dutch families. French men
have an average height of 1.74 meters. h en ask the question: “What pro-
portion of the total variance in the sample is a result of the men being
from French or Dutch parents?” In that case, both the genetic comple-
ment of the populations and the shared environmental ef ects of grow-
ing up in New York City would be responsible. h
is is not heritability in
