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Previous editions of this (and other) textbooks have used the photograph in (a) to illustrate the continuously varying nature of height. In the photo, taken around 1920, 175 cadets at the Connecticut Agricultural College lined up by height. In 1997, Professor Linda Strausbaugh asked her genetics students at the school, today the University of Connecticut at Storrs, to recreate the scene (b). They did, and confirmed the continuously varying nature of human height. But they also elegantly demonstrated how height has increased during the twentieth century. Improved nutrition has definitely played a role in expressing genetic potential for height. The tallest people in the old photograph (a) are 5'9' tall, whereas the tallest people in the more recent photograph (b) are 6'5" tall.

and b, then the lightest color would be genotype aabb; the darkest, AABB. The bell curve arises because there are more ways to inherit light brown eyes, with any two dominant alleles, than there are ways to inherit the other colors.

Traits molded by one or more genes plus the environment are termed complex traits (multifactorial traits). Height and skin color are multifactorial as well as polygenic, because environmental factors influence them: good nutrition enables a person to reach the height dictated by genes, and sun exposure affects skin color. Most of the more common illnesses, including heart disease, diabetes mellitus, hypertension, and cancers, are complex.

H How does polygenic inheritance make possible many variations of a trait?

How may the environment influence gene expression?

How can two genes specify five phenotypes?

matters of sex

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