HUMAN BIOLOGY

iNtroDUCtioN to GeNetiCs 381

wHy are some pHenotypes unpredictable?

• Observed traits can be difficult to predict when expression of the
  underlying gene or genes can vary.


• Examples include alleles that are incompletely penetrant and
  polygenic traits that result from the combined expression of two
  or more genes.


• With complex, multifactorial traits, the phenotype is shaped by
  more than one gene as well as by environmental factors.

taKe-Home messaGe

Figure 19.12 Animated! Earlobe attachment and height are examples of human traits that show continuous variation. A Human
earlobes run the gamut from being fully detached from the skin of the neck to being fully attached to it. B Female biology students
demonstrate continuous variation in height. Height is also a multifactorial trait because it is influenced by more than one gene as well
as by environmental factors, such as nutrition.

difficult to sort out these issues, many of the “good lifestyle

choices” you hear so much about these days—from not

smoking to eating a healthy diet and reducing stress—are

recommendations for ways each of us can limit the chances

a harmful gene or genes will be expressed.

the environment can affect phenotypes

There are many examples of human traits in which the
environment can help determine the exact phenotype
we observe. As just described, a person’s adult height is

programmed to a great extent by genes, but diet also may
play a role. If a young child’s diet is deficient in protein, or
a disease or injury prevents the normal release of growth
hormone, then the person will be shorter than his or her
genetic potential would allow. Tanning darkens the natural
color of a person’s skin, a change that may become perma-
nent in extreme cases. Heavy sun exposure also makes
inherited freckles more pronounced.
Many of us inherit genetic predispositions for certain
conditions, such as some cancers, obesity, hypertension,
even alcoholism and depression. Today a great deal of
scientific research is aimed at discovering the degree
to which genes and environmental factors contribute to
developing one of these health problems. Because it can be

5/35/25/15/04/11 5/4 5/5 5/6 5/7 5/8 5/9 5/10 5/11

Height (feet/inches)

Courtesy University of Florida News and Public Affairs

A © ruslanchik/Shutterstock.comB

© Hywit Dimyadi/Shutterstock.com © aastock/Shutterstock.com

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).