HUMAN BIOLOGY

378 Chapter 19

Rapid destruction of

sickle cells leads to anemia

Heart failure

Kidney failure

Abdominal pain

Paralysis

Heart failure

Pneumonia

Weakness

and fatigue

Impaired

mental function

Homozygous recessive individual

Abnormal hemoglobin

Sickling of red blood cells

Clumping of cells and

interference with blood

circulation leads to local

failures in blood supply

Normal HbA Sickle-cell HbS

val

his

leu

thr

pro

glu

glu

val

his

leu

thr

pro

val

glu

One amino acid

substituted in

hemoglobin

sickle cell

normal cell

or observable trait. Usually, however, the genetic founda-

tion of many human traits is more complicated.

one gene may affect several traits

Sometimes expression of a gene has effects on two or

more traits. This wide-ranging impact of a single gene is

called pleiotropy (ply-ah-trow-pee, after the Greek pleio-,

19.4

n    Some traits have clearly dominant and recessive forms. For

most traits, however, the story is not so simple.

n    Links to Red blood cells 8.1, ABO blood groups 8.4

Section 19.1 noted that genes are chemical instructions for

building proteins. A gene is “expressed” when its instruc-

tions are carried out and the cell makes the protein. In some

cases, the expression of a gene leads to a single phenotype,

Figure 19.8 A single genetic change leads to the many physical effects of sickle-cell anemia. A shows how an incorrect amino

acid has been substituted in the chain of amino acids making up the hemoglobin protein. The inset shows how the shape of a

sickled red blood cell differs from that of a normal red blood cell. B Tionne “T-Boz” Watkins, celebrity spokesperson for sickle-cell

anemia organizations. Right: The range of symptoms for a person who has inherited the mutated gene for hemoglobin’s beta chain

from both parents. There may be other effects as well. (© Cengage Learning)

single Genes, Varying effects

Ben Rose/Contributor/Getty Images

A

B

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