genes is for; that is, the protein the gene codes for and
the function of that protein.
Someday it will be possible to cure genetic diseases
by inserting correct copies of malfunctioning genes
into the cells of affected individuals. As of 2006, cer-
tain immune deficiencies (which previously would
have been fatal) seem to have been reversed in several
children by supplying correct copies of their faulty
genes. But these early successes have not been repli-
cated, and replication of results is an essential part of
the scientific method. Research must proceed slowly,
because human beings are the subjects, and so that any
potential risks of such therapy may be assessed and
reduced.
Other diseases that may eventually be cured or
controlled with gene therapy include cystic fibrosis
and hemophilia (for both of which clinical trials are
continuing), Parkinson’s disease, diabetes, muscular
dystrophy, and sickle-cell anemia. Much more
research and careful experimentation remain to be
done before gene replacement becomes the standard
treatment available to everyone with these genetic dis-
eases. This is a good example of how science must
work when results fall short of expectations.
Researchers and clinicians must pause and reevaluate
the underlying principles of their work and the meth-
ods they have chosen, decide what changes are neces-
sary, and try again.492 Human Development and Genetics
BOX21–7 SOLUTION TO GENETICS QUESTION
Question: Can parents who are both heterozygous for brown eyes have four children with blue eyes?
What is the probability or chance of this happening?
Answer: Yes. For each child, the probability or chance of having blue eyes is 1 in 4 (or 25%). To cal-
culate the probability of all four children having blue eyes, multiply the probability for each
child separately:
1st child 2nd child 3rd child 4th Child(^1) /
4
(^1) /
4
(^1) /
4
(^1) /
4
(^1) /
256
The probability is 1 in 256 (or 0.4%).
Table 21–6 HUMAN GENETIC TRAITS
Trait Dominant Phenotype Recessive Phenotype
ABO blood type AB, A, B O
Rh blood type Rh positive Rh negative
Hair color Dark Light (blond or red)
Change in hair color Premature gray Gray later in life
Hair texture Curly Straight
Hairline Widow’s peak Straight
Eye color Dark Light
Color vision Normal Color blind
Visual acuity Nearsighted or farsighted Normal
Skin color Dark Light
Freckles Abundant Few
Dimples Present Absent
Cleft chin Present Absent
Ear lobes Unattached Attached
Number of fingers/toes Polydactyly (more than 5 digits) 5 per hand or foot
Mid-digital hair Present Absent
Double-jointed thumb Present Absent
Bent little finger Present Absent
Ability to roll tongue sides up Able Unable