Essentials of Anatomy and Physiology

woman who has one of these recessive genes on one X
chromosome and a dominant gene for normal func-
tion on the other X chromosome will not express this
trait. She is called a carrier, however, because the gene
is part of her genotype and may be passed to children.
Let us use as an example, red-green color blind-
ness. The Punnett square in Fig. 21–10 shows that
Mom is carrier of this trait and that Dad has normal
color vision. A Punnett square for a sex-linked trait
uses the X and Y chromosomes with a lower case let-
ter on the X to indicate the presence of the recessive
gene. The possibilities for each child are divided

equally into daughters and sons. In this example, each

daughter has a 50% chance of being a carrier and

a 50% chance of not being a carrier. In either case, a

daughter will have normal color vision. Each son has

a 50% chance of being red-green color blind and a

50% chance of having normal color vision. Men can

never be carriers of a trait such as this; they either have

it or do not have it.

The inheritance of other characteristics is often not

as easily depicted as are the examples just discussed.

Height, for example, is a multiple gene characteristic,

meaning that many pairs of genes contribute. Many

pairs of genes result in many possible combinations

for genotype and many possible phenotypes. In

addition, height is a trait that is influenced by envi-

ronmental factors such as nutrition. These kinds of

circumstances or influences are probably important in

many other human characteristics.

Of the estimated 20,000 to 25,000 human genes,

almost all have been “mapped.” This means that they

have been precisely located on a particular chromo-

some. There is still much work to be done, however,

and the focus now is on learning what each of these

490 Human Development and Genetics

Table 21–4 HUMAN GENETIC DISEASES

Disease (Pattern

of Inheritance) Description

Sickle-cell anemia (R)

Cystic fibrosis (R)

Tay-Sachs disease (R)

Phenylketonuria

or PKU (R)

Huntington’s

disease (D)

Hemophilia (X-linked)

Duchenne’s muscular

dystrophy (X-linked)

Rrecessive; D dominant.

• The most common genetic disease among people of African ancestry. Sickle-cell hemoglo-
  bin forms rigid crystals that distort and disrupt RBCs; oxygen-carrying capacity of the blood
  is diminished.


• The most common genetic disease among people of European ancestry. Production of thick
  mucus clogs the bronchial tree and pancreatic ducts. Most severe effects are chronic respira-
  tory infections and pulmonary failure.


• The most common genetic disease among people of Jewish ancestry. Degeneration of
  neurons and the nervous system results in death by the age of 2 years.


• Lack of an enzyme to metabolize the amino acid phenylalanine leads to severe mental and
  physical retardation. These effects may be prevented by the use of a diet (beginning at
  birth) that limits phenylalanine.


• Uncontrollable muscle contractions begin between the ages of 30 and 50 years; followed by
  loss of memory and personality. There is no treatment that can delay mental deterioration.


• Lack of Factor 8 impairs chemical clotting; may be controlled with Factor 8 from donated
  blood.


• Replacement of muscle by adipose or scar tissue, with progressive loss of muscle function;
  often fatal before age 20 years due to involvement of cardiac muscle.

Table 21–5 ABO BLOOD TYPES:

GENOTYPES

Blood Type Possible Genotypes

OOO

AB AB

A AA or OA

B BB or OB