Human Genetics
Timothy G. Standish, Ph. D.
©1998 Timothy G. Standish
The Madness of King George III
Partly as a result of the erratic behavior of King
George III the American colonies decided to break
away from the United Kingdom
 Other members of King George’s family also
exhibited strange behavior with dire consequences.
These included Mary Queen of Scots and her son
James I both of whom were beheaded.
 Because madness seems to have run in the family, it
is thought to have a genetic basis
 Acute intermittent porphyria seems to be consistent
with the symptoms exhibited by George III

©1998 Timothy G. Standish
Human Heredity Is Not Unique
The genes of humans behave in the same way as
genes of other organisms
 Of the estimated 100,000 human genes, most are
identical in all humans
 The relatively small number of “polymorphic”
genes in humans account for only part of the
variability that we see between humans
 While each human (except for identical twins)
has a unique set of genetic information, variation
between humans also results from differences in
the environment

©1998 Timothy G. Standish
Politics and Genetics
Because we are talking about how humans are when
we are talking about human genetics, there can be
lots of controversy when traits are seen as relating to
race, gender or other sensitive issues
 This is particularly true when we start to talk about
the genetics of behavior
 Nazis and other extreme rightwing politicians see
human worth and behavior being based on genetics
 Communists and other extreme leftwing politicians
see humans as infinitely pliable and molded by their
environment not genetics

©1998 Timothy G. Standish
Simple Dominant/Recessive Traits
Many human genes are inherited as dominant or
recessive traits just like the traits Mendel studied
in peas
 Ear lobes provide an example of this:

Unattached
ear lobes are
inherited as a
dominant
trait.
Attached ear
lobes are
inherited as a
recessive
trait.
©1998 Timothy G. Standish
Simple Dominant/Recessive Traits

Having a bent little finger is a dominant trait
©1998 Timothy G. Standish
Dominant/Recessive Human Traits
Albinism - There are a number of different types
of albinism, but each is characterized by an
absence of pigment from the skin, eyes and hair.
Albino individuals typically have very pale white
skin, light blue or pink eyes, and light blond or
white hair. Albinism is a recessive trait.
 Brown teeth - Teeth have a brown color that is
not the result of poor oral hygiene or antibiotics
taken as a child. White teeth are dominant.

©1998 Timothy G. Standish
Dominant/Recessive Human Traits
Cleft chin - A noticeable indentation at the center
of the chin. For examples think of Kirk Douglas
and Michael Jackson (after plastic surgery).
Having a cleft chin is dominant to a smooth chin.
 Double-jointed thumbs - This is commonly
called a hitchhiker's thumb. The thumb can bend
back at almost 90 degrees. Hitchhiker's thumb is a
recessive trait, but it may vary in its expression.

©1998 Timothy G. Standish
Dominant/Recessive Human Traits
Hand folding - When the hands are folded either
the left or right thumb will be on top. Left thumb
on top is dominant.
 Mid-digital hair - Hair growing from the middle
section of each finger. Hair presence is dominant.
 PTC tasting - Phenylthiocarbamide (PTC) is a
bitter-tasting chemical that cannot be tasted by
some individuals. PTC tasters taste PTC as bitter,
non-tasters taste nothing when given PTC.
Tasting is dominant to the inability to taste PTC.

©1998 Timothy G. Standish
Dominant/Recessive Human Traits
Tongue rolling - Tongue rolling is the ability to
form a tube with your tongue. Rolling is dominant.
 Widow's peak - A sharp point in the hairline that
points toward the nose. Having a widow's peak is
dominant to a smooth hairline.

©1998 Timothy G. Standish
Sex-Influenced Human Traits
Baldness - Loss of hair from the scalp following
puberty. A sex-influenced trait that is most
commonly fully expressed (as a dominant trait) in
males, but carried on an autosome
 Index finger shorter than ring finger - The
index finger (next to your thumb) is longer than
the ring finger (next to your little finger). Check
the class data to see if the frequency is different
for the different sexes.

©1998 Timothy G. Standish
Sex-Linked Human Traits

Color blindness - Inability to distinguish between
colors of the same intensity. There may be two
types, red-green color blindness in which
individuals perceive red and green as the same
color, and complete color blindness in which all
colors are perceived as being the same and vision
is the functional equivalent of black and white
television. Both types of colorblindness are sexlinked traits.
©1998 Timothy G. Standish
Specific Human Traits
Dimples - Round indentations in the cheeks when
smiling, not lines or clefts.
 Freckles - Small patches of darker pigmented skin
on various parts of the body and most visible in
those areas commonly exposed to the sun. These
may be present on both dark- and light-skinned
individuals.
 Hair whorling - These may be referred to as
cowlicks. Areas in which hair grows from your
scalp in a whorl instead of in a given direction.

©1998 Timothy G. Standish
Specific Human Traits
Myopia - Short sightedness, the inability to see far
off objects without correction by glasses or
contacts
 Polydactyly - Having more than five digits on
each hand and foot.
 S-methyl thioester smeller - S-methyl thioesters
are produced in the urine after consumption of
asparagus. Some individuals cannot smell this
substance. If you smell a strong odor on urination
after eating at least 5 asparagus spears, you are an
S-methyl thioester smeller.

©1998 Timothy G. Standish
Specific Human Traits

White forelock - A white patch of hair at the
front of the scalp.
©1998 Timothy G. Standish
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Multiple Alleles
Eye color is determined by more
than one gene
Thus eye color appears to vary on
an almost continuous scale from
brown to green to gray to blue
Eye color is determined by two
genes, one controls texture of the
iris which refracts light to make
blue. A second determines relative
abundance of melanin. When a
small amount of melanin is
present, green eyes result, while
brown and black eyes result from
relatively increasing amounts of
melanin
©1998 Timothy G. Standish
Multiple Alleles
Hair color is determined by
more than one gene
 Thus hair color appears to
vary on an almost
continuous scale from black
to brown to blond to red
 The brown and black
pigment is melanin
 The red pigment is an ironcontaining molecule

©1998 Timothy G. Standish