©1998 Timothy G. Standish

Human GeneticsHuman Genetics

Timothy G. Standish, Ph. D.

©1998 Timothy G. Standish

The Madness of King George IIIThe 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 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 UniqueHuman 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 GeneticsPolitics 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 right wing 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 TraitsSimple 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:

Attached ear lobes are inherited as a recessive trait.

Unattached ear lobes are inherited as a dominant trait.

©1998 Timothy G. Standish

Simple Dominant/Recessive TraitsSimple Dominant/Recessive Traits

Having a bent little finger is a dominant trait

©1998 Timothy G. Standish

Dominant/Recessive Human TraitsDominant/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 TraitsDominant/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 TraitsDominant/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 TraitsDominant/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 TraitsSex 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 TraitsSex 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 sex linked traits.

©1998 Timothy G. Standish

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.

Specific Human TraitsSpecific Human Traits

©1998 Timothy G. Standish

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 asparagas spears, you are an S-methyl thioester smeller.

Specific Human TraitsSpecific Human Traits

©1998 Timothy G. Standish

White forelock - A white patch of hair at the front of the scalp.

Specific Human TraitsSpecific Human Traits

©1998 Timothy G. Standish

Multiple AllelesMultiple 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 AllelesMultiple 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 iron containing molecule