Human Genetics By: K Marsh

Genetics Vocabulary Go Over Quick Genetics Review Genotype- the two alleles in an organism Phenotype- outward appearance of an organism Allele- when there is more than one type of expression for a gene (ie humans have a freckles allele and a no freckles allele).

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 thought to have a genetic basis Acute intermittent porphyria seems to be consistent with the symptoms exhibited by George III

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

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 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

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.

Simple Dominant/Recessive Traits Having a bent little finger is a dominant trait

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. Some albanism is related to pleiotropy (will be discussed later). 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.

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.

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.

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.

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.

Specific Human Traits Myopia - Short sightedness, the inability to see far off objects without correction by glasses or contacts is a recessive trait. Polydactyly - Having more than five digits on each hand and foot is a dominant trait. 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.

Beyond Simple Genetics Polygenic Pleiotropy Sex-Linked Co-dominance Incomplete Dominance Multiple Alleles

Polygenic Human Skin Color Polygenic inheritance is a pattern responsible for many features that seem simple on the surface. Many traits such as height, shape, weight, color, and metabolic rate are governed by the cumulative effects of many genes. 8 different genes code for melanin and skin pigmentation

Polygenic 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

Polygenic 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

Pleiotropy Pleiotropy occurs when a single gene influences multiple phenotypic traits. Sickle-cell anemia, is an example.  A single gene results in irregularly shaped red blood cells that painfully block blood vessels, cause poor overall physical development, as well as related heart, lung, kidney, and eye problems. 

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).

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 sex linked traits.

Co-Dominance Two or more alleles are dominant, they both express their traits phenotypically. Examples: Dalmations, Holstein Cows, & Zebras. Both black and white colors are shown at the same time.

Incomplete Dominance Neither allele in a genotype is fully expressed, there is a blending of both traits. Example: A pink carnation flower is created from parents that are white and red for flower color.

Multiple Alleles Multiple alleles are when there are three or more alleles for a given trait (gene). Example: Blood Types: There are A, B, and O blood types. Hand out Blood typing permission slips.