Chapter 12: Patterns of Heredity & Human Genetics

Section 12.1: Mendelian Inheritance of Human Traits

I. Making a Pedigree

*Pedigree - a “genetic family tree”; a graphic representation of genetic inheritance.

A. How a Pedigree Works

1. Symbols:

a) circles - represent females

b) squares - represent males

2. Shading:

a) entirely shaded - when a circle or square is shaded in

or colored in it represents individuals that possess the

trait being studied.

b) half - shaded - a circle or square that is only half -

shaded represents a carrier.

*carrier - a carrier is a heterozygous individual;

they may or may not show the trait.

3. Parents & Generations

a) parents - parents are represented by a horizontal line

connecting a circle and a square.

b) parents and offspring - parents are connected to their

offspring by a vertical line.

c) generations - each horizontal row of circles and squares designates one generation. The most recent generation is at the bottom of the pedigree, the oldest is at the top.

4. Numbering:

a) Generations - generations are numbered by Roman Numerals, I, II, III, IV, etc. from top to bottom.

b) Individuals - individuals are numbered or “counting” numbers; 1, 2, 3, 4 etc.

Pedigree

Other Pedigree Symbols:

Analyzing a Pedigree:

II. Simple Recessive Heredity & Recessive Disorders

*most genetic disorders are caused by recessive alleles.

*in order to have the disease, the individual must be “homozygous recessive.”

A. Cystic Fibrosis (CF)

*most common genetic disorder among Caucasian

Americans.

1. Affected - 1/ 20 white Americans is a carrier

1/ 2000 have the disease

2. Cause - defective protein

in plasma membrane of cells

in lungs and digestive tract.

3. Signs/ Symptoms -

formation and accumulation

of thick mucus in the lungs

and digestive tract.

4. Treatment - drug therapy, special diets, physical therapy,

breathing treatments.

Lifespan:

no treatment = 5 < years

with treatment = 27+ years

Other images of Cystic Fibrosis:

B. Tay-Sachs Disease

1. Affected - allele for Tay-Sachs is found primarily in the Amish and Jewish populations.

2. Cause - recessive allele that causes the absence of an enzyme that breaks down lipids stored in tissues of the central nervous system, therefore lipids accumulate in these cells.

3. Signs/ Symptoms - destruction of

central nervous system, seizures,

blindness, degeneration of nerves.

4. Treatment - drug therapy, only

live a few years; no cure.

Tay-Sachs Pedigree:

C. Phenylketonuria (PKU)

1. Affected - allele is mainly found in individuals of Norway and Swedish descent.

2. Cause - absence of an enzyme that converts the amino acid phenylalanine into tyrosine; therefore phenylalanine in PKU babies cannot be broken down and accumulates in the brain and nervous system.

3. Signs/ Symptoms - build up of phenylalanine and by products in the body causing severe damage to the central nervous system; mental retardation.

*Baby appears to be healthy at birth b/c mother’s enzymes prevented build up of phenylalanine during pregnancy. Once the baby begins to drink milk, which is rich in phenylalanine, the amino acid starts to accumulate and mental retardation occurs.

4. Treatments - babies are now tested for PKU at birth and if they have PKU they are put on special formula diets until their brain develops, thus preventing mental retardation from the build up of the amino acids.

*Mothers that are homozygous recessive for PKU must limit the amount of phenylalanine taken in during pregnancy, thus diet foods should be limited since they tend to have a high content of phenylalanine ex) equal

D. Sickle Cell Anemia

1. Affected - mainly affects African American population

1/ 400 - has it

1/ 10 - is a carrier

2. Cause - substitution of a single

amino acid in the hemoglobin of

RBC (red blood cells).

3. Signs/ Symptons - lethargy,

anemia, brain damage, heart failure,

organ damage.

4. Treatments - blood transfusions; drug therapies, possible gene therapy treatments.

III. Simple Dominant Heredity/ Dominant Traits & Disorders

*Only one dominant allele is needed for the individual to exhibit the trait.

A. Simple Dominant Traits

1. Tongue rolling - dominant - can roll tongue (R )

recessive - cannot roll tongue (r )

2. Hapsburg Lip - protruding lower lip

dominant - protruding lip

recessive - no protruding lip

3. Free/ Attached Ear Lobes -

dominant - free lobes (F)

recessive - attached ear lobes (f)

4. Hitchhiker’s Thumb -

dominant - bent 30 degrees or

more

recessive - straight

5. Widow’s Peak -

dominant - has peak (W)

recessive - no peak (w)

B. Huntington’s Disease

* lethal genetic disorder

1. Cause - rare dominant allele; usually would cause death before birth, but because it is late acting (onset is between 30-50 yoa) it can be passed on to the next generation without being known.

2. Signs/ Symptoms -

breakdown of certain areas of the

brain, leading to motor

degeneration.

3. Treatments - some drugs are available to slow the progression of the disease but there is no cure. Genetic tests can be done to see if an individual has the HD allele.

Normal HD