NOTES: 14.1 -14.2 HUMAN HEREDITY

• Key Terms

– Autosomal Recessive

– Autosomal Dominant

– Pedigree

– Tay-Sachs– Tay-Sachs

– Cystic Fibrosis

– Phenylketonuria

• Key Concepts

– How to read and interpret a pedigree

– How diseases are caused

Recessive and Dominant Alleles:

• Some common genetic disorders are autosomal

recessive

– This means that you need two recessive alleles (on

any of the 44 chromosomes—NOT the sex any of the 44 chromosomes—NOT the sex

chromosomes) to express the disease

• EX: Cystic Fibrosis

• Other genetic disorders are autosomal dominant

– Only one allele is needed for the trait to be

expressed

• EX: Huntington’s Disease

Autosomal Recessive Disorders:

• In order to develop an autosomal recessive trait, an individual must have the genotype: “aa”

• To be born with a homozygous recessive • To be born with a homozygous recessive genotype, both parents must be heterozygotes (“carriers”)…or homozygous recessive themselves (although with most disorders, that is not the case)…WHY NOT?

From gene to molecule:From gene to molecule:From gene to molecule:From gene to molecule:

• In both cystic fibrosis and sickle cell anemia, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorderprotein, causing a serious genetic disorder

Cystic Fibrosis:

• Caused by a recessive allele on chromosome #7– It is an autosomal genetic disorder

• Causes digestive and respiratory problems

• Death around 20-30 years of age• Death around 20-30 years of age

• How does it happen?– Three bases are deleted from the protein, which

removes one amino acid

– The protein cannot fold properly anymore, and is destroyed

– Result: airway is clogged with mucus

CF Example:

• Cystic fibrosis heterozygotes (Ff)

– just one copy of the normal (dominant) allele is

enough to supply the cell with the proper proteins to

function.

– Because of this, the normal allele is considered – Because of this, the normal allele is considered

dominant over the recessive allele

• Therefore, a person who is heterozygous does

not suffer from Cystic Fibrosis

Sickle Cell Anemia:● One DNA base has been changed

• Amino acid is valine, instead of glutamicacid

• Result = abnormal hemoglobin

• The abnormal hemoglobin forms crystal-• The abnormal hemoglobin forms crystal-like structures that change the shape of the red blood cells

Sickle Cell Anemia:

• The abnormal red blood cells are shaped like a “sickle” or a half-moon;

• These RBCs slow blood flow, block small blood vessels, and result in tissue damage and pain.and pain.

Sickle Cell Anemia:

• Three genotypes possible:

1) HH = all healthy hemoglobin

2) Hh = ½ healthy hemoglobin; ½ 2) Hh = ½ healthy hemoglobin; ½ abnormal hemoglobin;

**CODOMINANCE!!

3) hh = all abnormal hemoglobin

Sickle Cell Anemia:

• Individuals who are Hh do not have serious health problems and can lead relatively normal lives, but-

• They do show some signs of sickle cell • They do show some signs of sickle cell anemia if the availability of oxygen is reduced

(i.e. high altitude;

strenuous exercise)

Pedigrees

• Graphical

representation

recording the line of

ancestors

– Family tree

– Used for breeding

animals (e.g. dogs,

cats, horses)

– Trace genetic

disorders

Pedigree Chart:

• Shows how a trait is transmitted

from generation to generation

• Each row is a generation

• Circles represent females• Circles represent females

• Squares represent males

– Shaded in: person expressesthat trait

– Half shaded in: person is only a

carrier

– Clear: person does not carry or express that trait

Symbols to Pedigrees

Tay-Sachs Disease:

• Autosomal recessive disorder

• Recessive allele results in the absence of an enzyme that normally breaks down lipids in the central nervous systemlipids in the central nervous system

• Without this enzyme, the lipids accumulate in the nervous system and the affected individual dies

Phenylketonuria (PKU):

• Autosomal recessive disorder;

• Absence of an enzyme to break down the amino acid phenylalanine

• The accumulation of phenylalanine causes damage to the nervous systemdamage to the nervous system

• By avoiding phenylalanine in

the diet, affected infants

can avoid the symptoms

of the disorder

Autosomal Recessive Traits &

Pedigrees:

• May skip a generation

• Affected individuals are born to 2 carriers

• Males and females affected equally

• KNOWN carriers will be half-shaded in…it • KNOWN carriers will be half-shaded in…it is not always possible to know if an individual is a carrier

What about autosomal dominant

human traits?

• A single dominant allele inherited from 1 parent is all that is needed for a person to show the dominant trait.

Simple Dominant Traits:

• Tongue rolling

• Unattached earlobes

• Hitchhiker’s thumb

• Hair in the middle section of fingers• Hair in the middle section of fingers

• Ability to taste PTC

Autosomal Dominant Traits &

Pedigrees:

• Typically seen in every generation, affecting multiple people

• Affected individuals are born to affected parent(s)parent(s)

• Males and females affected equally

• There are NO “carriers”!

(you either have it – AA or Aa –

or you don’t – aa)

Autosomal Dominant Disorder:

HUNTINGTON’S DISEASE

• Rare, but lethal, dominant allele;

• Results in a breakdown of parts of the brain;

• Onset between the ages of 30 and 50;

• No known treatment or cure;• No known treatment or cure;

• There is a test available to see if you have it –

may help with the decision to start a family;

• Every child of an affected individual has a 50/50

chance of being affected (and passing it down)

Sex-Linked Traits and Pedigrees:

• only (or mostly) males are affected;

• affected males are born to “carrier” females;

• typically not seen in all generations• typically not seen in all generations

Queen Victoria’s Legacy in

Royal Families of Europe