NOTES: 14.1-14.2:

HUMAN HEREDITY

Human Genes:

● The human genome is the complete set of

genetic information

-it determines characteristics such as eye

color and how proteins function within cells

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

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 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:

• 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 disorder

What makes an allele dominant

or recessive?

• Gene = sequence of DNA that codes for a

protein

• Usually:

– Dominant alleles code for the correct production of

that protein

– Recessive alleles code for no / wrong protein

production

– Heterozygous condition — the normal (dominant)

allele will still cause correct protein production

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

• 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

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 glutamic acid

• Result = abnormal hemoglobin

• 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.

Sickle Cell Anemia:

• Three genotypes possible:

1) HH = all healthy hemoglobin

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

anemia if the availability of oxygen is

reduced

(i.e. high altitude;

strenuous exercise)

Pedigree Chart:

• Shows how a trait is transmitted

from generation to generation

• Each row is a generation

• Circles represent females

• Squares represent males

– Shaded in: person expresses

that trait

– Half shaded in: person is only a

carrier

– Clear: person does not carry or

express that trait

Tay-Sachs Disease:

• Autosomal recessive disorder

• Recessive allele results in the absence of

an enzyme that normally breaks down

lipids 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 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

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

• Ability to taste PTC

Autosomal Dominant Traits &

Pedigrees:

• Typically seen in every generation,

affecting multiple people

• Affected individuals are born to affected

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;

• 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)

• Can’t trace every human trait through a pedigree because some genes are polygenic– Shape of your eyes

– Shape of your ears

– Height

– Eye color

• Also, phenotype is influenced by your environment (ex: nutrition & exercise)– Average height is 10 cm more than it was in the

1800’s in the US & Europe due to nutritional improvements

– Genes that are denied a proper environment in which to reach full expression in 1 generation, can achieve full potential in a later generation (genes are inherited, the environment is not)

REVIEW:Human Blood Groups

Blood Groups• Human blood comes in a variety of

genetically determined blood groups

• Using the wrong blood during a blood transfusion can be fatal

• A number of genes help determine blood type but we will focus on two:

– ABO blood groups

– Rh blood groups

Blood Groups — Rh factor:

• The Rh blood group is determined by a

single gene with 2 alleles — positive and

negative

• The “positive” allele is dominant

• You need two Rh- alleles (Rh-/Rh-) to be

Rh negative

Rh Factor• The Rh factor genetic information is

also inherited from our parents, but it is

inherited independently of the ABO

blood type alleles.

– There are 2 different alleles for the Rh

factor known as Rh+ and Rh-.

Mother Father Child

Rh- Rh+ Rh+

Rh- Rh- Rh-

The “Rh Issue”… Mom = Rh- Baby #1 = Rh+

ABO Blood Groups

• This is a case of multiple alleles

• There are 3 alleles for this gene—A, B, and O.

• AND…A and B are CODOMINANT!

• O is recessive to A and B

• Alleles A and B produce antigens (which are carbohydrates) on the surface of red blood cells

• O produces NO antigens

ABO Blood Groups—the wrong blood can be FATAL

• Antigens are recognized by the immune system and induce an immune response

• If the wrong blood is transfused, the body will respond to these antigens by producing antibodies– Antibodies are named for what they attack

• Antibodies bind to the foreign molecule (the antigen) and blood clumping will occur, which leads to blood clotting, which leads to death

ABO Blood Groups

• If you have blood type A, then you have:– The “A” antigen on the surface or your RBCs

– You have anti-B antibodies

– You can receive type A blood and type O blood

• Remember: Your antibodies are named for what they attack—so if you received type AB or type B blood then clumping would occur.

ABO Blood Groups

• Try this cross

• Cross a person who’s genotype is IAIA with a person who is IAIB.

• Give the possible genotypes and phenotypes

• Cross a person who’s genotype is IAIA with a person who is IAIB.

• Gametes = IA, IA and IA, IB

• Cross:

IA

IA

IAIBIB

IA

IAIAIAIA

IAIB

• Genotypes= ½ IAIA, ½ IAIB

• Phenotypes = ½Type A, ½ Type AB

Human Genes & Chromosomes:

Vocabulary:

• Sex-linked gene

• Sex-influenced gene

Key Concepts:

• Why are sex-linked

disorders more common in

males than in females?Lou Gehrig: died at

37 of ALS

Human Genes & Chromosomes:

• Chromosomes 21 & 22 are the smallest human

autosomes

– Chromosome 22 contains approximately 43 million

DNA bases (approx. 545 genes)

– Chromosome 21 contains approximately 32 million

DNA bases (approx. 225 genes)

• 1 of these genes is associated with ALS (Amyotrophic Lateral

Sclerosis)

• Causes a progressive loss of muscle control due to the

destruction of nerves in the brain and spinal cord

Remember…

• Humans have 46 chromosomes (23 pair)

• 2 of them are sex chromosomes they determine what sex you are

– XX = female

– XY = male

• 44 of them are autosomes they do not determine what sex you are

Who determines the sex of a

child?

The mother or the father?

THE FATHER!!!!

Why does the father determine the sex of the offspring???

• Mom is XX, she can donate either one X

chromosome or the other X chromosome

• Dad is XY, he can donate either an X

chromosome or a Y chromosomes.

• If the offspring receives the father’s X, it is

female

• If the offspring receives the father’s Y, it is male

The Y chromosome:

• If a Y chromosome is present, the person is male

• X chromosomes contain genes necessary for growth / development

• No cases of a person born with being 45,0Y

– Probably spontaneously aborted (miscarriage)

Sex Linked Genes:

• Genes carried on the X or Y chromosome are

“sex-linked” because they are on the sex

chromosomes

• Many sex-linked genes are found

on the X-chromosome

Sex Linked Genes:

• All X-linked traits are expressed in males

• WHY???????

• Males only have 1 copy of the X chromosome, while females need 2 copies of the defective gene

Human Sex-Linked Gene

Disorders:

1. COLORBLINDNESS:

• 3 human genes associated with color vision

are located on the X-chromosome

• In males, a defective version of any one of

these produces colorblindness

• Females must receive 2 copies of the allele to

be colorblind

XCXCXCXc

XCYXcY

More Human Sex-Linked Gene

Disorders…

2. HEMOPHILIA

• 2 important genes on the X-chromosome that

code for proteins that control blood clotting

• A recessive allele in either of these 2 genes

may lead to hemophilia

-“bleeders disease”

-injections of normal clotting

proteins prevent death

Sex-Linked Traits and Pedigrees:

• only (or mostly) males are affected;

• affected males are born to “carrier”

females;

• typically not seen in all generations

Queen Victoria’s Legacy in

Royal Families of Europe

Sex-Influenced Genes:

• A person’s phenotype is affected by their sex

(internal environment is different…hormones!)

• Ex: Pattern Baldness

WOMEN: MEN:

BB: not bald BB: not bald

Bb: not bald Bb: bald

bb: bald bb: bald

X-Chromosome Inactivation:

• Females have 2 X-chromosomes… if 1 is

enough for males, how does the cell

“adjust” to the extra x-chromosome in

females?

• One X-chromosome is randomly

switched off

– Condenses and is called a Barr body

X-Chromosome Inactivation –

an example:

• Tortoise-shell cats:

XBY = brown male

XbY = orange male

X-Chromosome Inactivation –

an example:

• Tortoise-shell cats:

XBXB = brown female

XbXb = orange female

XBXb = tortoise-shell female

-Can a male cat have tortoise-shell fur?

Tortoise-shell cats!

(a.k.a. “Torties”)

XBXb