Download AP Bio Ch. 14 Mendel

AP BIOLOGY
Mendel and the Gene Idea
Gregor
Mendel:
The Father
Of Genetics

TRUE BREEDING VS. HYBRID
True breeding organisms produce
offspring of the same variety when they
self-pollinate.
Hybridization is the crossing of two
true-breeding varieties.
MONOHYBRID CROSSES
Track the inheritance of a single characteristic such
as flower color.
P1 generation- original parents
F1 generation- hybrid offspring of the P1
generation.
F2 generation- offspring resulting from
interbreeding of the hybrid F1 generation.


MENDEL’S IDEAS
Different alleles of a gene account for variations
in inherited characteristics.
For each trait, an organism inherits two alleles,
one from each parent.
If the two alleles differ, the dominant allele will be
expressed in the organism’s appearance.
The two alleles for each characteristic segregate
during gamete production, with the egg or sperm
each getting one of the two alleles.
DOMINANT AND RECESSIVE ALLELES
Alleles are alternate versions of a gene
Dominant alleles are expressed in the
organism’s appearance, while the recessive
allele is masked.
An organism must receive a recessive allele
from each parent for the trait to be
expressed.
HOMOZYGOUS VS. HETEROZYGOUS
An organism is said to be homozygous
when it has two identical alleles for a
trait. Ex) TT, tt
Heterozygous means having two
different alleles for a given trait
Ex) Tt
PHENOTYPE VS. GENOTYPE
Phenotype is the physical and
physiological expression of an organism’s
traits.
Phenotype can be expressed in how an
organism look, behaves, or functions.
Genotype is the actual genetic makeup an
organism has; the genes it has for the trait
TESTCROSS
A test cross involves the breeding of an
organism of unknown genotype with a
homozygous recessive individual to
determine the unknown genotype.
The ratio of resulting phenotypes gives clues
to the unknown genotype.

DIHYBRID AND TRIHYBRID CROSSES
A dihybrid cross is a cross between
parents that have two different traits.
A trihybrid cross is between parents
differing in three traits.

MENDEL’S LAWS
Law of Segregation- Allele pairs
separate during gamete formation, and then
randomly reform pairs during fertilization.
Law of Independent Assortment- Each
allele pair segregates independently during
gamete formation; applies when genes for
two traits are located on the same
chromosome.
GENETICS AND PROBABILITY
Mendelian inheritance reflects the rules of
probability.
Rule of multiplication- used to determine the
chance that two or more independent events will
occur together in a specific combination. Multiply
the probability of one event by the probability of
the other happening.
Rule of addition- the probability of an event that
can occur in two or more different ways is the
sum of the separate probabilities of those ways.

INCOMPLETE DOMINANCE
Incomplete dominance is a type of
inheritance in which the F1 hybrids have an
appearance that is intermediate between the
phenotypes of the parents.
Example: Red flowers crossed with white
flowers results in pink offspring

Incomplete Dominance
CODOMINANCE
Codominance is a phenotypic situation in
which both alleles are expressed in a
heterozygous organism.
Example: Cross of white
chickens and black chickens
results in speckled offspring
CODOMINANCE
TAY-SACHS DISEASE
 In Tay-Sachs disease, brain cells lack
an enzyme to metabolize
gangliosides. Lipids accumulate in the
brain causing damage and eventually
death.
 Although it is considered to be
caused by a recessive allele,
individuals that are heterozygous for
Tay-Sachs have an enzyme-activity
level between normal and those with
the disease even though they lack
outward symptoms of the disease.
ACHONDROPLASIA
 Achondroplasia is a disorder
caused by a dominant allele.
 Individuals affected by this
disorder have short stature, a
form of dwarfism.
 Heterozygotes have the dwarf
phenotype.
 The condition affects 1/10,000
individuals in the general
population.
 The majority of people are
homozygous for the recessive
allele.
HUNTINGTON’S DISEASE
 Huntington’s disease is a
degenerative disease of the nervous
system that does not become
apparent until the person reaches
adulthood. It is possible that they
would have had children before
they were aware that they had the
disease.
 Offspring could find out if they
inherited the disease by DNA
analysis.
IS A DOMINANT ALLELE ALWAYS MORE
COMMON IN A POPULATION?
 For example, the allele for
polydactly (having extra fingers
or toes) is dominant, but occurs
in only 1/400 babies born.
 The recessive allele is more
common (399/400 have the
normal number of digits)
 No, a dominant allele does not
always occur more frequently in
a population than the recessive
allele.
MULTIPLE ALLELES
 Multiple alleles are more than two alleles that represent a
gene.
 Different phenotypes result from different combinations
of the alleles.
 Example: ABO blood groups
Phenotypes include type A, B, AB, and O blood
 The genes coding for the glycoproteins (antigens) on the
surface of red blood cells, which determine blood type,
are inherited from parents

PLEIOTROPY
Most genes have multiple phenotypic
effects, a property called pleiotrophy.
Pleiotropic alleles are responsible for the
multiple symptoms associated with certain
hereditary diseases in humans, such as
cystic fibrosis and sickle-cell disease.
WHAT IS EPISTASIS?
Epistasis is the situation where one gene alters
the expression of another gene that is
independently inherited.
Example: Fur color in mice
BB,Bb = black
bb= brown
CC, Cc = pigment is deposited
cc= no pigment
 Any animal with cc will be white, no matter what
color fur was coded for
Epistasis
In
Mice
POLYGENIC INHERITANCE
Polygenic inheritance is an additive effect of
two or more genes on a single phenotypic
character.
Skin color is a result of polygenic
inheritance.
Three separately inherited genes affect the
darkness of skin. Dark is dominant over
light.
ENVIRONMENTAL IMPACT ON HUMAN
PHENOTYPE
 Human phenotypes can be
influenced by environmental
factors.
 For example: nutrition
influences growth and height,
exercise alters physical build,
sun exposure darkens the skin,
and experience improves
performance on intelligence
tests.
NORM OF REACTION
The “norm of reaction” for a genotype is the
range of phenotypic possibilities for a single
genotype, as influenced by the environment.
Generally, norms of reaction are broadest for
polygenic characters.
Polygenic characteristics are“multifactorial”
because many factors, both genetic and
environmental, collectively influence phenotype.

Environmental effect on Phenotype
FAMILY PEDIGREES
Pedigrees are family trees describing the
interrelationships of parents and children
across generations.
Pedigrees can be used to trace the
inheritance of characteristics.
CARRIERS
Some human disorders are recessive traits.
A person heterozygous for the trait does
not have the disease, but are considered to
be “carriers” because they can pass the
recessive allele to their offspring.
CYSTIC FIBROSIS
 Cystic fibrosis affects 1/2500 whites
of European descent. 1/25 is a
carrier.
 The disease is caused by a defect in
a gene that codes for a membrane
protein functioning in chlorine ion
transport between cells and
extracellular fluid.
 Symptoms include high
extracellular Cl- conc., thick mucus
build up in the pancreas, lungs,
digestive tract, and other organs,
bacterial infection, and eventually
death.
SICKLE-CELL DISEASE
Sickle-cell disease affects 1/400 African-
Americans.
It is caused by the substitution of a single amino
acid in the hemoglobin protein of red blood cells,
causing them to have a “sickle” shape.
1/10 African-Americans are heterozygous for the
sickle-cell allele.
Individuals with the sickle-cell trait (carriers) have
an increased resistance to malaria.

LETHAL ALLELES
Lethal dominant alleles are much less
common than lethal recessive alleles.
Lethal dominant alleles are not masked in
heterozygotes. Individuals often do not
survive to reproduce.
A person who is a carrier for a lethal
recessive allele may not know it until they
have children.
MULTIFACTORIAL DISEASES
 Multifactorial diseases are those
that have both a genetic as well
as an environmental influence.
 Examples include heart disease,
diabetes, cancer, alcoholism, and
mental illnesses such as
schizophrenia and manicdepressive disorder.
PKU
 PKU is phenylketonuria, a
condition caused by lack of
enzymes to metabolize the
amino acid phenylalanine.
 Buildup of phenylalanine and
phenylpyruvate accumulate in
the blood, causing mental
retardation.
 Problems can be prevented by
early identification, and a special
diet low in phenylalanine
CARRIER RECOGNITION
Carrier recognition is the process of
determining if an individual is a carrier of a
particular genetic disorder.
PRO: allows people with family histories of
the disorder to make informed decisions
about having children
CON: carriers could be discriminated against
if the info is used in the wrong way