Chapter 12
Mendel & Heredity
Section 12.1
Genetics – study of heredity; how traits
are passed from parents to offspring
I. Gregor Mendel “Father of Genetics”
A. History of
1. Lived in a monastery in Austria in 1842
2. In 1851, moved to the University of
Vienna where he studied science and
mathematics
3. Returned to monastery, taught, & kept a
garden (most famous for studies on peas)
B. Initial Observation
1. Mendel crossed (mated) purple flowered
peas plants with white flowered pea
plants.
2. All the offspring had purple flowers.
3. Two of these first offspring were crossed,
and some of their offspring had purple
flowers, but a few had white flowers!
II. Features of Pea Plants
A. Vocabulary
1. Characters – physical features that are
inherited
• Flower color in peas is a character
2. Trait – one of several possible forms of a
character
•
The flower colors of purple or white are traits
3. Hybrid – offspring of a cross between
parents with different traits
•
If a purple flowered peas crosses with a white
flowered pea, its offspring would be a hybrid
B. Pea Plant Characters – Mendel observed
7 characters (do not need to write down)
1. Height of plant’s stem (tall or short)
2. Position of flower on stem (mid-stem or end
of stem)
3. Pod Color (green or yellow)
4. Pod Appearance (smooth or bumpy)
5. Seed Texture (round or wrinkled)
6. Seed Color (green or yellow)
7. Flower Color (purple or white)
C. Pollination – Mendel manually took
pollen from an anther (male) &
transferred it to the stigma (female).
1. Self-Pollination: a flower will pollinate
itself or another flower on the same plant
2. Cross Pollination: a flower will pollinate a
flower on a different plant
***Mendel uses both forms of pollination in is
experiments.
III. Mendel’s Experiment
A. Mendel first used true-breeding plants.
1. True-Breeding – a plant that is pure for a
trait; when self-pollinated, its offspring will
always have that same trait
•
A true-breeding purple flowered plant will always
produce purple flowered plants!
2. Mendel formed 14 strains that were truebreeding.
3. These true-breeding plants were labeled the
P1 generation since they were the original
parents.
B. Two P1 generation plants of contrasting
traits were cross-pollinated. Their
offspring were labeled the F1 generation.
C. An F1 generation plant was allowed to
self-pollinate. Its offspring were labeled
the F2 generation.
D. An Example of Mendel’s Results
P1 = True-Breeding Purple X True-Breeding White
F1 = 100% were Purple Flowers
F2 = 75% Purple flowers & 25% White flowers
* = 3 Purple : 1 White Ratio
**Mendel crossed all 14 traits and always received
a 3:1 ratio in the F2 generation
E. Calculating Ratios
1. Probability – chance something will happen
Probability = # of times an event happens
# of times it could of happened
Ex: In an F2 generation, Mendel recorded 705
purple flowered plants and 234 white flowered
plants.
Purple: happened 705 times out of 939;
705/939 = 75.1 %
White: happened 234 times out of 939;
234/939 = 24.9%
Section 12.2
I. Explaining Mendel’s Experiments
A. Alternate Versions of Genes = alleles
1. For each character, there are 2 possible
traits
2. Each trait is controlled by a specific gene
– Purple flowers is controlled by one gene
for flower color & white flowers is
controlled by a different gene for flower
color
– The purple gene is called the purple
allele
B. Dominant & Recessive Alleles
1. Dominant Allele – the allele whose trait is
always expressed
2. Recessive Allele – not expressed when a
dominant allele is present.
P = purple allele
p = white allele
2 Dominant alleles (PP) = purple flowers
1 Dominant 1 Recessive (Pp) = purple flowers
2 Recessive alleles (pp) = white flowers
3. We receive one allele for each character
from each parent.
• So 2 alleles are needed to determine our
traits
II. Mendel’s Laws
A. Law of Segregation
1. A pair of alleles are segregated
(separated) during meiosis.
2. This means one allele on one
chromosome will be separated from the
other allele on the other homologous
chromosome during meiosis to make
gametes.
Law of
Segregation
B. Law of Independent Assortment
1. Alleles are separated randomly and
independently into gametes
Section 12.3
I. Vocabulary
A. Genotype – all the alleles/genes inherited;
an individual’s genetic information
•
Example –Pp is the genotype for a purple
flowered plant
B. Phenotype – the physical expression of
one’s genotype
•
Example – the color purple is the phenotype of
a plant with a genotype of Pp.
C. Homozygous – genotype with 2 identical alleles
•
Example: PP = homozygous dominant
pp = homozygous recessive
D. Heterozygous – genotype with 2 different alleles
•
Example: Pp = heterozygous
II. Punnett Squares – model/tool that predicts
the outcomes of a genetic cross
A. Monohybrid Cross – cross between
individuals involving one trait (just flower
color)
1. Example: In pea plants, purple is dominant over
white flowers. Determine the probabilities of
possible outcomes for a cross between a
homozygous dominant plant and a homozygous
recessive plant.
B. Types of Dominance
1. Incomplete Dominance – an allele does not
completely dominate another; phenotype
will be a blend.
Ex: In snapdragons, the red allele (R)
does not completely dominate the
white allele. A heterozygote (Rr) will
have pink flowers.
2. Codominance – both alleles for a trait are
fully expressed (spots, AB blood type in
humans)
A heterozygous plant
(Rr) would display a
phenotype of spots 
3. Multiple Alleles – a trait has 3 or more
possible alleles (an individual still has only 2
alleles for the trait).
Ex: Blood type in humans. There are 3
alleles a person can inherit: the A allele,
B allele, and O allele.
Type A = AA or AO
Type B = BB or BO
Type AB = AB (codominance)
Type O = OO