Chapter 12
I. Mendel: The Father of Genetics
Genetics: Study
of heredity.
Heredity: Passing
of traits from
parents to
offspring.
Mendel used the garden pea :
Has many traits, two clearly
different forms; easy to tell apart.
Mating can easily be controlled
Useful Features of Peas
Small, grows easily, matures
quickly, and produces lots of
offspring.
Self-pollinates (easy breeding)
Seeds inherit all traits from the
plant that bore them!
Mendel’s First Experiments
In your book, p. 269!
Please read about Mendel’s experiment
and simplify the steps by explaining them
in your own words.
(Pair-Share)
Mendel studied crosspollination, produces seeds
that are the offspring of two
different plants.
* Mendel began with purebred
plants (the result of selfpollination).
Hybrids – org. produced by
crossing parents with different
traits. (ex. Purple flower X
white flower)
True-Breeding = All offspring
displays same form of a trait.
P generation = parents.
F1 generation = offspring of
the p generation.
F2 generation = offspring of
the F1 generation.
In familiar terms please… 
P generation (grandpa)
F1 generation (dad)
F2 generation (baby)
All F1 plants showed
only one form of the trait.
(i.e. all offspring had
purple flowers)
All of the different trait
had disappeared!!
When the F1 plants
self-pollinate, the
missing trait
reappeared in the F2
generation!
Summary
Mendel found that for
all seven traits that he
studied, a 3:1 ratio was
observed in the F2
generations.
Mendel’s Theory
1.For each inherited trait,
an individual has two
copies of the gene—one
from each parent.
2.There are alternative
versions of each gene,
called ALLELES.
Mendel’s Laws
3. When 2 different alleles occur
together, one may be
completely expressed, while the
other “disappears”.  do NOT
blend!
Dominant: The expressed form
of a trait.
Recessive: Is not expressed if a
dominant allele is present.
Important Terms
Using the Frayer Model (reading
strategy) explain the relationship
between the following genetics
terms, beginning on p. 273:
• Genotype vs. Phenotype
•Homozygous vs. Heterozygous
•Dominant vs. Recessive
Mendel’s Laws
1. Law of Segregation: When
an organism produced
gametes, each pair of alleles
is separated and each
gamete has an equal chance
or receiving either one of
the alleles.
Mendel’s Laws
2. Law of Independent
Assortment: during gamete
formation, alleles for each
trait segregate
independently.
•inheriting one trait doesn’t affect
your inheritance of another trait
Types of Crosses
Monohybrid = involves one trait (Ex.
seed color)
Dihybrid = involves two traits (Ex. Seed
colod and seed shape)
Punnett Squares
•Model that predicts the likely
outcomes of a genetic cross.
•Shows all of the genotypes
that could result!
Pedigrees
A pedigree is a family history that
shows how a trait is inherited over
several generations.
Circles = females
Squares = males
Shaded circle or square = trait
expressed.
Circle or square not shaded = trait
not expressed.
Autosomal traits =Located
on chromosomes other than
X and Y. Will appear in
both sexes equally.
A sex-linked trait is a trait
whose allele is located on
the X chromosome.
Most sex-linked traits are
recessive.
Why would a sexlinked disorder show
up more in males than
in females?
Working sex-linked
Punnett Squares:
A. Blood Groups
ABO
 Red
Groups
Blood Cells (RBC) can
carry two antigens, A & B
 Presence & absence of A & B
antigens produces 4 possible
blood types: A, B, AB, O
B. Rh Groups
 + if Rh antigen is present on
RBC, - if not.
Phenotype
Genotype
A
B
AB
O
IAIA, IA i
IBIB, IB i
IAIB
ii
Traits NOT controlled by simple
dominant-recessive alleles
Ex.
Height, weight, hair
color, skin color in humans
Polygenic traits: Traits
controlled by multiple
genes.
Incomplete
Dominance =
Offspring displays a trait
intermediate between the
two parents.
Ex. Red + white = pink in
snapdragons.
Ex. Straight + Curly =
wavy hair in Caucasians.
Codominance
= Two
dominant alleles are
expressed at the same time.
Ex. Red hair + white hair =
roan hair in horses & cattle
 Multiple
Alleles = Genes with 3 or
more alleles.
 Ex. ABO blood group
Traits Influenced by the
Environment
 Acidity
of soil determines flower
color in hydrangea
 Temperature affects coat color of
arctic fox.
 Fur on the ears, nose, paws, and
tail is darker in Siamese cats
because body temperature is
cooler.
Traits Caused by Mutation
1. Sickle Cell Anemia = A
recessive disorder caused by
a mutated allele that
produces hemoglobin.
 RBC take on a disk-like shape
when oxygen levels are low.
Results
in poor blood
circulation.
1:500 African Americans
Protects affected people
from malaria.
2. Hemophilia
Recessive
Sex-linked
Impairs
blood’s ability to
clot.
1:10,000 white males
3. Huntington’s Disease



Caused by a dominant allele on an autosome.
Symptoms begin to show in 30’s or 40’s.
Forgetfulness and irritability, loss of muscle
control, eventually death.
Was brought to US by two brothers from
England in 1600’s, who fled allegations of
witchcraft. The “strange” behaviors was
actually caused by the onset of this disease.
4. Cystic Fibrosis
 Recessive
autosomal
 Mucus clogs organs, including
lungs and liver
 1:2,500 whites
5. Tay-Sachs Disease
 Recessive
 Deterioration
of central nervous
system in infants
 Caused by a defective form of a
brain enzyme.
 1:3,500 Ashkenazi Jews
6. PKU
 PKU
babies lack an enzyme that
converts phenylalanine into the
amino acid tyrosine. As a result,
phenylalanine build up in the
body, causing mental retardation.
 Early detection and a special diet
can prevent mental retardation.