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Chapter 3
Genetics:
The science of heredity
MENDEL
! Gregor Mendel was a priest and teacher in
the 1850 s
!  Mendel is famous for his observations and
studies of pea plants and their traits
(physical characteristics)
!  Mendel is known as the father of genetics,
which is the study of heredity (the passing of
traits from parents to offspring)
Mendel s Experiments
!  Pea plants were easy to study because
most of their traits exist only in two(2)
forms (ex - either tall or short, no
medium heights)
!  Pea plants produce a large number of
off spring in one generation = easy to
collect lots of data
Mendel s Experiments
!  Mendel started by cross-breeding
purebred plants with opposite forms of
traits (ex - crossing a tall plant with a
short plant)
!  Purebred plant - one that always
produces offspring with the same form
of the trait as the parent
Mendel s Experiments
!  All of the offspring(F1 generation) from the
cross of two purebred parents(P generation)
showed only one of the traits!
!  One of the traits seemed to disappear
!  Then, he crossed two of the F1
generation…………..
Mendel s Experiments
!  HOLY COW!!!!!!!!!!!
!  Even though all of the F1 generation
showed only one form of the trait, their
offspring (the F2 generation) showed a
mix of the two forms of the trait!
! 3/4 showed one and 1/4 showed the other
Mendel s Experiments
!  Mendel studied a
number of different
traits and all of his
experiments showed
similar results.
Mendel s Results
!  Individual factors control the
inheritance of traits
!  The factors exist in PAIRS
!  The female contributes one factor
while the male contributes the other
!  One factor in the pair can MASK (or
hide) the other factor
Modern Mendel
 The factors that control traits = GENES
 Different forms of genes = ALLELES
 One allele is inherited from each parent
 Some alleles are DOMINANT - the trait
always shows up when this allele is present
!  Some alleles are RESESSIVE - the trait is
masked or covered up when the dominant
allele is present and will only show up if the
dominant allele is not present
!
!
!
!
Which allele is dominant and
which allele is recessive?
The F1
generation
plants are
called
HYBRIDS.
Hybrids have
two (2)
different
alleles for a
given trait
Genetic Shorthand
!  Use letters to represent alleles of traits
!  Dominant alleles are represented by a capital
letter (ex - F for purple flower color)
!  Recessive alleles are represented by lower
case letters (ex - f for white flower color)
! FF = two dominant alleles
! ff = two recessive alleles
! Ff = one dominant and one recessive allele
Section 3.2 - Probability
Vocabulary
!  probability
! Punnett square
!  phenotype
!  genotype
!  homozygous
!  heterozygous
! Codominance
GENETICS TEST THIS
FRIDAY!! REVIEW
NOTES.
Paper Pet Assignment
1.  Choose partner
2.  By end of slide show be prepared
to draw a Punnet square for the
combination of your 2 paper pets.
3.  Determine the ratio AND the
percentage for the different
genotypes and phenotypes that
are possible in the offspring of
your 2 paper pets.
Probability
! Probability - is the
likelihood that a
particular event will
occur
!  Mendel was the 1st
scientist to recognize
that the principles of
probability can be used
to predict the
outcomes of genetic
crosses
Practice
!  Suppose you were to
toss a coin 20 times.
Predict how many
times the coin would
land heads up and
how many tails up.
!  Test your hypothesis
and record data.
!  How did your results
compare to your
prediction? Compare
your results to the
overall class results.
http://mathslover.hubpages.com/hub/How-to-calculate-probabilty
Punnett Squares
! Punnett squares are used to show all
possible genetic outcomes of a genetic
cross and to determine the probability
of a particular outcome
What we can see and what we can t
! Phenotype - an organisms physical
appearance or physical traits
! Genotype - an organisms genetic
makeup or allele combinations
! Homozygous - purebred, two identical alleles
for a given trait (TT)
! Heterozygous - hybrid, two different alleles for
a given trait (Tt)
Codominance
!  Codominance is when neither allele is
recessive or dominant
!  Both forms of the trait are seen in the
hybrid
Solving Crosses
(problems)
Steps for Solving a Genetics
Problem:
1.  Start by drawing a box. Divide it into 4
squares.
2.  Write the male parent’s alleles along
the top of the square and the female
parent’s alleles along the left line.
3.  Copy the alleles into the boxes
R ____
r
____
as shown.
R RR R r
4.  The completed Punnett ____
r rR r r
____
square shows all
possible allele combinations.
Let s Solve Together
!  Short hair (L) is dominant to long hair (l)
in mice. Give the probabilities for the
different phenotypes if a short-haired
mouse (male) is crossed with a longhaired mouse (female)?
!  Genotype ratio: ½ Ll: ½ ll
!  Phenotype ratio: ½ short hair: ½ long
hair
Let s Solve Together
!  In humans, brown eyes (B) are dominant
over blue (b)*. A brown-eyed man marries
a blue-eyed woman and they have three
children, two of whom are brown-eyed and
one of whom is blue-eyed. Draw the
Punnett square that illustrates this
marriage. What is the man’s genotype?
What are the genotypes and phenotypes
of the children?
Coin Crosses
!  Use coins to model Mendel’s cross
between 2 Tt pea plants
1. Place a small piece of masking tape on
each side of 2 coins.
2. Write a T (for tall) on one side and a t (for
short) on the other.
3. Toss both the coins together 20 times.
Record the letter combinations that you
obtain from each cross.
4. Report fraction AND percentage of
crosses that would results in a tall plant.
Paper Pet Assignment
1.  Pick one paper pet to be male and
one to be female.
2.  Record the alleles for the male and
female.
3.  Draw a Punnet square for the
combination of your 2 paper pets.
4.  Determine the ratio AND the
percentage for the different
genotypes and phenotypes that
are possible in the offspring of
your 2 paper pets.