Chapter
10
Introduction to Genetics
Why Study Heredity ?
Studying heredity allows us
to figure out what our
children may look like…
Studying heredity allows us to
figure out the possible blood
types of our children. Such
information can mean life or
death for an unborn child…
Why Study Heredity ?
Studying heredity helps explain
why males tend to be red/green
colorblind much more frequently
than females
Why do men have a distinct
pattern of balding, when women
don’t ? Studying heredity helps
us explain this occurrence
Why Study Heredity ?
Studying heredity allows us to identify how
certain terrible diseases are passed on from
parents to children
Studying heredity allows us to
look for certain traits
throughout our family tree
Introduction to
Genetics
Chapter 10.2
Mendel’s Principles
Gregor Mendel


1842 – Entered a monastery
in Austria

Worked in the garden of the
monastery

Raised pea plants
1851 – Entered University
of Vienna to

studied Math and Science

Explained trends in pea plant
data
Mendel’s Principles
Pea Plants

Pea plants reproduce
through sexual reproduction

Contain both male (anthers)
and female (stigma)
reproductive structures

Pollination: pollen uniting
with egg

Pea plants are able to selfpollinate (pollinate
themselves)
Pea Plants
Mendel’s Principles
Self-pollination
• male structures fertilize the female
structures of the same plant
Cross-Pollination
• male structures of a plant fertilize
the female structures of another
plant
Mendel’s Principles
Mendel’s Experiments
• Mendel wanted to
study the plants
traits over many
generations.
• Mendel controlled
which plants
fertilized which
Mendel’s Principles
Mendel’s Experiments

Mendel obtained purebreed,
true-breeding plants

Looked at 7 traits
Mendel’s Principles
Mendel’s Experiments

Crossed true breeding plants
with contrasting traits.

The trait not present in the
F1 generation seemed to “reappear” in the F2 generation

Mendel concluded that
“factors” or genes must
come in pairs !

We call these “factors”
alleles
Mendel’s Principles
Mendel’s Results and Conclusions

Dominant factor, or
gene can mask a more
recessive gene

trait that “re-appeared”
was ¼ of the population

3 dominant traits for
every 1 recessive trait
Mendel’s Experiment
Visual
Mendel’s Conclusions
Visual
Predicting Genetic Crosses
Probability
Probability =
Number of times an event is expected to happen
Number of chances for that event to happen
The probability of a coin
landing heads up is:
1
2
What is the probability that a coin will land heads up,
if the previous 5 tosses it landed tails up ?
IT IS STILL….. 1/2
Each toss is independent, or unaffected by what came before it !!!
Predicting Genetic Crosses
Probability
What is the probability of tossing a coin heads up 5 times in a row ?
This question asks us to figure out a series of events
happening not simply the next event, unlike the last
question
In order to predict the probability of a series of events
occurring together, we must use….
THE RULE OF MULTIPLICATION
This rule states that you must multiply the likelihood
of each separate event occuring
½ X ½ X ½ X ½ X ½ = 1/32
Is the answer to the question above
Probability
Visual
Predicting Genetic Crosses
Chromosomes and Genes
• Genes are small
segments of DNA
on chromosomes
• Information for
different traits
The flower color gene in the diagram
above has two forms:
• A chromosome
can carry 100s to
1000s of genes
P-purple & p-white
The different forms of genes are referred to as alleles
Predicting Genetic Crosses
Genotype and Phenotype
Genotype
P
p
• The genes for a
particular trait
Phenotype
• Physical
appearance of
the trait
Genotype: Pp
Phenotype: Purple
In the diagram
above, P=purple
flowers p=white
flowers
Pp = Purple flowers
Predicting Genetic Crosses
Monohybrid Punnett Squares
Punnett squares are charts used to predict the traits of
offspring from a set of parents
MOM
D
A
D
B
b
b
Bb
bb
b
Bb
bb
The boxes within a
punnett square represent
the possible zygotes
that a couple can make
Capital letters are always written first within the grid of a
punnett square
Monohybrid means that the punnett square is used to
predict ONE trait
Monohybrid Cross
Visual
Predicting Genetic Crosses
Monohybrid Punnett Squares
Setting up genetic word problems:
I.
Designate letter and what they represent
T = Tall
The letter used is typically the
t = short
first letter of the dominant trait
II. Determine possible genotypes and the phenotype
that each represents
TT = Tall height
Tt = Tall height
tt = short height
Predicting Genetic Crosses
Monohybrid Punnett Squares
Setting up genetic word problems:
III. Determine the genotypes of the parents being crossed
Example:
Homozygous dominant
Homozygous recessive
Hybrid
Pure dominant
Pure recessive
Heterozygous
Homo = same
= TT
= tt Hetero = different
= Tt
= TT
= tt
= Tt
TT x Tt shows that a Homozygous dominant is being
crossed with a hybrid
Predicting Genetic Crosses
Monohybrid Punnett Squares
Setting up genetic word problems:
IV. Complete the punnett square to show possible gene combinations
T
t
T
TT
Tt
T
TT
Tt
V. Determine the genotypic and phenotypic ratios
Genotypic ratio Phenotypic ratio -
2-TT:2-Tt gives a ratio of 1-TT:1-Tt
100 % Tall
Predicting Genetic Crosses
Dihybrid Punnett Squares
A dihybrid cross invloves using a punnett square to predict
how offspring will inherit ________
TWO traits
The F.O.I.L. method of distribution needs to be used to
show how each of the four alleles from each parent
could combine
Parent #1 (AaBb)
AB
Keep the
order of
letters
consistent,
still always
putting the
capital
letters first
X
Ab
Parent #2 (AaBb)
aB
ab
AB AABB AABb AaBB AaBb
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb
aaBB aaBb
ab AaBb Aabb
aaBb aabb
Genes still
segregate, so you
never have two of
the same letter in
a given egg or
sperm (this can’t
read “aa”)
Predicting Genetic Crosses
Dihybrid Punnett Squares
Crossing two heterozygotes for one trait yielded a 3:1
ratio for Mendel’s peas
Crossing two heterozygotes for two traits yields a
9:3:3:1 ratio
9 Dominant for both traits
3 dominant for first trait, recessive for the second
3 recessive for first trait, dominant for the second
1 recessive for both traits
Mendel’s Laws
The Law of Independent
Assortment
The Law of Segregation

Two alleles which
control a trait are
separated



Random Distribution of
alleles

Gametes
Parent – Tt


Eggs or sperm has either
a T or t
Not Both!

Flower color gene has no
connection to plant
height
YyRr




YR
Yr
yR
yr
Possible
Gametes
Produced
during
meiosis
Predicting Genetic Crosses
Testcross
While most phenotypes are visible, genotypes cannot be seen
To determine the GENOTYPE of an organism with a
dominant trait, you need to perform a
TESTCROSS
A testcross involves mating a homozygous recessive
organism with the organism of unknown genotype
The “testcross”
homozygous
recessive organism
B
b
b
Bb
bb
b
Bb
bb
If this mating produces any organisms with recessive
traits, you can determine that the unknown genotype is
heterozygous for the dominant trait
Test Cross
Visual