Genetics - The study of
heredity, how traits are passed from
parent to offspring
x
o
r
=
or
The study of heredity started
with the work of Gregor Mendel
and his
pea plant garden
Mendel was an Austrian Monk that
lived
in the mid 1800’s
Mendel’s Laws of
Heredity
Why we look the way
we look...
What is heredity?
• The passing on of
characteristics (traits) from
parents to offspring
• Genetics is the study of
heredity
Gregor Mendel
• Mendel used pea plants to
discover the mechanism of
heredity – how traits get
passed from parents to
offspring.
Why Mendel used peas...
• They reproduce sexually
• They have two distinct, male
and female, sex cells called
gametes
• Their traits are easy to
isolate
Mendel crossed the Plants
• Fertilization - the uniting of
male and female gametes
• Cross - combining gametes
from parents with different
traits
Questions
•
•
•
•
•
•
What
What
What
What
What
What
did Mendel cross?
are traits?
are gametes?
is fertilization?
is heredity?
is genetics?
What Did Mendel Find?
• He discovered different laws
and rules that explain
factors affecting heredity.
Phenotype & Genotype
• Phenotype - the way an organism
looks
• Example - red hair or brown hair
• genotype - the gene combination
of an organism
• AA or Aa or aa
Heterozygous & Homozygous
• Heterozygous - if the two
alleles for a trait are different
(Aa)
• Homozygous - if the two alleles
for a trait are the same (AA or
aa)
Dihybrid vs Monohybrid
• Dihybrid Cross - crossing
parents who differ in two
traits (AAEE with aaee)
• Monohybrid Cross - crossing
parents who differ in only
one trait (AA with aa)
Questions...
• What is the phenotype?
• What is the genotype?
• What is homozygous?
• What is heterozygous?
• What is monohybrid
crossing?
Mendel’s cross between tall pea
plants yielded all tall pea plants.
His cross between small pea
plants yielded all small pea plants.
X
Short
plants
Tall plants
=
X
=
Mendels’ cross between tall pea
plants and small pea plants yielded
all tall pea plants.
x
=
Mendel then crossed these second
generation tall pea plants and
ended up with 1 out 4 being small.
x
=
What Did Mendel Find?
• He discovered different laws
and rules that explain
factors affecting heredity.
Mendel’s work led him to the
understanding that traits such as
plant height are carried in pairs
of information not by single sets
of information.
• Carrying the information are
chromosomes
• Chromosomes are made up of
sections called genes
• Genes are made up of DNA
Rule of Unit Factors
• Each organism has two
alleles for each trait
• Alleles - different forms of
the same gene
• Genes - located on
chromosomes, they control
how an organism develops
Rule of Dominance
• The trait that is observed in
the offspring is the dominant
trait (uppercase)
• The trait that disappears in
the offspring is the
recessive trait (lowercase)
Questions...
• What do we call the trait
that is observed?
• What case (upper or lower) is
it written in?
• What about the one that
disappears?
• What case is it written in?
Law of Segregation
• The two alleles for a trait
must separate when gametes
are formed
• A parent randomly passes
only one allele for each trait
to each offspring
Law of Independent
Assortment
• The genes for different
traits are inherited
independently of each other.
Questions...
• How many alleles are there
for each trait?
• What is an allele?
• How many alleles does a
parent pass on to each
offspring for each trait
Segregation - Alleles separate
Heterozygous
parents
Tt
Tt
t
T
Gametes
T
t
Gametes
Fertilizaiton
Parent
1
t
T
TT
Tt
F2 Generation
Parent
t
2
T
Tt
tt
Probability
The likelihood that a particular
event will occur is
called_______.
• Probability
Probability
The probability that a single coin
flip will come up heads is…
• a. 100 percent
• b. 75 percent
• c. 50 percent
• d. 25 percent
Probability
The probability that a single coin
flip will come up heads is….
• a. 100 percent
• b. 75 percent
• c. 50 percent
• d. 25 percent
Probability
• Is the following sentence true
or false?
• The past outcomes of coin flips
greatly affect the outcomes of
future coin flips.
• False
Probability
• Why can the principles of
probability be used to predict
the outcomes of genetic
crosses?
• The way in which the alleles
segregate is completely random,
like a coin flip.
Punnett Squares
• How do geneticists use Punnett
squares?
• Punnett squares can be used to
predict and compare the genetic
variations that will result from
a cross.
Genetics & Punnett Squares
First let’s look at two basic kinds
of genes:
–Dominant - A gene that is
always expressed and hides
others
–Recessive - A gene that is only
expressed when a dominant gene
isn’t present
Dominant and Recessive Genes
• A dominant gene will always
Widows
mask a recessive gene.
Peak
• A “widows peak” is
dominant, not having a
widows peak is
recessive.
• If one parent contributes a
gene for a widows peak,
and the other parent
doesn’t, the off-spring will
have a widow’s peak
Genetics
Punnet Square - A tool we use for
predicting the traits of an offspring
• Letters are used as symbols to
designate genes
• Capital letters are used
for dominant genes
• Lower case letters are
for recessive genes
• Genes always exist in pairs
used
Genetics
A Widows Peak, dominant, would be
symbolized with a capital “W”,
while no widows peak, recessive,
would be symbolized with a
lower case “w”.
Father-No Widows Peak ‘w’
Mother-Widows Peak ‘W’
Genetics
All organisms have two copies of each
gene, one contributed by the
father, the other contributed by
the mother.
Homozygous - Two copies of the
same gene
Heterozygous - Two different genes
Genetics
For the widows peak:
WW - has a widows peak
Homozygous dominant
Ww - has a widows peak
Heterozygous
ww - no widows peak
Homozygous recessive
Since Herman has no widows peak, he
must be “ww”, since Lilly has a
widows peak she could be either
“WW” or “Ww”
Definitely Homozygous recessive
ww
Either Heterozygous
Ww
or Homozygous dominant
WW
Genetics
We can use a “Punnet Square” to
determine what pairs of genes Lilly has
• A Punnet Square
begins with a box 2
x 2
• One gene is called
an “allele”
Assume Lilly is heterozygous
Ww
Assume Herman is homoozygous
recessive
ww
• One parents pair is
W
w
w
Ww
ww
w
Ww
ww
split into alleles on
top, the other along
the side
• Each allele is
crossed with the
other allele to
predict the traits of
the offspring
Genetics
Notice that when Lilly is crossed with Herman,
we would predict that half the offspring
would be “Ww”, the other half would be
“ww”
Half “Ww”, Heterozygous, and will
have a widows peak
Half “ww”, Homozygous, and
will not have a widows peak
W
w
w
Ww
ww
w
Ww
ww
Genetics
Another possibility is that Lilly
might be “WW”, homozygous
dominant.
Assume Lilly is homozygous
dominant
WW
W
W
w
Ww
Ww
w
Ww
Ww
Assume Herman is homoozygous
ww
Notice that
all the
offspring are
heterozygous
and will have
a widows
peak
Genetics
So which is true? Is Lilly homozygous
dominant (WW) or is she
heterozygous (Ww)?
W
w
w
Ww
ww
w
Ww
ww
W
W
w
Ww
Ww
w
Ww
Ww
If Lilly were
heterozygous, then
1/ of their
2
offspring should
have a widows
peak, 1/2 shouldn’t
W
w
w
Ww
ww
w
Ww
ww
If Lilly were
homozygous, all of
their children will
have a widows
peak
W
W
w
Ww
Ww
w
Ww
Ww
Recall that Herman and Lilly had
another offspring, Marylin. She
had no widows peak, therefore,
Lilly must be heterozygous.
Genetics & Punnett Squares
Now let’s look at two other basic
kinds of genes, Incomplete
dominance and Codominance:
–Incomplete dominance - Genes
that work together to produce a
third trait where the alleles are
blended
–Like a red flower crossed with a
white flower produces a pink
flower
Genetics & Punnett Squares
–Codominant - Genes that work
together to produce a third
trait where both alleles
contribute to the trait
–Like a red flower crossed with a
white flower produces a red and
white flower
Genetics
Hair color can be an example
Prince
Charming
is blond
Snow
White has
black hair
Genetics
What color hair
will the
offspring of
Prince
Charming and
Snow White
have?
Genetics
Hair color is different from widows
peak, no color is truly dominant.
In Fairy tales…
– Brown and blond are two, true
traits
– Homozygous conditions produce
either brown or blond hair
– Heterozygous conditions produce
red hair
Genetics
For Snow White to have brown hair she
must be homozygous dominant, “BB”, a
blond Prince Charmin must be
homozygous recessive, “bb”.
B
B
b
Bb
Bb
b
Bb
Bb
Genetics
All the offspring from Prince Charming
and Snow White will therefore be
heterozygous, “Bb”, and since hair
color is codominant….. all their
children will have red hair.
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That’s all for now!