Chapter 4
Lesson 2
Modeling Inheritance
Learning Objective:

Using your family history to predict
genetic outcomes with phenotypes.
Modeling Inheritance

Two tools can be used to identify and
predict traits among genetically related
individuals.
• Punnett square – using family histories to
•
predict genotypes and phenotypes.
Pedigree – Creating a map using family
histories to predict phenotypes.
One more thing about Punnetts




Human Gender
XX = girl
XY = boy
You ALWAYS have a
50% chance of
having a boy or girl
EVERY time.
X
X
X
XX
XX
Y
XY
XY
Pedigrees



A pedigree shows genetic traits that
were inherited by members of a family.
Pedigrees focus more on phenotypes.
They are also used with Punnett
Squares to determine genotypes.
Pedigree chart
1
2
Normal
Affected
Female
3
9
10
4
11
5
6
7
12
8
13
Male
Married
14
Children
Pedigrees to evaluate

Go to page 201 and do problems #7 - 9

#7 = Not affected = dd = C

#8 = affected = Dd = B

#9 = not affected = dd = C
Draw a pedigree

One couple has a son and a daughter
with normal pigmentation. Another
couple has one son and two daughters
with normal pigmentation. The daughter
of the first couple has three children with
the son of the second couple. Their son
and one daughter have albinism; their
daughter has normal pigmentation.
Answer

Albino
Female

Male
1st couple
1
2
6

2nd couple
3
7
8
4
5
Is albinism, recessive
or dominant?
It is recessive.
What must the
genotype be of #2 and
#3? (hint: make a
Punnett Sq.)
Answer


Both parents must
be Aa.
Not all their children
are albino.
A
a
A
AA
Aa
a
Aa
aa
Learning Objective – Part 2


By chance, Mendel studied traits only
influenced by one gene with two alleles.
However, we know now that some
inherited traits have
There are more complex patterns.
Incomplete Dominance

Incomplete
dominance produces
a THIRD phenotype
that is a blend of
parents phenotype.
R = red flowers
W = white flowers
Red x white = 100%
pink
R
R
W
RW
RW
W
RW
RW
Codominance


When BOTH alleles
can be observed in
the phenotype.
Black horses (BB)
are codominant to
white horses (WW).
The heterozygous
horses (BW) is an
appaloosa horse.
B
B
W
BW
BW
W
BW
BW
Codominance continued

Blood types are
unique because they
are an example of:
•
Codominance: A and
B are both dominant.
AND
• Multiple Alleles: There
are three different
alleles.
Multiple Alleles



Some genes only have two alleles, like
in Mendel’s experiments.
However, there are genes that have
more then two alleles.
But remember you can still only inherit
two of the alleles. One from each
parent.
Multiple Alleles continued



In a Labrador
retriever, coat color
is determined by one
gene with 4 alleles.
Black is dominant to
chocolate = B or b.
Yellow is recessive
epitstatic = E
(when present, it blocks the
black and chocolate alleles)
Phenotype
Possible
genotypes
Black
BBEE, BdEE,
BBEe, BbEe
Chocolate
bbEE
bbEe
Yellow
BBee
Bbee
bbee
Sex-linked


Because the Y chromosome is shorter
than the X chromosome it has fewer
genes.
Therefore if you are a boy you only get
one copy of these genes. So you get
what your Mom gives you.
Sex-linked continued

Examples in humans
include:
•
•
Colorblindness
Hemophilia
Polygenic Inheritance



When multiple genes determine the
phenotype.
Many phenotypes are possible
Examples in humans include:
• Height
• Weight
• Skin color
Polygenic - Skin color



There may be 100 different genes
involved and many mutations.
Melanin is a pigment responsible for skin
color and is a natural sunblock.
Lighter skin allows for more absorption
of UV rays from sunlight. This is
important for making vitamin C.
Polygenic - Skin color
Polygenic - Skin color
Polygenic - Skin color

The long standing, but unproven
hypothesis is that . . .
• If you lived near the equator you needed
•
darker skin to protect you from the intense UV
rays . . .
If you lived farther north you needed lighter
skin to help you get more UV rays because
there is less sunlight.
Genes and the Environment

Environment can affect an organisms
phenotype.
• Genes effect heart disease, but so do diet and
•
•
exercise.
Genes affect skin color, so does exposure to
sunlight.
Go to page 189 and write down, on the left
side of your notebook, two more examples of
how the environment can affect genes.
Learning Objective
To learn about different types of genetic
disorders.
Genetic Disorders


If a change occurs in a gene, the
organism with the mutation may not be
able to function as it should.
An inherited mutation can result in a
phenotype called a genetic disorder.
Sickle Cell Anemia


This disease affects millions of people
world wide.
About 2 million Americans have the
sickle cell trait. About 1 in 12 are African
Americans.
Sickle Cell Continued
Sickle Cell Continued
Sickle Cell Continued
Sickle Cell Continued
T-Boz

Many people came to know
more about SCD through the
efforts of Tionne “T-Boz”
Watkins of the R&B group, TLC.
Many people with SCD grow up
knowing that this disease could
greatly impact their life, but at
35, T-Boz is going strong and is
an inspiration to many. Not only
did she sing and dance her way
to stardom with the pop group
TLC, she gave birth to a child at
30. “Everything I ever wanted to
do, I did. I'm all for taking over
the disease instead of letting it
take over me."