CHAPTER 9
Mendel and the Gene Idea…
and a little bit about human genetics.
What is INHERITANCE?
The study of how physical traits
are passed from generation to
generation.
How are they passed on???
• GENES within DNA.
• DNA is the UNIT OF HEREDITY.
• DNA is the molecule that is transmitted
from parent to offspring that determines the
physical outcome or plan.
• DNA is instructions to make proteins.
• PROTEINS are enzymes, hormones, and
provide structure & color!!!!
GREGOR MENDEL:
•The “father of genetics”
•Austrian Monk
•First to explain patterns of
inheritance.
•Analyzed sweet pea plants
over 12 years.
•Called the genes- “factors”
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Mendel studied the
inheritance patterns for
physical traits like:
1.) flower petal color
or
2) pea pod color
or
3) pea shape.
Ex. round or wrinkled peas
Purple or White flowers
Mendel’s Experiments: First he
created “pure breeding” strains of
the plant through self-pollination.
Parent Generation:
• Pure for one of two contrasting traits
• Result of many generations of self-pollination
Genetic Cross #1
Mendel chose two plants pure for the two
contrasting traits, (P1) generation, and crossed
them (bred them together).
Mendel’s
Experiments:
(P1) Purple x (P1) White =
First Filial Generation (F1):
• Offspring of parent generation
• All F1 plants looked like only
one of the parents (in this case
they all looked purple)
• These were HYBRIDS
Genetic Cross #2:
Mendel then crossed two of the
F1 plants (ewww! plant incest)
Mendel’s Experiments:
Ppurple x Pwhite = F1purple
F1purple x F1purple =
Second Filial Generation
(F2):
• The traits from both parents
reappear in these offspring
• Some PURPLE
• Some white
Mendel found the ratio of
expression to be 3:1.
IMPORTANT CONCLUSIONS:
1. If a trait exists in two contrasting forms one is
dominant, one is recessive. The recessive one
disappears in the F1 generation (ex. White).
2. Factors controlling traits occur in pairs.
3. The DOMINANT FACTOR prevents the recessive
factor from being expressed.
4. Recessive factors are only expressed when both
factors in the pair are recessive.
5. When pea plants reproduce, a factor pair is
segregated (split) and each factor ends up in a
separate gamete. Mendel’s law of segregation.
Figure 14.4 Mendel’s law of segregation (Layer 2)
In modern terms:
1. Mendel’s factors are called
genes. Some genes are
dominant, others recessive.
2. Different forms of a single
gene are called alleles.
3. Genes occur in pairs, at the
same position on two
chromosomes.
4. The gene position is called Alleles are alternate versions
of a gene found on homologous
the locus.
Chromosomes.
5) The two chromosomes that contain the same genes are
called: homologous.
6) If both alleles in a gene pair are the same, we say the pair
is: homozygous (dominant or recessive)
7) If both alleles in a gene pair are different, we say the pair
is: heterozygous.
8) When solving inheritance problems, use capital letters to
represent dominant genes.
9) When solving inheritance problems, use lowercase letters
to represent recessive genes.
10) A genotype is the combination of genes an individual
possesses.
11) A phenotype is the appearance of an individual as the
genes are being expressed.
The Punnett Square
• Graphic tool used to solve inheritance problems
• Predicts the possible gene combinations inherited by
the offspring and their probability of occurring.
• For example, a monohybrid cross: involves one
character/trait.
ex. Heterozygous X homozygous recessive (on board)
Segregation of alleles and fertilization as chance events
If an individual is
Heterozygous (Pp), flipping
a coin- landing either heads
or tails is similar to the
probability of creating a
gamete containing either the
dominant or recessive gene
in it…
Both have a 50% probability
or 1 out of 2 chances of
occurring.
Fertilization and creating of zygotes is the probability of the
2 gametes happening together. 1/2 x 1/2 or 1/4
A test-cross:
is used to determine the genotype of an individual with
a dominant phenotype (cross the individual in question
with an individual with a recessive phenotype).
ex.
____
X
homozygous recessive
--->
offspring
DIHYBRID CROSS
• a two character/trait cross, with two gene
pairs.
• Results in 4 (not 2) phenotypes/looks.
• Each parent can create a variety of
gametes… how many?
ex. AaBb X AaBb
There are 4 possible kinds of gametes:
Mendel’s Law of Independent Assortment states when the
two gene pairs are located on non-homologous
chromosomes they segregate independently of each
other.
Aa Bb
Results = 4 possible gamete combinations. (use FOIL)
first: AB
outer: Ab
inner: aB
last: ab
(see board)
Arrogant, Beautiful X Arrogant, Beautiful
Man
Woman
Both are Heterozygous for both genes…
AaBb
X
AaBb
What are their chances of having: arrogant, beautiful
children? Modest, beautiful? arrogant, ugly? Etc.
AaBb X AaBb
AaBb X AaBb
AB
AB
Ab
aB
ab
Ab
aB
ab
AaBb X AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
aB
ab
AaBb X AaBb
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
aB
ab
Phenotype Ratio
9:3:3:1
9/16 Arrogant, Beautiful
3/16 Modest, Beautiful
3/16 Arrogant, Ugly
1/16 Modest, Ugly
Figure 14.7 Testing two hypotheses for segregation in a dihybrid cross
Now, it can’t be that easy can it?
No it can’t.
BESIDES THIS PATTERN OF INHERITANCE, called
DOMINANCE or COMPLETE DOMINANCE, where
having just ONE dominant allele results in the dominant
phenotype, THERE ARE ALSO THE FOLLOWING PATTERNS:
1. Incomplete dominance
2. Codominance
3. Multiple Alleles
4. Polygenic Inheritance
5. X linked inheritance
1)Incomplete Dominance: Both
alleles are the same strength
• Heterozygous individuals…express both
alleles, The resulting phenotype is a mix
(blend) of the two.
• For example: Carnation flower color
• The alleles are also written differently:
AA’ or CR CW
Figure 14.9 Incomplete dominance in snapdragon color
2)Codominant Genes:
• In a heterozygous individual both alleles
affect the phenotype in separate
distinguishable ways.
• For example: Roan Horses have red and
white hairs
• Another example: A, B, and AB blood
groups in humans.
3) Polygenic inheritance: When
one phenotype is under the
control of multiple gene pairs
• For example: Human Skin Color
• Alleles are… Dark and light
• At least… three genes control the color of your
skin
• The phenotype is a result of… the cumulative
effects of the dominant genes
• Human skin color exists… as a gradient
Figure 14.12 A simplified model for polygenic inheritance of skin color
4) Multiple Alleles: When more
than two forms of the gene exist
• For example: Human Blood Types (ABO blood groups)
• Blood cells have a carbohydrate marker that is found on
the surface of red blood cells.
• These markers are called: A substance or B substance.
• They are recognized by antibodies present in the blood
serum of individuals for foreign carbohydrate markers.
• There are two types of RBC antibody: Anti-A & Anti-B
• Antibodies are a class of biological molecules called:
Immunoglobulins
• The genes for creating these antibodies are determined by
the genes for creating the A or B substance.
Blood Type Genotype Antigens
on surface
(phenotype)
of RBC
A
B
AB
O
Plasma
Antibodies
Donate
Receive
Blood Type
A
B
AB
O
Genotype
IA IA
IAi
IB IB
IBi
IA IB
ii
Antigens
on surface
of RBC
Plasma
Antibodies
Donate
Receive
Blood Type Genotype Antigens on
surface
A
B
AB
O
Plasma
Antibodies
IA IA
IAi
IB IB
IBi
IA IB
Substance
A
Anti-B
Substance
B
Anti-A
AB
none
ii
none
Anti-A
Anti-B
Donate
Type A
& AB
Type B
& AB
TYPE
AB
All
blood
types
Receive
Type A
&O
Type B
&O
A, B,
AB & O
Type O
Figure 14.10 Multiple alleles for the ABO blood groups
Figure 14.10x ABO blood types
• Remember, the IA & IB alleles are
CODOMINANT.
• With human blood type, another marker called the
Rh factor is denoted as + or -.
• So, type O -, is the worst to be in terms of getting
a blood transfusion… but is the best donor.
• “universal donor”