Lecture 3
Mendelian Genetics
Phenotype and Genotype
Genotype and Phenotype
• Genotype – genetic constitution of an
•
-
organism: Total set of alleles of an individual
Describe the following :
AA: Homozygous Dominant (Wild Type)
Aa: Heterozygous
aa: Homozygous Recessive (double mutant)
• Phenotype – observable characteristic
• Genotype and environment
• Contribution of environment varies between
genes
• Can be controlled by many genes
• Random developmental events
Mendel’s Experimental Design
Mendelian Genetics
• Modern genetics began with
Gregor Mendel’s quantitative
genetic experiments
• Austrian monk
• Mathematician
• Numerical and observational
data
• Several generations
Stamen
Carpel
Mendelian Genetics
• Heritable, obvious traits
• Simple crosses at first
• Used peas because:
• Easy to grow and available
• Many distinguishable
characteristics
• Self-fertilization
• True breeding peas
Pea Traits
Monohybrid Crosses and Mendel’s Principle of
Segregation
Dominant and Recessive Alleles
• Pairs—the factors that control each trait exist in
pairs.
• Female parent—contributes one factor (♀)
• Male parent—contributes one factor (♂)
• Together these make a pair
• Genes—are the factors that control traits.
• Genes consist of pairs of alleles. One that
comes from the mother parent and one that
comes from the father parent.
Dominant and Recessive Alleles
• Alleles—the different forms of a gene (such as tall
or short, wrinkled or smooth).
• Dominant allele—one whose trait always shows up
when the allele is present.
• Recessive allele—is masked (or covered up) when
the dominant allele is present. Recessive alleles only
show up if a dominant allele is not present.
EXAMPLES
recessive
dominant
Recessive is the green box and dominant is the black box. Each of your
parents has a pair of alleles that they can share. If they only give one…
answer the following questions.
dominant
dominant
?
dominant
recessive
dominant
?
dominant
recessive
recessive
?
recessive
Breeding Crosses
• Initial cross is the P generation
• Parents
• Progeny of parents is first filial
generation
• F1 generation
• Inbreeding of first generation
creates second filial generation
• F2 generation
Monohybrid Crosses
• Cross between true-breeding
individuals with one different trait
• Mendel’s first crosses
• Resembled only one of the parents
• Planted progeny and allowed selffertilization
• Revealed both phenotypes
aa
AA
Aa
Aa
Aa
[S]
[W]
Smooth seeds (allele A) are
completely dominant to
wrinkled seeds(allele a)
Monohybrid Cross
• Mendel determined
that
• Particulate
factors for genes,
each contains a
set of two
• Transmitted by
both parents
• Alternate forms
called alleles
• True breeding
forms contains
identical set
GENETIC MAKEUP (ALLELES)
P PLANTS
Gametes
AA
aa
All A
All a
F1 PLANTS
(hybrids)
All Aa
Gametes
1/
2
1/
2
A
Eggs
A
A
a
Sperm
AA
F2 PLANTS
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 AA : 2 Aa : 1 aa
a
a
Aa
Aa
aa
Monohybrid Cross
• F1 generation had both alleles
• Only one expresses
• One allele masks
• Dominant
• Recessive
• Identical alleles – homozygous
• Different alleles - heterozygous
Monohybrid Cross
Principle of Segregation
• Recessive characteristics
are masked
• Reappear in F2
• Members of a gene pair
(alleles) segregated during
gamete formation
How cells carry characteristics ?
Aa male
Formation of sperm
F1 genotypes
Aa female
Formation of eggs
1
2
1
2
1
2
A
A
A
a
A
1
4
F2 genotypes
1
2
a
A
a
1
4
A
a
1
4
a
a
1
4
Branch Diagrams
• Punnett squares can
become messy with
more than one gene
• Use branch diagram
to figure out genotype
and phenotype
expected frequency
Test Cross
• Mendel did several
crosses
• Followed over several
generations
TESTCROSS:
GENOTYPES
B_
• Selfing also very
important
• Allowed plants to
reveal their genotype
and not just their
phenotye
bb
Two possibilities for the black dog:
BB
or
Bb
B
B
b
GAMETES
b
OFFSPRING
Bb
All black
b
Bb
bb
1 black : 1 chocolate
Test
Cross
Dihybrid and Trihybrid Crosses and
Mendel’s Principle of Independent
Assortment
The Principle of Independent
Assortment
• Factors for different
traits assort
independently of one
another
• Genes are inherited
independently of each
other
• Segregate randomly in
gametes
• Dihybrid Cross
Branch Diagram of Dihybrid Cross
P
h
e
n
o
t
y
p
e
Genotype vs.
Phenotype
Test Cross With Dihybrid
Trihybrid Cross
Tribble Traits Activity
Statistical Analysis of Genetic
Data: The Chi-Square Test
Probability & Genetics
Statistical Analysis
• Data from genetics is quantitative
• Use statistics to show deviation of
observed results from predicted results
• Chance factors cause deviations
• Null-hypothesis – no difference between
the predicted and observed
• If not accepted then have to come up with a
new hypothesis for deviation
Genetics & Probability
• Mendel’s laws:
• segregation
• independent assortment
reflect same laws of
probability that apply to
tossing coins or rolling dice
Probability & genetics
• Calculating probability of
making a specific gamete
is just like calculating the
probability in flipping a
coin
• probability of tossing
heads? 50%
• probability making a P
gamete…
P
50%
Pp
p
PP
P
100%
P
Probability & genetics
• Outcome of 1 toss has no
impact on the outcome of the
next toss
• probability of tossing heads each
time?
50%
• probability making a P gamete
each time? 50%
P
Pp
p
Calculating probability
Pp x Pp
male / sperm
P
p
sperm
egg
offspring
P
P
PP
P
p
1/2 x 1/2 =
female / eggs
1/2 x 1/2 =
P
PP
Pp
p
Pp
pp
Pp
1/4
P
1/2 x 1/2 =
p
1/4
1/4
1/2
p
p
1/2 x 1/2 =
pp
1/4
2005-2006
Rule of multiplication
• Chance that 2 or more independent events
will occur together
• probability that 2 coins tossed at the same
time will land heads up
1/2 x 1/2 = 1/4
• probability of Pp x Pp  pp
1/2 x 1/2 = 1/4
Calculating dihybrid probability
• Rule of multiplication also applies to
dihybrid crosses
• heterozygous parents — YyRr
• probability of producing yyrr?
• probability of producing y gamete = 1/2
• probability of producing r gamete = 1/2
• probability of producing yr gamete =
1/2 x 1/2 = 1/4
• probability of producing a yyrr offspring =
1/4 x 1/4 = 1/16
Rule of addition
• Chance that an event can occur
2 or more different ways
• sum of the separate probabilities
• probability of Pp x Pp  Pp
sperm
egg
offspring
P
p
Pp
1/2 x 1/2 =
p
P
1/2 x 1/2 =
1/4
Pp
1/4
1/4
+ 1/4
1/2
Chi-square test
• Test to see if your data supports your hypothesis
• Compare “observed” vs. “expected” data
• is variance from expected due to “random
chance”?
• is there another factor influencing data?
• null hypothesis
• degrees of freedom
• statistical significance
Chi-Square Test
• Goodness of fit test
• How much observed number deviates from the
expected number
Chi - Square Test
Mendelian Genetics in Humans
Pedigree Analysis
• Inheritance patterns
are studied using
family trees
• Pedigree analysis
• Phenotypic records
• Proband is where gene
was discovered
Examples of Human Genetic Traits
•
Most genetic disorders
are recessive
•
•
Homozygous recessive
expression
•
•
Due to lack of function
Dominant usually
selected out
Albinism
Characteristics of Recessive
Inheritance Traits
• Most have normal
heterozygous parents
• Heterozygotes have 3:1 ratio
• When both parents have the
trait then all progeny have
the trait
• Cystic Fibrosis, Sickle Cell
Anemia, Tay Sachs
Characteristics of Dominant
Inheritance Traits
• Gain of function mutations
• New property of the mutant
gene
• No loss of function
• Must have one parent with
disease
• Does not skip generations
• Will transmit to half its
progeny
• Huntingtons disease, Marfan
syndrome, achondroplasia