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Mendelian
Genetics
Chapter 11
Part 1
pp. 189-197, 202-204
Genetic Theories
1. Blending Theory traits were like paints and mixed
evenly from both parents.
2. Incubation Theory only one parent controlled the traits
of the children.
Ex: Spermists and Ovists
3. Particulate Model parents pass on traits as discrete units that
retain their identities in the offspring.
INTRODUCTION TO GENETICS
2
Fruit and Flower of the Garden
Pea
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Flower Structure
stamen
anther
filament
stigma
style
a.
ovules in
ovary
carpel
5
• Mendel crossed pea
plants that differed
in certain
characteristics and
traced the traits
from generation to
generation
• This illustration shows
his technique for
cross-fertilization
that results in hybrid
offspring
Figure 9.2C
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
White
1 Removed
stamens
from purple
flower
Stamens
Carpel
PARENTS
(P)
2 Transferred pollen
Purple
from stamens of
white flower to
carpel of purple
flower
3 Pollinated carpel
matured into pod
4 Planted
seeds
from pod
OFFSPRING
(F1)
Self-pollination
• One flower as both parents.
• Natural event in peas.
• Results in pure-bred offspring where the
offspring are identical to the parents.
Results - Summary
• In all crosses, the F1 generation
showed only one of the traits regardless
of which was male or female.
• The other trait reappeared in the F2 at
~25% (3:1 ratio).
Mendel’s 4 Conclusions:
1. Various forms of genes exist
6
Mendel’s 4 Conclusions:
2. For each characteristic, we inherit 2 alleles (one
from each parent)
3. If alleles are different, in complete dominance,
there is a dominant and recessive allele
GENE LOCI
P
GENOTYPE:
Figure 9.4
a
B
P
a
b
PP
aa
Bb
HOMOZYGOUS
for the
dominant allele
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
HOMOZYGOUS
for the
recessive allele
DOMINANT
allele
RECESSIVE
allele
HETEROZYGOUS
Mendel’s 4
Conclusions:
GENETIC MAKEUP (ALLELES)
P PLANTS
4. Law of Segregation:
alleles separate during
meiosis and rejoin during
fertilization of sperm and
egg. Remember Anaphase
I (separation of tetrads)
Gametes
PP
pp
All P
All p
F1 PLANTS
(hybrids)
Gametes
All Pp
1/
2
Eggs
F2 PLANTS
Phenotypic ratio
3 purple : 1 white
Genotypic ratio
1 PP : 2 Pp : 1 pp
Figure 9.3B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
1/
P
P
P
PP
p
Pp
p
Sperm
p
Pp
pp
2
MENDEL’S LAW OF SEGREGATION
During the formation of gametes (eggs or sperm), the two alleles
responsible for a trait separate from each other. Alleles for a trait
are then "recombined" at fertilization, producing the genotype
for the traits of the offspring.
Rr
R
r
We can show Mendel’s idea
in the form of punnet
squares:
1.What is the
probability
that a
couple will
have a boy?
10
Modern Genetics View
• Each trait in a pea plant is controlled by two
alleles (alternate forms of a gene)
• Dominant allele (capital letter) masks the
expression of the recessive allele (lower-case)
• Alleles occur on a homologous pair of
chromosomes at a particular gene locus
– Homozygous = identical alleles
– Heterozygous = different alleles
15
Genotype versus Phenotype
• Genotype
– Refers to the two alleles an individual has for a specific
trait
– If identical, genotype is homozygous
– If different, genotype is heterozygous
• Phenotype
– Refers to the physical appearance of the individual
16
Patterns of Inheritance
• Complete dominance (Mendel’s focus)
• Incomplete dominance
• Codominance
• Multiple Alleles
• Pleiotropy
• Epistasis
• Polygenic Traits
11
Fig. 14-UN2
Degree of dominance
Example
Description
Complete dominance
of one allele
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
PP
Pp
C RC R
Codominance
Multiple alleles
Pleiotropy
Heterozygotes: Both
phenotypes expressed
In the whole population,
some genes have more
than two alleles
One gene is able to
affect multiple
phenotypic characters
CRCW CWCW
IAIB
ABO blood group alleles
IA , IB , i
Sickle-cell disease
Examples of
COMPLETE DOMINANCE
1. Class Example: Brown eyes are dominant over blue.What is
the probability that Mr. and Mrs. Brooks will have a child
with brown eyes if both are hybrids?
2. Class Example:The ability to roll the tongue is dominant to
the lack of this ability. Mr. Brooks is recessive and Mrs.
Brooks is heterozygous.
3. Class Example: Right handedness is dominant to left.
Tommy is right-handed but not a hybrid and his brother is
left handed.
What is the phenotypic ratio?
If Tommy has a child in the future, what handedness will this child be?
What was the probability that Tommy would have the genotype he has?
What was the probability that Tommy would have the phenotype he has?
10
Punnett Square Showing Earlobe Inheritance
Patterns
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Parents
Ee
Ee
eggs
E
e
EE
Ee
Ee
e e
Punnett square
spem
E
e
Offspring
Allele key
E = unattached earlobes
e = attached earlobes
Phenotypic Ratio
3
unattached earlobes
1
attached earlobes
20
Mendel’s Law of Probability
Multiplication
• Allows us to easily calculate probability, of
genotypes and phenotypes among the offspring
• Punnett square in next slide shows a 50% (or ½)
chance
– The chance of E = ½
– The chance of e = ½
• An offspring will inherit:
–
–
–
–
The chance of EE =½x½=¼
The chance of Ee =½x½=¼
The chance of eE =½x½=¼
The chance of ee =½x½=¼
Mendel’s Law of Probability: Addition
• Rule of Addition is that the probability of an event that can
occur in two or more independent ways is the sum of the
separate probabilities of the different ways….applies to
heterozygous being produced (Ee)
•
An offspring will inherit:
– The chance of EE =½x½=¼
– The chance of Ee =½x½=¼
– The chance of eE =½x½=¼
– The chance of ee =½x½=¼
The probability that a heterozygous
offspring will be produced is 1/4 + 1/4 =
1/2.
14
EXAMPLES
Monohybrid using Rules of
multiplication and addition
AA x Aa……Probability of Aa offspring?
Aa x Aa…..Probability of Aa offspring?
15
What if you don’t
know each parent’s
genotype?
TEST CROSS –
designed to reveal
the genotype of an
organism when you
do not know
Example using a TEST CROSS
• The ability to roll the tongue is
dominant to the lack of this
ability. Mr. Smith is a tongue
roller while Mrs. Smith is not.
What are the possible
probabilities of the next
generation?
17
15
Mendel’s Law: The LAW of INDEPENDENT
ASSORTMENT is revealed by tracking 2 traits
• By looking at two
characteristics at
once, Mendel
found that the
alleles of a pair
segregate
independently of
other allele pairs
during gamete
formation
– This is known
as the law of
independent
assortment
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
DIHYBRIDS
Analysis of two traits simultaneously
• Trait: Handedness (right vs. left) and Eye Color
(brown vs. blue)
• Use ‘H’ for handedness and ‘E’ is for eye color. Right
handedness is dominant to left and brown eyes are dominant
to blue. Mom is heterozygous for both traits and dad is a
hybrid for both traits.
– WHAT IS THE PHENOTYPIC
RATIO?
Rules of Probability Applies
to Dihybrid Crosses too
• For a dihybrid cross, HhEe x HhEe,
what is the probability of an F2
having the genotype
»HHEE?
»HhEe?
»hhee?
Let’s Try a Trihybrid
• Determine the probability that the
offspring would produce recessive
phenotypes for at least 2 of the 3
characters.
– Heterozygous purple flowers and yellow seeds
and round seeds crossed with heterozygous
purple and recessive green and wrinkled.
21
Fig. 14-UN1
Incomplete Dominance
Class Example: A cross between a red four o clock and a white four o
clock produces a pink one. Cross a red four o clock with a pink four o
clock.
Codominance
• Two alleles affect the phenotype in separate and distinguishable ways.
• Neither allele can mask the other and both are expressed in the
offspring and not in an “intermediate” form.
• In a study conducted by the University of Michigan, pitch black
feathered birds crossed with gray feathered birds produced a spotted
(black and gray), bird. Show a cross between 2 speckled birds.
• 1) In cattle, roan coat color (mixed red and white
hairs) occurs in the heterozygous (RW) offspring of
red (RR) and white (WW) homozygotes. When two
roan cattle are crossed, the phenotypes of the
progeny are found to be in the ratio of 1 red:2
roan:1 white. Which of the following crosses could
produce the highest percentage of roan cattle?
• A) roan x roan
• B) red x white
• C) white x roan
• D) red x roan
• E) All of the above crosses would give the same
percentage of roan.
Multiple Alleles
ABO Blood System
Multiple Allelic Traits
35
Rh Factor associated with
Blood Types demonstrates
Complete Dominance
29
Pleiotropy
• Most genes have multiple
phenotypic effects. The
ability of a gene to affect an
organism in many ways is
called pleiotropy.
Epistasis
• Epistasis occurs when a gene
at one locus alters or
influences the expression of a
gene at a second loci. In this
example, C is for color and the
dominate allele must be
present for pigment (color) to
be expressed.
In a certain breed of plants, B
produces blue flowers and is
dominant over b which produces
green flowers. Another gene
determines in which cells the pigment
will be synthesized. Allele M allows
complete synthesis of the pigment
throughout the flowers but the mutant
allele m prevents pigment
production.
You cross two plants that are BbMm.
•
Polygenetic Inheritance
• Qualitative variation
usually indicates
polygenic inheritance.
This occurs when there
is an additive effect from
two or more genes.
Pigmentation in humans
is controlled by at least
three (3) separately
inherited genes.
18
40
Fig. 14-UN2
Degree of dominance
Example
Description
Complete dominance
of one allele
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance
of either allele
Heterozygous phenotype
intermediate between
the two homozygous
phenotypes
PP
Pp
C RC R
Codominance
Multiple alleles
Pleiotropy
Heterozygotes: Both
phenotypes expressed
In the whole population,
some genes have more
than two alleles
One gene is able to
affect multiple
phenotypic characters
CRCW CWCW
IAIB
ABO blood group alleles
IA , IB , i
Sickle-cell disease
Fig. 14-UN3
Relationship among
genes
Epistasis
Example
Description
One gene affects
the expression of
another
BbCc
BbCc
BC bC Bc bc
BC
bC
Bc
bc
9
Polygenic
inheritance
A single phenotypic
character is
affected by
two or more genes
AaBbCc
:3
:4
AaBbCc