Mendellian Inheritance and Gene Action

MENDELIAN INHERITANCE
AND GENE ACTION
-BY PROF. T. V. RATHOD

Mendelian
Inheritance
&
Heredity;
Gene action
“Pass the Peas
like we use to
say it!”
Image: Sweet pea flower;
Giligone; Gregor Mendel, Wiki

Genetics Terminology: Chromosomes & Genes
• ________ -
Complete complement
of an organism’s DNA.
• Cellular DNA is
organized in
___________.
• ______ have specific
places on chromosomes.
Image: Chromosome & gene, Graham Colm,
National Human Genome Research Institute

So who was Mendel?
• Once upon a time (1860's), in an Austrian monastery,
there lived a monk named Gregor Mendel.
• Mendel spent his spare time breeding pea plants.
• He did this over & over & over again, and noticed patterns
to the inheritance of traits, from one set of pea plants to
the next.
• By carefully analyzing his pea plant numbers, he
discovered three laws of inheritance.
• Mendel's Laws are as follows:
1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment
• In his work, the words "chromosomes" or "genes" are nowhere to
be found. The role of these things in relation to inheritance &
heredity had not been discovered yet.
• What makes Mendel's contributions so impressive is that he
described the basic patterns of inheritance before the
mechanism for inheritance (namely genes) was even discovered!
The dude
was a
total
GENIUS!
Image: Gregor Mendel, Mendel's Principles of Heredity: A Defense
by Bateson, William; Spicoli from Fast Times at Ridgemont High

First, a little more genetics terminology.
Then…
Mendel's Laws
1. Law of Dominance
2. Law of Segregation
3. Law of Independent Assortment
Image: Gregor Mendel, Mendel's Principles
of Heredity: A Defense by Bateson, William.

Mendel’s experiments
• Convenience of handling
• Controlled mating
• Short life cycle
• Large number of fertile
off-springs
• Presence of variation
Characters chosen by Mendel for his study
Character Dominant form Recessive form
1. Plant height Tall Dwarf
2. Seed texture Round Wrinkled
3. Seed colour Yellow Green
4. Flower colour Violet White
5. Pod colour Green Yellow
6. Pod shape Inflated Constricted
7. Position of flowers Axial Terminal

Genetics Terminology
• Genotype: the genes of an
organism (all your genes)
• Phenotype: an organism’s traits
(expression of your genes)
• Allele: variations of a gene
• Represented with letters for
the different types of alleles
(PP, Pp, pp)
• homozygous: pair of identical
alleles for a character (PP, pp)
• heterozygous: two different
alleles for a gene (Pp)

• Character: heritable feature (i.e.,
fur color)
• Trait: variant for a character (i.e.
brown)
• True-bred: all offspring of same
variety
• Hybridization: crossing of 2
different true-breds
• Hereditary variation: refers
to the differences in the inherited
traits.
• Environmental Variation: It is
entirely due to environment.
We label the different
generations of a cross as:
• P generation (parents)
• F1 generation (1st filial generation)
• F2 generation (2nd filial generation)

___ ___ ___ ___
___ ___
Q: Would “Harriet” be able to roll her tongue?
Dominant & Recessive
Genotypes & Phenotypes
____________ genotype:
Both recessive alleles must be present (rr).
___________ genotype:
At least one dominant allele is present (R-).
Character: Tongue Rolling
Being able to roll your tongue is a dominant
phenotype.
Harry: Being able to roll your tongue is the dominant trait
(phenotype). Q: How would we represent the genotype of he
was homozygous dominant?
Hermione: Not being able to roll your tongue is the
recessive (phenotype). Q: What would be the recessive
genotype?

• In a cross of parents that
are pure for contrasting
traits, only one form of the
trait will appear in the next
generation.
• Offspring that are hybrid
for a trait will have only the
dominant trait in the
phenotype.
• States that in a hybrid one
factor of the allelomorphic
pair expresses itself
completely over the other.
1. Mendel’s Law of _________
Image: Simple Inheritance, complete
dominance, Magnus Manske

2. Mendel’s Law of ________
• A pair of alleles /
allelomorphs is brought
together in a hybrid (F 1)
they remain together
without contaminating each
other
• they separate or segregate
from each other into a
gamete in a complete and
pure form during the
formation of gametes.
• The alleles for each
character segregate
(separate) during gamete
production (_______).
• Alleles for a trait are
recombined at fertilization,
becoming genotype for the
traits of the offspring.
Image: Independent assortment and
segregation diagram, Mariana Ruiz.
Table showing how
the genes exchange
according to
segregation or
independent
assortment during
meiosis and how this
translates into
Mendel's laws.

• Alleles for different traits
are distributed to sex cells
(& offspring) independently
of one another.
• The factors in an
allelomorphic pair separates
independently to the
separation of factors in the
other allelomorphic pair.
Remember…Mendel came up with
this stuff BEFORE we know
about the existence of DNA,
genes, chromosomes.
WOW!
3. Mendel’s Law of _____ ______
Image: Independent assortment and
segregation diagram, Mariana Ruiz.
Diagram of how the
genes exchange
according to
segregation or
independent
assortment during
meiosis and how this
translates into
Mendel's laws.

Mendel’s Laws:
1. Law of Dominance:
- States that in a hybrid one factor of the allelomorphic
pair expresses itself completely over the other.
2. Law of Segregations:
- During the formation of gametes (eggs or sperm), the two alleles
(hereditary units) 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.
3. Law of Independent Assortment:
- The factors in an allelomorphic pair separates independently to the separation
of factors in the other allelomorphic pair.
Image: Gregor Mendel, Mendel's Principles of
Heredity: A Defense by Bateson, William

Monohybrid cross
(cross with only 1 trait)
 Problem:
 Using this is a several step process, look at
the following example
 Tallness (T) is dominant over shortness (t)
in pea plants. A Homozygous tall plant (TT)
is crossed with a short plant (tt). What is
the genotypic makeup of the offspring?
The phenotypic makeup?

1. Determine alleles of
each parent, these are
given as TT, and tt
respectively.
2. Take each possible
allele of each parent,
separate them, and
place each allele either
along the top, or along
the side of the punnett
square.
Tt Tt
Tt Tt

Here we have some more
interesting results:
First we now have 3
genotypes (TT, Tt, & tt)
in a 1:2:1 genotypic
ratio. We now have 2
different phenotypes
(Tall & short) in a 3:1
Phenotypic ratio. This is
the common outcome
from such crosses.

Dihybrid crosses
 Dihybrid crosses are made when phenotypes
and genotypes composed of 2 independent
alleles are analyzed.
 Process is very similar to monohybrid crosses.
 Example:
 2 traits are being analyzed
 Seed colour(Yy) with yellow being dominant
green,
 Seed shape (Rr) with round being dominant
to wrinkled.

Dihybrid cross example
The cross with a pure-breeding (homozygous) Yellow, Round seed plant
with a pure-breeding green,wrinkled plant should look like this.

Reasons for success of Mendel
• The experiments were very well designed & conducted with great care and skill.
• The choice of his experimental material.
• Mendel studied the inheritance of only characters at a time.
• The characters he chose were well defined and simple.
• The seven characters selected by Mendel showed qualitative inheritance.
• The contrasting forms of each of case one form was completely dominant over
other.
• His knowledge on mathematics was a definite asset for the interpretation of his
findings.
• He maintained particulars of pedigree records, which gave him the exact
ancestry of any given plant.

Figuring Out Patterns of Inheritance
A Punnett square is a tool for
diagramming the possible
genotypes of offspring.
• Let do a Punnett square for the
trait of round seed shape is
dominant over wrinkled seed
shape.
• Round seed shape
- dominant phenotype
- Q: What is gentoype?
• Wrinkled seed shape
- Recessive phenotype
- Q: What is genotype?
F1 (Progeny)
- Dominant phenotype [round seed shape]
- Q: What is F1 genotype?
Male parennt Genotype:
Female
parent
Genotype:
Generation Parental
Parents Female X Male
Phenotype Round X Wrinkled
Genotype RR X rr
Gametes R r
Generation F1 Rr
(Heterozygous) Round
F2 F1 XF1
R
R r
r
RR
Round
Rr
Round
Rr
Round
rr
Wrinkled

So far, we’ve discussed
Simple Inheritance &
Punnett Squares…
But, of course, genetic is
much more complicated
than that.
Let’s explore:
• Complete dominance
• Incomplete dominance
• Co-dominance
• Over dominance
A Punnett square

• Complete dominance:
• The phenotype produced by a
heterozygote is identical to
that produced by the
homozygotes for the
concerned dominant allele.
• The dominant allele in such a
situation is said to be
completely or fully dominant.
Eg: In garden pea, round seed shape is
completely dominant over wrinkled.
Round x Wrinkled
RR rr
F1 Rr
r
Wrinkled
R
Round
Rr
Round

• Incomplete dominance: In many
cases, the intensity of
phenotype produced by
heterozygote is less than that
produced by the homozygote for
the concerned dominant allele.
• Therefore the phenotype of
heterozygote falls between
those of the homozygotes for
the two concerned alleles.
• Such a situation is known as
Incomplete or partial dominance
and the dominant allele is called
incompletely dominant or
partially dominant.
Eg : In Mirabilis jalapa (Four ‘O’ clock plant)
a partially dominant allele ‘R’ produces red
flowers in homozygous state, while its
recessive allele ‘r’ produces white flowers
in homozygous state. When a red (RR)
flower type plant is crossed with white
(rr) flower type plant, the hybrid (Rr) has
pink flowers.
Red x White
RR x rr
F1 Rr
r
White
R
Red
Rr
Pink

• Co-dominance: Both the
alleles of a gene express
themselves in heterozygotes.
• As a result, heterozygotes
for such genes possess the
phenotypes produced by both
the concerned alleles.
• The coat colour of short horned
breed of cattle presents an
excellent example of
codominance. Roan colour is that
which has patches of red and
white colours.
Red x White
CR C R x Cr C r
F1
Cr
White
CR
Red
CR Cr
Roan

Co-domiance : of human blood
- Has three alleles: A, B & O
- AB co-dominant, O recessive
- Genotype represented using
IA, IB & i
Phenotype Genotype
Type A IAIA or IAi
Type B IBIB or IBi
Type AB IAIB
Type O ii
Image: ABO blood type, InvictaHOG

ABO Blood Type
You make antibodies against the
antigens of other blood types. .
– Q: Which blood type can
accept anyone's blood.
– Q: Which blood type is known
as the “universal donor. Why?
Image: ABO blood type, InvictaHOG
Phenotype Genotype
Type A IAIA or IAi
Type B IBIB or IBi
Type AB IAIB
Type O ii

• Over dominance: In case of some
genes, the intensity of character
governed by them is greater in
heterozygotes than in the two
concerned homozygotes.
• This situation is known as over-
dominance.
• True over dominance is known in case
of very few genes.
• Over-dominance is not the property
of an allele but is the consequence
of heterozygous state of concerned
gene.
 Transgressive segregation: The
appearance of individuals in F2 or
subsequent generation which exceed the
parental types with reference to one or
more characters is known as
transgressive segregation. Or
 The segregants which fall outside the
range of both the parents are called
transgressive segregants and the
phenomenon is called transgresive
segregation.
Eg: white eye gene (W) of Drosophila
exhibits overdominance for some of
the eye pigments such as
sepiapteridine and Himmel blaus.
These two eye pigments are present
in low concentration in the recessive
homozygotes (ww), while the
dominant homozygotes (WW) have
relatively higher concentrations of
these pigments. However, the flies
heterozygous for this gene (Ww)
have an appreciably higher
concentration of these two pigments
than the two homozygotes.

Exceptions to Mendel’s laws:
• Paramutations and polyploidy are exceptions to the law of
segregation or law of purity of gametes.
• Linkage is an exception to Mendel’s second law i.e. law of
independent assortment.
• Incomplete dominance is an exception to the principle of
dominance.
• Pleiotropism is an exception to the principle of unit characters.
• Modification of F2 ratios due to incomplete -dominance, co-
dominance, lethalfactors, interaction of factors, epistatic
factors are all exceptions.

GENE ACTION
• Gene action refers to the manner in which genes control
the phenotypic expression of various characters in an
organism.
• Alleles of the gene may interact with one another in a
number of ways to produce variability in their phenotypic
expression.
• The dominant and recessive relationship is fundamental
and is essentially constant with each pair of alleles.

Gene action can be of the following types:
1. Based on the dominance effect:
a) Complete dominance
b) Incomplete dominance
c) Co-dominance
d) Over dominance
e) Pseudo-dominance
2. Based on lethal effects :
a) Dominant lethals
b) Recessive lethals
3. Based on epistatic action :
a) Epistatic factors
b) Supplementary factors
c) Duplicate factors
d) Complementary factors
e) Additive factors
f) Inhibitory factors
4. Based on number of genes involved:
a) Monogenic
b) Digenic
c) Oligogenic
d) Polygenic
5. Based on pleiotropism
a) Pleiotropic gene action

TYPICAL DIHYBRID RATIO
9:3:3:1

Based on the dominance effect:
• Complete dominance
• Incomplete dominance
• Co-dominance
• Over dominance
• Pseudo-dominance:
• Expression of recessive allele of the
gene in the hemizygous state /
condition either due to sex linkage
(Eg: colour blindness in human
beings) or chromosomal aberrations
(deletion in heterozygotes) is known
as pseudo-dominance.

Based on lethal effect:
• Dominant Lethal gene action: A
lethal gene affecting coat colour in
mice was discovered by French
geneticist Cuenot in 1905.
• He found that,
• Yellow coat colour in mice was
produced by a dominant gene ‘Y’
• recessive allele ‘y’ determines the
normal black / grey coat colour
• all the mice with yellow coat colour
were heterozygous Yy
• and he was unable to found a mouse
homozygous for ‘Y’ allele (YY).
• The dominant allele ‘Y’ is lethal and
hence it causes death of homozygous
‘YY’ embryos at an early stage of
development.

• Recessive lethals:
• Albino seedling character in plants such
as rice and barley is governed by
recessive alleles.
• Whenever these alleles are in the
homozygous state the seedlings are near
white or almost white and totally devoid
of chlorophyll.
• Albino seedlings survive only as long as
the food material stored in the seeds is
available to them because they are not
able to carry out photosynthesis.
• The heterozygotes, however are normal
green and are identical with the dominant
homozygotes in their phenotype as well as
their survival.
• Segregation of such genes produces 3
green : 1 albino seedling if they are
counted within a week from germination.
• However, if the plants are counted at
maturity, there will be only green plants in
the progeny.
Female x male
Green x Green
GG Gg
F1 Gg
On selfing of Gg individuals
Gg x Gg
F2
G
G g
g
GG
Green
Gg
Green
Gg
Green
gg
albino
(dies)

Based on epistatic gene action:
• When expression of one gene
depends on presence / absence of
another gene in an individual, it is
known as gene interaction.
• Interaction of genes at different
loci that affect the same character
is called epistasis.
• The term epistasis was first used by
Bateson in 1909 to describe two
different genes which control the
same character,
• out of which one masks / suppresses
the expression of another gene.
• Gene that masks the action of
another gene is called epistatic gene
• while the gene whose expression is
being masked is called hypostatic
gene.
Epistatic gene interaction can be of the
following types.
(i) Epistatic factors - 12 : 3 : 1
(ii) Supplementary factors - 9 : 3 : 4
(iii) Duplicate factors - 15 : 1
(iv) Complementary factors - 9 : 7
(v) Additive factors - 9 : 6 : 1
(vi) Inhibitory factors - 13 : 3

Epistatic factors - 12 : 3 : 1
• Also referred to as masking gene
action.
• In this, dominant alleles of two
genes affecting the same
character produce distinct
phenotypes when they are with
homozygous recessive state of the
other gene.
• But when dominant alleles of both
the genes present together, the
expression of one gene masks that
of other.
• When both the genes are present
in the recessive state, a different
phenotype is produced.
• Thus, in this case, both the genes
express themselves when their
dominant alleles are present
togather, but the expression of
one is so intance or strong that
expression of other gene cannot
be observed.

Supplementary factors - 9 : 3 : 4
• Dominant allele of one of the
two genes governing a character
produces a phenotypic effect.
• However, the dominant allele of
the other gene does not produce
a phenotypic on its own.
• But when it is present with the
dominant allele of the first
gene, it modifies the phenotypic
effect produced by that gene.

Duplicate factors - 15 : 1
• Character showing duplicate
gene action are determined by
two completely dominant genes,
which produces the same
phenotype whether they are
alone (i.e., with the recessive
allele of other gene) or
together;
• the contrasting phenotype is
produced only when both the
gene are in homozygous
recessive state
TV
TV
Tv
Tv
tV
tV
tv
tv
TTVV TTVv TtVV TtVv
TTVv TTvv TtVv Ttvv
TtVV TtVv ttVV ttVv
TtVv Ttvv ttVv ttvv
TTVV
Triangular
ttvv
Ovate
TtVv
All triangular
F1 (TtVv) x F1 (TtVv)
x
F1 generation

Complementary factors - 9 : 7
• In this, the production of one of
the two phenotypes of a trait
require the presence of
dominant alleles of both the
genes controlling the concerned
trait.
• When any one of the two or
both the genes are present in
the homozygous recessive state,
the contrasting phenotype is
produced.

• In this, one of the two completely dominant genes produces the
concerned phenotype, while its recessive allele produces the
contrasting phenotype.
• The second dominant gene, called inhibitory gene, has no effect of its
own on the character in question;
• However, it can stop the expression of the dominant allele of the first
gene.
• As a result when the two dominant genes are present together, they
produce the same phenotype as that produced by the recessive
homozygote of the first gene
Inhibitory factors - 13 : 3

• The two completely dominant genes controlling a character produced
identical phenotypes when their dominant alleles are present with
homozygous recessive condition of the other gene.
• But when dominant alleles of both the genes are present together,
their phenotypic effect is enhanced as if the effect of the two genes
were cumulative or additive
Additive factors - 9 : 6 : 1

Polygenic gene action
• In general, one gene controls or affects a single character. But some
characters are known to be controlled by more number of genes.
• Such genes are called poly genes and the phenomenon is called polymerism. Eg :
Yield in plants.

Pleiotropic gene action
• In general, one gene affects a single character.
• But some of the genes are known to affect or control more than one character.
• Such genes are called pleiotropic genes and the phenomenon is known as
pleiotropism.
• Many fold phenotypic expressions of a single gene is called pleiotropism or
pleiotropic gene effects.
• Eg: White eye gene effects the shape of sperm storage organs and other
structures in Drosophila.
• Good example of pleiotropism has been reported in wheat.
• A gene governing awns in Ona’s variety of wheat also increases the yield as well
as seed weight.

Confused?
Here are links to fun resources that further
explain genetics & heredity:
• Genetics & Heredity Main Page on the Virtual Cell
Biology Classroom of Science Prof Online.
• “Pass the Peas” song by James Brown performed by the
James Brown Tribute Tour.
• Independent Assortment animation, step-through and
quiz, WHFreeman.
• “I Think I’m Going Bald” song by Rush.
• Mendel’s Experiments animation, step-through and quiz,
Freeman, S, Biological Science, Second Edition, Pearson Prentice Hall,
Inc.© 2006 Pearson Prentice Hall, Inc.
• “Behind Blue Eyes” song by The Who.
• “She Blinded Me With Science” music video Thomas
Dolby.

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