Lecture 6:
Mendelian Monohybrid
Inheritance (Segregation)
Clara Yona
Department of Biosciences
Solomon Mahlangu College of Science and
Education

Mendelian Genetics Definitions
• Allele - one alternative form of a given allelic pair; tall and dwarf are the
alleles for the height of a pea plant; more than two alleles can exist for
any specific gene, but only two of them will be found within any
individual
• Allelic pair - the combination of two alleles which comprise the gene
pair
• Homozygote - an individual which contains only one allele at the allelic
pair; for example DD is homozygous dominant and dd is homozygous
recessive; pure lines are homozygous for the gene of interest
• Heterozygote - an individual which contains one of each member of the
gene pair; for example the Dd heterozygote

Defn cont…
• Genotype - the specific allelic combination for a certain gene or set of
genes
• Phenotype - literally means "the form that is shown"; it is the outward,
physical appearance of a particular trait
• Dominant - the allele that expresses itself at the expense of an alternate
allele; the phenotype that is expressed in the F1 generation from the cross
of two pure lines
• Recessive - an allele whose expression is suppressed in the presence of a
dominant allele; the phenotype that disappears in the F1 generation from
the cross of two pure lines and reappears in the F2 generation

Defn cont…
• Monohybrid cross - a cross between parents that differ at a
single gene pair (usually AA x aa)
• Monohybrid - the offspring of two parents that are
homozygous for alternate alleles of a gene pair

Animals

Mechanism (1): All in the Genes
commons.wikimedia.org/wiki/Image:DNA_double_helix_vertikal.PNG
• The genetic make-up of
an organism is known as
its genotype.
• An organism’s genotype
and the environment in
which it lives determines
its total characteristic traits
i.e. its phenotype.
Phenotype
Genotype

Mechanism (2): DNA
Watson and Crick and
their model of DNA
www.chem.ucsb.edu/~kalju/chem110L/public/tutorial/images/WatsonCrick.jpg
en.wikipedia.org/wiki/DNA
DNA
replication
• The double-helix
structure of DNA
was discovered
in 1953.
• This showed how
genetic information
is transferred from
one cell to another
almost without error.

Gregor Johann Mendel
(1822-1884)
"father of modern genetics"

Gregory Mendel
• 1st scientist to develop the fundamental principles that became the modern
science of “genetics”
• Mendel study was based on simple patterns of inheritance
• Demonstrated that heritable properties are parceled out in separate units
(independently inherited)
• When doing his works, was selective growing the common pea plants
(Pisum sativum)
• At the time there was no knowledge of chromosomes, cell structure,
fertilization, mitosis and meiosis, which were discovered after his lifetime

Mendelism vs Darwinism
• In 1865 Mendel proposed that invisible internal units
(factors) of information account for observable traits, and that
these "factors" - which later became known as genes - are
passed from one generation to the next.
• Mendel's work remained unnoticed, languishing in the shadow
of Darwin's more sensational publication from five years
earlier, until 1900, when Hugo de Vries, Erich Von
Tschermak, and Carl Correns published research
corroborating Mendel's mechanism of heredity.

… Mendelism vs Darwinism
• In 1859, Charles Darwin (1809 - 1882) hypothesized that
animal populations adapt their forms over time to best exploit
the environment, a process he referred to as "natural selection."
As he traveled in the Galapagos Islands, he observed how the
finch's beaks on each island were adapted to their food sources.
He theorized that only the creatures best suited to their
environment survive to reproduce.
• Charles Darwin's landmark book, “The Origin of Species," was
published in London. It effectively drowned out all other
scientific voices, including Mendel’s, for decades.

Charles Darwin

• Although he was not a world renowned scientist, Mendel was
the 1st to trace the characteristics of successive generations of a
living thing
• While his research was with plants, the basic underlying
principles of heredity that he discovered also apply to people
and animals
• The impact of genetic theory is no longer questioned in
anyone's mind
• Many diseases are known to be inherited, and pedigrees are
typically traced to determine the probability of passing along an
hereditary circles

Mendel studies
• Based on seven traits of peas
• These were qualitative traits and could be measured and
assigned value
• These characteristics were visible and were used to study the
effects of reproduction

The subject for his research, chose the common garden pea
plants as focus of his research because:
(i) they have a number of sharply contrasting characters
(ii) they are easy to culture
(iii) they can be grown easily in large numbers
(iv) they produce many offspring per mating
(v) they are capable of self-fertilization
(vi) they have a number of varieties in type and
(vii) their reproduction can be manipulated.

SEED SHAPE
Round Wrinkled

POD COLOUR
Green Yellow

SEED COLOUR
Yellow Green

POD SHAPE
Inflated Constricted

FLOWER COLOUR
Purple White

FLOWER POSITION
Axial
(stem)
Terminal
(at tip)

STEM LENGTH
Tall Dwarf

Pea- plant characters studied by Mendel
Purple White
Axial (stem) Terminal
(at tip)
Yellow Green
Round/Smooth Wrinkled
Inflated Constricted
Green Yellow
Tall Dwarf

Mendel Experiments
(i) Number of different traits that can be
studied
(ii) Be self-fertilizing with a flower structure
that limits accidental contact
(iii) Offspring of self-fertilized plants should
be fully fertile

PEA FLOWER SEX STRUCTURE
Female sex structure
(Pistil)
Male sex structure
(Stamen)

Pea plant
• Have both male and female reproductive organs
• Mendel could open the flower buds and remove stamen before they were
ripe
• He used pollen from another plant and dusted the pistil to affect effective
cross-pollination
• As a result, they can either self-pollinate themselves or cross-pollinate
with another plant
• In his experiments, he was able to selectively cross-pollinate purebred
plants with particular traits and observe the outcome over many
generations

Mendel carried out some cross-fertilization

Removed stamens
from purple flower
1
2
3
Transferred pollen
from stamens of
white flower to
carpel of purple
flower
4 Planted seeds
from pod
Pollinated carpel
matured into pod
White
Stamens
Carpel
Parents
(P)
Purple
Offspring
(F1)

• Mendel was able to selectively cross-pollinate purebred plants with
particular traits and observe the outcome over many generations
• With clear-cut differences it is possible to cross or self pollinate the
plant and examine the characteristics of their offspring
• This was the basis for his conclusions about the nature of genetic
inheritance

Mendelian Experiments
• A cross between individuals differing in single character is a
monohybrid cross or monohybrid inheritance
Monohybrid test cross

F2 = F1 x F1

The reappearance of the recessive trait in ¼ of the F2,
suggests genes come in pairs that separate in the formation
of sex cells.
Three genotype (YY, Yy and yy)
Two phenotype (Yellow & Green)
3 Yellow 1 Green

Mendel Experimental Results
(i) F1 offspring showed only one of the two parental
traits, and always the same trait
(ii) Results were always the same regardless of which
parent donated the pollen
(iii) The trait not shown in the F1 reappeared in the F2 in
about 25% of the offspring
(iv) Traits remained unchanged when passed to
offspring: they did not blend in any offspring but
behaved as separate units
(v) Reciprocal crosses showed each parent made an
equal contribution to the offspring

Conclusions from monohybrid experiment
• Evidence indicated that, factors could be hidden or unexpressed, these
are the recessive traits
• The term phenotype refers to the outward appearance of a trait, while
the term genotype is used for the genetic makeup of an organism
• Male and female contributed equally to the offsprings' genetic
makeup
• Upper case letters are traditionally used to denote dominant traits,
lower case letters for recessives

Mendel’s first law
Mendel's First Law - the law of segregation; during gamete
formation each member of the allelic pair separates from the
other member to form the genetic constitution of the gamete
The reappearance of the recessive characteristic in F2 generation
shows that recessive genes are not lost or modified in the F1.

Principle of Segregation in Modern
Genetics

(a) Gamete formation/ meiosis
Give rise into four daughter cells.

(b) Gene segregation

Thank You