6.1.1 Linking Mendel`s Findings to Modern Genetics

Blueprint of Life
Topic 6: Linking Mendel’s Findings to Modern Genetics
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
 describe outcomes of monohybrid crosses involving
simple dominance using Mendel’s explanations
 describe the aspects of the experimental techniques used by
Mendel that led to his success
 Outline the reasons why the importance of Mendel’s work
was not recognised until some time after it was published
Monohybrid: a monohybrid is an individual that has
contrasting factors for one characteristic (mono = one; hybrid
= mixed or contrasting). Monohybrid inheritance is therefore
the inheritance of a single pair of contrasting characteristics.
The characteristics of an
organism are determined by
factors that occur in pairs.
Only one member of a pair
of factors can be represented
in any gamete (segregation).
Offspring inherit one factor
from each parent.
When two hybrids breed,
statistically they will produce
a ratio of three offspring
showing the same trait as the
parents (termed the dominant
trait) to one offspring showing
the contrasting recessive trait.
Mendel called the traits that pass from one generation to the
next ‘factors’, today we call them ‘genes’ and we call contrasting
forms of the same gene ‘alleles’. For example, tall and short are
alleles of the gene for height. However, chromosomes and genes
were not discovered for another 35 years.
A letter of the alphabet is used to
represent each ‘factor’ (e.g. stem
length in plants). The different forms
are distinguished by using a capital
letter for the dominant form (e.g. T )
and a lower case version of the same
letter for the recessive form (e.g. t).
Note: capital and lower case versions
of the same letter signify dominant
and recessive forms of the same
Explaining the Outcomes of
Monohybrid Crosses
This is how we would
demonstrate how Monohybrid
offspring are created when
pure-breeding parents with
contrasting characteristics are
Explaining the Outcomes of
Monohybrid Crosses
All gametes from pure-bred tall
will contain the factor ‘T’;
similarly gametes from purebred short will all contain ‘t’.
Each hybrid F1 individual
inherits one factor from each
parent (Tt).
Explaining the Outcomes of
Monohybrid Crosses
The monohybrid plants of the
first filial generation all
resemble the parent possessing
the dominant characteristic.
The factor that is expressed is
dominant in preference to the
other factor (recessive), which
is hidden.
Explaining the Outcomes of
Monohybrid Crosses
When the hybrid plants
produce gametes, the factors
for tallness and shortness
segregate or separate, with the
result that one half of the
gametes contain the factor for
tallness (T) and the other for
shortness (t). During
fertilisation, the gametes fuse,
each contributing one factor to
the resulting F2 offspring.
Explaining the Outcomes of
Monohybrid Crosses
In the plants of the second filial
generation, the dominant
characteristic appears three
times more frequently than the
recessive characteristic. That is,
as a result of a monohybrid
cross, the average ratio of
dominant to recessive offspring
observed is 3:1.
Explaining the Outcomes of
Monohybrid Crosses
The cross of gametes from the
F1 hybrids is complex, so it
could also be shown more
simply using a Punnett square.
A Punnett square is the
alternate way of representing
Mendel’s monohybrid cross.
Explaining the Outcomes of
Monohybrid Crosses
Mendel’s explanations as
outlined in this figure can be
applied to any monohybrid
cross involving factors such as
those he describes, i.e. factors
that are inherited as ‘discrete
units’ and not dependent on
whether they come from the
male or female parent.
Mendel’s Laws
Mendel’s model of inheritance
is based of the following
■ Each characteristic or trait in
an individual is controlled by a
pair of inherited factors.
■ Mendel’s factors (characters
or traits from parents) pass as
unmodified ‘units’ (individual
‘Mendelian’ factors today are
called ‘genes’) to successive
generations according to set
Mendel’s Laws
■ Individuals have two factors
for each characteristic and they
may have two factors the same
(in pure-breeding individuals)
or the two factors could differ
(in hybrid individuals).
■ The trait that is expressed
(appears) in the hybrid
individuals is dominant,
whereas the one that is hidden
or masked is the recessive trait
(Mendel’s first law—
Mendel’s Laws
■ During gamete formation,
the pair of factors for a trait
segregate (separate) and each
gamete receives only one
factor for the trait (Mendel’s
first law—segregation).
■ When the inheritance of
more than one trait is studied,
the pairs of factors for each
trait separate independently of
the other pairs of factors
(Mendel’s second law—
independent assortment).
Mendel’s Laws
Mendel’s ratios provided
supporting evidence for his
model and the establishment of
his universal laws.
Mendel’s Further Experiments
Mendel carried out similar experiments involving monohybrid
crosses for the other six characteristics that he studied in pea
plants. He made predictions based on his model, which he
tested with further crosses, all of which validated his previous
Mendel’s Further Experiments
Mendel carried out more complex experiments involving
dihybrid crosses to establish his second law—Mendel’s law of
independent assortment.
Mendel’s Further Experiments
In Mendel’s second law, he
established further ratios that show
that when individuals with two or
more pairs of unrelated, contrasting
characteristics are crossed (e.g. tall
plants with yellow pods × short
plants with green pods), the
different pairs of factors separate
out independently of each other.
That is, tall is not always inherited
with yellow and short with green—
some tall green offspring and some
short yellow offspring will result.
Mendel’s Experimental Techniques
Mendel’s scientific experiments were well controlled, he tested
only one variable at a time and the first-hand data that he
gathered was quantitative, leading to his successful analysis of
results. He drew valid conclusions, which eventually became
known as Mendel’s laws.
Mendel’s Experimental Techniques
His techniques were:
■ Valid and reliable: Mendel
changed only one variable at a
time and controlled all others.
He used large sample sizes and
repeated his experiments for
different traits. He analysed his
results mathematically to
identify patterns and trends and
then applied appropriate
formulae to draw valid
Mendel’s Experimental Techniques
His techniques were:
■ Accurate: He reduced the
possibility of experimental error—
all experiments were conducted in
a controlled environment (a
greenhouse). Those crosses that
relied on self-fertilisation (e.g.
to establish pure-breeding lines)
were conducted by keeping the
plants isolated from any others,
ensuring that no accidental
crosspollination would occur.
Mendel’s Experimental Techniques
In plants that required crosspollination, Mendel removed
the stigma of some and the
anthers of others and then
manually transferred pollen
from the anthers of one plant
to the stigma of another,
preventing errors arising from
accidental self-pollination.
Mendel Ignored
Mendel published his work on genetic inheritance in garden
peas in 1866 in the publication Proceedings at the Natural
History Society of Britain. Despite its later significance and its
originality, his work made little impression on anyone for almost
35 years.
Mendel Ignored
In 1900 his work was
rediscovered and its value
recognised independently by
three different cytologists. They
read his original paper,
experimented and confirmed
his findings and so publicised its
significance to the biological
Mendel Ignored
So why was Mendel ignored for all
of those years? There are several
possibilities, but the truth is
probably a combination of all of
■ Mendel’s work was too
progressive—it appeared to be
based on very little known
background. At that time, very
little was known about cells;
chromosomes, mitosis and meiosis
were unknown and studies of
genetics did not exist.
Mendel Ignored
■ He presented his papers to a very small group of scientists
(about 40) at two meetings (February and March 1865) of The
Natural Science Society of Britain (founded by his colleagues)—
a fairly low profile gathering of scientists in the province of
Mendel Ignored
■ His work differed quite radically from previous research and
the scientists to whom he presented may not have understood
it—they certainly did not recognise its significance. The
accepted belief at the time was the ‘blending’ of characteristics
in the offspring of contrasting pure-breeding parents (e.g. a tall
parent crossed with a short one was thought to give offspring of
medium height). Mendel’s use of mathematics and statistics to
analyse results and make predictions in biology was also
different to the norm of that time and may not have been
Mendel Ignored
■ He had no established reputation or recognition in the
broader scientific world because he had done no prior significant
research and had no interaction with other well-known
scientists. As a result, his standing as a ‘scientist’ may have been
doubted. He was also known to be fairly shy and may not have
had the confidence or the desire to try to convince others of his
-Students to complete Mendel's experimental Technique

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