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Biology 1-3a & 1-3b
Mendelian Genetics
1-3: Mendelian Genetics
• Genetics is the study of genes,
variation in genes throughout
generations, and heredity (the
ways in which genes are past on
from generation to generation) in
living things.
1-3: Mendelian Genetics
• Gregor Mendel
(1822-1884) was an
Austrian Monk with
scientific training in
mathematics, physics
and biology.
• His experiments laid
the foundation for
everything we know
about genetics today.
• He is often called the
Father of Genetics
1-3: Mendelian Genetics
• Before we take a closer look at
exactly what Mendel did to receive
such a major title, we need a little
more background…
1-3: Mendelian Genetics
• We inherit 23 chromosomes from our mother
and 23 chromosomes from our father.
• These chromosomes contain genes for the
same type of information, so essentially we
have two versions of every gene in our DNA.
• Each version of the gene we inherit is called
an allele.
– So you inherit one allele from your mother and
one from your father
1-3: Mendelian Genetics
1-3: Mendelian Genetics
Mendel’s Experiments:
• In order to understand what Mendel
did in his experiments, we need a basic
understanding of plant reproduction:
• The male reproductive part of the flower is
called the stamen.
– From the anther located at the top of the stamen,
pollen (plant sperm) is released.
• The female reproductive part of the flower is
called the pistil.
– The sticky top of the pistil, called the stigma,
collects the pollen which moves down into the
ovary to create the
next generation of
pea plants.
• Now, because pea plants have BOTH male and
female reproductive parts, it is possible for
them to self-pollinate themselves.
• Also, because the pollen grains are usually
distributed by either the wind or pollinators,
such as bees, it is very difficult to know which
plants are reproducing with each other
• For his experiments, Mendel needed to
control exactly what plants were reproducing
with each other…
• In order to control the
breeding, Mendel cut the
stamen off of half of the
flowers
• He then brushed the
pistil of these stamenless flowers with pollen
that he had collected
from a chosen plant
• Finally, the flowers were
covered with a bag to
prevent stray pollen
grains from finding their
way in.
1-3: Mendelian Genetics
Mendel’s Experiments – Phase One
• The first thing Mendel did was to
isolate true breeding strains of peas
with distinctive traits.
– To do this he crossed pea plants with, for
example, purple flowers with other purple
flower pea plants until all of their offspring
were consistently purple.
1-3: Mendelian Genetics
Mendel’s Experiments –
Phase Two
• Once he had isolated his
true breeding strains, he
began to cross-breed
them and analyze what
their offspring looked like.
• The first cross was
between a true breeding
white flowering plant and
a true breeding purple
flowering plant
– ALL of their offspring (the F1
generation) were purple!
1-3: Mendelian Genetics
Mendel’s Experiments
– Phase Two
• Next, he took two of
the offspring from
this first breeding
test and bred them
to each other
– Yes, it’s technically
incest, but they’re
flowers!
• 75% of the new
offspring were purple,
but 25% were white
again!
• Now keep in mind that the prevalent thought
of the day was that if a purple flower and a
white flower were bred, they should create a
light purple flower
• For Mendel to have produced white flowering
plants by breeding two purple flowering
plants….well, these were very strange results
indeed!
1-3: Mendelian Genetics
Mendel’s Experiments –
Phase Two
• Mendel’s next step was to
take plants from the F2
generation and breed them
with each other to try and
determine if they were “true
breeding plants” or not
– He discovered that the white
plants WERE true breeding, but
only 1/3 of the purple plants
were.
– The other 2/3 of the purple
plants would product a mix of
purple and white offspring
• Mendel repeated this
type of experiment
with six other traits,
examining thousands
of pea plants
– In all cases, one trait
disappeared in the F1
generation and then
reappeared in the F2
generation.
What in the world does this all mean?
1-3: Mendelian Genetics
Mendel’s Logic:
• If Mendel crossed purple peas with white peas
he got all purple peas
• When these purple peas were crossed with each
other he got 3 purple to 1 white
• He decided to use symbols to represent what
happened. He used a P for purple and a W for
white. He represented the cross as follows:
• In the parent generation
PxW
• In the F1 generation
all W
• In the F2 generation
3P to 1W
1-3: Mendelian Genetics
Mendel’s Logic:
• Mendel reasoned that if there were no “W” traits present
in the F1 generation, they must have died out and should
never reappear
• When the W trait reappeared in the next generation,
Mendel reasoned that it must have been present but not
seen in the previous generation.
– He concluded that there must be two factors for every trait present in
each individual, but that one factor could be masked by another
•
•
•
•
His new theory was represented in the cross as follows:
In the parent generation
PP x PW
In the F1 generation
all PW
In the F2 generation
1PP to 2PW to 1WW
• Ok, confusion about all the letters aside, what
Mendel had discovered was the existence of
two “factors” (what we now know as genes or
alleles) for each genetic trait!!
• Even Mendel realized that all the lettering was
confusing, so he developed some new
terminology for genetics…
1-3: Mendelian Genetics
Genetics Terminology:
• The appearance of an organism is its phenotype
– i.e. purple or white
• The genetic makeup of an organism is its
genotype
– i.e. WW or PP or PW
• The original, true-breeding strain is called the
parental (P) generation
• The first generation of offspring are the F1
generation
• The second generation of offspring are the F2
generation
1-3: Mendelian Genetics
Genetics Terminology:
• The ability of one factor to mask the appearance
of another makes it dominant.
• The ability of a factor to be masked by another
makes it recessive.
• To reduce the amount of letters used, when
choosing letters for the genotype use the letter of
the dominant characteristic
– True breeding purple = PP
– True breeding white = pp
• Now that we know the terminology, how do
we predict the genetic makeup of future
generations?
• By using a PUNNET SQUARE
1-3: Mendelian Genetics
The Punnet Square:
• Maternal and
paternal genotypes
are written on the
outsides
• Fill in the boxes
based on the alleles
present
1-3: Mendelian Genetics
The Punnet Square:
• Show the P
generation cross
between a true
breeding purple and
a true breeding white
plant
– What would the
phenotypes of the
offspring be?
– What would the
genotypes of the
offspring be?
1-3: Mendelian Genetics
The Punnet Square:
• Show an F1
generation cross
from the previous
question
– What would the
phenotypes of the
offspring be?
– What would the
genotypes of the
offspring be?
A bit more vocabulary…
• If an individual has two copies of the same allele,
they are said to be homozygous
– Eg. PP or pp
• If an individual has two different copies of an
allele, they are said to be heterozygous
– Eg. Pp
A bit more vocabulary…
• If an individual has two copies of the same
DOMINANT allele, they are said to be
homozygous dominant
– Eg. PP
• If an individual has two copies of the same
RECESSIVE allele, they are said to be homozygous
recessive
– Eg. pp