The science of genetics explains why you have
inherited certain traits from your parents.
Geneticists study heredity.
By the end of this unit you will
understand how traits were
passed on to you.
Gregor Mendel was an
Austrian monk who first
predicted how traits were
transferred from one
generation to the next.
Up until the time of Mendel (1822-1884) the
actual mechanisms governing inheritance were
unknown. Darwin had no knowledge about how
traits were passed on.
Mendel’s simple laws and
ideas were published in 1866
but largely went unrecognized
until 1900, which was long
after his death.
Knowledge of the complex genetic mechanisms
finally came as a result of careful laboratory
breeding experiments carried out over the last
century and a half.
One possible explanation of heredity is a “blending”
hypothesis that Darwin implicated in his work.
This is the idea that genetic material contributed
by two parents mixes in a manner analogous to
the way blue and yellow paints blend to make
green.
This may be true for some traits which are governed by
incomplete dominance, a complex form of inheritance pattern, but
for most it is not.
X
=
An alternative to the blending model is the
“particulate” hypothesis of inheritance: the
gene idea
Parents pass on discrete heritable units known as
genes. Mendel knew nothing about genes, or
alleles. He called these “particles” governing
inheritance, “unit factors”. How did Mendel come
up with it?
Gregor Mendel documented a
particulate mechanism of
inheritance through his
experiments with garden
peas
Describe the “blending” hypothesis of inheritance
vs. the particulate hypothesis.
Blending has traits from mother and father mixing
equally to produce a medium range within the offspring,
whereby particulate has each parent contributing a
discrete unit called a gene, which may or may not be
observable in the offspring.
Mendel chose the garden pea (Pisum sativum) for his
experiments for various reasons.
• because they can be grown easily and
quickly
• reproduce in large numbers
• their reproduction can be manipulated.
The transfer of pollen (male gamete) to the
female reproductive organ (ovary) is called
pollination.
?
Pollination
Plants
Animals
When Mendel wanted to breed two
plants, he dusted the female organ
of one plant with pollen from the
plant he wished to cross it with.
In a typical breeding experiment:
 Mendel mated two contrasting, true-breeding varieties
in a process called hybridization
The true-breeding parents are called the “P” generation
True breeding individuals are those who when
bred together, always produce offspring of the
same type.
The hybrid offspring of the P
generation are called the F1
generation (F stands for filial)
When F1 individuals selfpollinate the F2 generation is
produced
Through careful control, Mendel was able to use his
experiments to show how traits were transferred from
one generation to the next.
• He only analyzed one trait
at a time. For this reason,
they were called monohybrid
crosses.
The Traits Mendel Studied:
• seed shape (round or wrinkled)
• seed color (green or yellow)
• flower color (purple or white)
• flower position (Axial or terminal)
• pod color (green or yellow)
• pod shape (inflated, or constricted)
• plant height (tall or short)
Why do geneticists studying heredity through experimentation use
organisms such as peas and fruit flies?
These organisms reproduce and produce new generations
quickly; they reproduce abundantly with many offspring; they are
easily manipulated to reproduce.
What is a true-breeding individual?
An organism that when bred with another true-breeding
individual, will only produce the same type of offspring as
themselves.
What is hybridization?
When true-breeding individuals with contrasting traits are
crossed, the result is a hybrid.
What is a monohybrid cross?
A cross between two individuals that only looks at one trait.
As you can see, Mendel was extremely lucky in
that each of the traits he studied had two clearly
distinct forms.
Mendel’s studies found that when crossed, one trait of a
pair seemed to disappear in the F1 generation, and then
reappear in the F2 plants.
P1 Generation
Round X Wrinkled= Round
Round X Wrinkled= Round
x
=
F1 Generation
When Mendel crossed
these two individuals, the
results were:
Round X Round= Round, and wrinkled both
x
=
Because of this curiosity, Mendel came up with his rule
of unit factors.
The rule of unit factors:
• Mendel concluded that each organism has two
factors that control each of its traits. We now call
The gene for
these factors alleles.
flower color
For example, each of Mendel’s
pea plants had two alleles of the
gene that determined its seed
shape. A plant could have
inherited two alleles for round,
two alleles for wrinkled, or one
for each trait.
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
An organism’s two alleles for each trait are inherited from each of
its parents. One from female, and one from male.
Because of Mendel’s work, he was also able to come up with a
rule of dominance.
Mendel called the observed trait the dominant trait, and
the trait that disappeared, the recessive trait.
Mendel concluded that the allele for round seeds was
dominant to the allele for wrinkled seeds, and when the
two different alleles were presented together, the round
one would always win!
Round x Wrinkled = Round
X
=
When recording the results of crosses, it is customary to use the same
letter for the different alleles of the same gene. Usually, one would use
the dominant allele’s letter for both traits, too. You would use an upper
case to represent the dominant allele, and a lower case for the
recessive. This is not always the case, however, as with fruit flies. se/se;
how/how…etc
The dominant allele is always written first, so the alleles for seed
shape would look like this: R=round, r=wrinkled
In pea plants with purple flowers, the genotype
may not be immediately obvious.
A testcross allows us to determine the genotype of an
organism with the dominant phenotype, but unknown
genotype
A testcross crosses an individual with the dominant
phenotype (and unknown genotype) with an individual
that is homozygous recessive for a trait
Parents: pp x PP or Pp
If the resulting offspring are all purple, then the
genotype in question must be homozygous
dominant.
If the resulting offspring are both purple and
white, then the genotype in question must have
been heterozygous.
Put the “rule of unit factors” in your own words.
Mendel said that each organisms contains two factors that
govern each trait…one from mother, and one from father. It is the
combination of these two “units” that determine the phenotype of
the offspring.
Put Mendel’s rule of dominance in your own
words.
Mendel said that when these two “units” were presented
together, one may “mask” the other, and dominate over it. This
would be the dominant trait. The masked trait would be called the
recessive trait.
Describe what a testcross is used to determine.
A testcross would be used to determine an unknown
genotype, when the phenotype is dominant. The cross ALWAYS
involves a homozygous recessive individual x unknown genotype.
Mendel crossed his parental generation (P) of truebreeding peas, and studied his results.
He was then curious about what would happen if he crossed the
offspring from that first monohybrid cross with each other.
Round X Round = Both round, and wrinkled
Offspring appeared in a three to one ratio looking
like this: 3 round, and 1 wrinkled
x
=
The results were surprising, but they helped him
to determine one of his two laws. The Law of
Segregation.
• Every individual has two alleles of each gene.
• When gametes are produced through meiosis, they
each receive one of these alleles.
• During fertilization when gametes unite, they
randomly pair to produce four possible
combinations of alleles.
This law is best expressed
R
r
by observing a simple
hereditary tool called a
R RR
Rr
Punnett Square, which is a
tool used by geneticists to
r
Rr
rr
predict possible genetic
outcomes.
The results of Mendel’s crosses were to prove that not all pea
plants with round seeds were the same. Some with round seeds
when crossed with one another, would produce only offspring with
round seeds.
Others when crossed, had the potential to produce both round,
and wrinkled seeded offspring.
He concluded that two organisms can look alike,
but still have different underlying allele
combinations.
• The way an organism looks and
Round
behaves is known as PHENOTYPE
• The allele combination an organism
contains is known as GENOTYPE
Rr
An organism is homozygous for a trait if both of
its alleles are exactly the same.
RR, or rr
An organism with the genotype of RR, is said to be homozygous dominant,
because of the presence of the dominant allele.
An organism with the genotype of rr is said to be homozygous recessive,
because of the presence of the recessive allele
An organism is heterozygous for a trait if both of
its alleles are different.
Rr
Organisms with this heterozygous allele combination will
express the dominant trait in their phenotype.
Mendel’s Law of Segregation says what?
•
•
•
Every individual has two alleles (unit factors) for each gene
When the individual produces gametes, these unit factors
separate randomly into the different sex cells.
During fertilization, these traits reunite in four possible
combinations.
What tool is best used to illustrate this law?
A simple punnett square
The last of Mendel’s laws is best expressed when observing
something called a dihybrid cross.
While most of Mendel’s crosses were monohybrid in
nature, it took the dihybrid cross to show his Law of
Independent Assortment.
Whereas Mendel’s monohybrid crosses used only one trait when
studying genetic outcome, his dihybrid crosses used two.
Because of this particular cross, Mendel was able
to determine that the two traits were still inherited
independently of each other.
The dihybrid cross that Mendel first looked at studied the two traits
of pea seed shape, and color.
Mendel crossed a round yellow
pea, with a wrinkled green pea
X
How do you think
he showed a
dihybrid cross such
as this in a Punnett
Square?
The gametes from RrYy Parent
RY
ry
RRYY RRYy RrYY
Ry
rY
RRYy RRyy RrYy Rryy
rY
Ry
RrYY
ry
RY
RrYy Rryy
RrYy rrYY
rrYy
RrYy
rrYy
rryy
As you can see, the gametes will separate from each
other, and then recombine or assort independently
from one another in four different ways.
In this cross,
remember all the
alleles sort
themselves
independently from
one another, so
you must show all
possible alleles.
The parents are both
heterozygous for seed
shape and color.
R = Round
r = wrinkled
Y = Yellow
y = green
Using the Law of Independent Assortment as a
guide, determine the possible gametes formed
from these parents.
VVPp VP, and Vp
ZzBb ZB, Zb, zB, and zb
MMPP MP
ffMm fM, and fm
NNtt Nt
So, how many squares would you have to make to do a
tri-hybrid cross? 64 How does one determine probability
in a far more complex cross…say one that uses four
traits? There is a simpler mathematical way for doing a
tri-or even a quadri-hybrid cross.
In the following cross, the traits observed are: Red/brown eyes (alleles R,r);
Normal/vestigial wings (alleles V, v); Long/short sex combs (alleles L, l); and
Normal/ebony body color (alleles E, e).
These characteristics are found within fruit flies (Drosophila
melanogaster), a long-time favorite of the heredity scientists.
Here is the cross we will be performing:
RrVvLlEe X RrVvLlEe
Question: What is the probability of producing offspring with the
following genotype: RRvvLLEe?
RrVvLlEe X RrVvLlEe =
RRvvLLEe
First, take each trait for each parent individually. Look at eye color first.
Offspring genotype is RR. Both parents, however, are Rr (they have red eyes,
however they are both heterozygous.) What is the probability that a cross of
this trait alone, will produce an RR individual? If you have to do a small Punnett
Square, you can…but eventually, you should be able to see it in your mind.
The chances are 25%, or 1/4
Now do the same thing for each trait, individually, determining the
probability of each cross.
Vv x Vv = vv (again=25% or ¼)
Ll x Ll = LL (again = 25% or ¼)
Ee x Ee = Ee (= 50% or ½)
At this point, your task is
simple. Multiply all the
fractions together, and your
result is the probability of
producing that genotype!
So... ¼ x ¼ x ¼ x ½ = ?1/128
The probability of producing the above genotype is 1/128!
Challenge:
Given the following cross:
RRVvLLEe X RrvvLlee
What is the probability of the following combination?
RrVvLLee?
½ x ½ x ½ x ½ = 1/16
RRVvLLEE?
½x½x½x0=0