Parents pass certain traits to their offspring.
Landon (left) at 6 months
and Gavin (right) at 3.5
months.
On the left, cousins;
on the right, brothers!
Notice the similarities
and differences!
PART 1
Key Concepts
• Genes are particulate and are inherited
according to Mendel’s laws.
• Mendel learned about heredity by
conducting experiments.
• Genes are carried on chromosomes.
• Alleles and genes interact to produce
phenotypes.
•Your combination of traits received for each
characteristic are unique to you.
•Your mother and father each contribute to your genetic
makeup.
•For example: the color of your hair, the size of your feet
and the shape of your nose are some of your traits.
•Heredity- The passing of
traits from parents to
offspring.
•Genetics-The branch of
biology that studies heredity.
•Geneticists-Biologists who
study heredity.
• Look at the
photographs to
the right.
• What traits have
these babies
inherited from
their parent?
•Remember that
traits are passed
from parents to
offspring
(inherited) by the
passing of DNA
from parents to
their offspring.
•Called the “Father of Genetics”
•In the late 1800’s Mendel began
studying the passing of traits from
parents to offspring.
•He, too wanted to know why certain
patterns of traits showed up in
living things.
•Analyzed pea plants to look for
patterns in how traits were passed
from parents to offspring.
While trying to develop new color variations of flowers
Mendel began experimenting…..
• He studied 7 characteristics (genes), each with 2
contrasting traits (alleles) available.
• CHARACTERISTIC-a distinguishing quality that an
organism exhibits.
• Ex: height, hair color, eye color, skin color.
• TRAIT- specific hereditary options available to be
inherited for each characteristic.
• Ex: tall height/short height, brown hair/blonde
hair, brown eyes/blue eyes, Dark skin/ light skin.
CHARACTERISTICS
1. Plant Height
TRAITS
Tall vs. Short
2. Seed Color
Yellow vs. Green
3. Seed Shape
Round vs. Wrinkled
4. Pod Color
5. Pod/Flower Location
6. Pod Shape
7. Flower Color
Green vs. Yellow
Axial vs. Terminal
Inflated vs. Constricted
Purple vs. White
• He started by growing plants that were PURE-having a
trait and always passing that trait to their offspring.
•Then, Mendel bred plants
that had different traits to
see what the offspring would
look like.
•The first plants he crossed
he called the “Parental
Generation” (P)
•In his first crosses, Mendel found that only one
of the two traits appeared in the offspring
plants (F1 generation – children of P
generation).
•For example, when he crossbred tall pea
plants with short pea plants, the offspring
(F1) were always tall.
•After his first crosses, Mendel took those
offspring plants (F1) and crossed them.
•In these second crosses, both traits showed
up again in the F2 generation.
(F2 GENERATION-offspring of the F1
generation).
•The same results happened in every experiment. One
trait, like being tall, was always present in the first
generation (F1).
•The other trait, like being short, seemed to go away;
only to reappear again in the second generation.
•This happened with every set of traits that
Mendel studied in the same proportions..
EXAMPLES:
Plant Height
Cross
Tall x Short
Seed Color Cross
Yellow x Green
Parent
P
F1
F2
Seed Shape
Cross
Round vs.
Wrinkled
All Tall Plants
All Yellow Plants
All Round Plants
¾ Tall
¼ Short
¾ Yellow
¼ Green
¾ Round
¼ Wrinkled
MENDEL’S 3 CONCLUSIONS:
• 1. Law (Principle) of Dominance and Recessiveness
• 2. Law (Principle) of Segregation
• 3. Law (Principle) of Independent Assortment
From these experiments, Mendel
discovered that traits from one parent
may hide traits from the other parent.
He also found that certain traits would
always show themselves over other traits.
• These traits he called
DOMINANT-can
mask or dominate the other trait and is displayed
most often.
• The hidden or masked traits he called RECESSIVEthe trait that can be covered up; is displayed less
often.
• Ex: the trait (allele) for tall is dominant over the
trait (allele) for short, so the short allele would
be the recessive allele.
• Mendel hypothesized that something in the
pea plants was controlling the characteristics
that came through
• He called these controls “factors”
(We now know that these
characteristics are controlled by
Genes- pieces of DNA that code for a characteristic
(protein that is made).)
•Mendel also figured out that for each
trait, the plant had two factors, or a pair
of factors, controlling the expression of
each trait.
•Each pair consists of alternate forms
(alleles) of the same gene; one from
mother and one from father.
•Dominant traits were controlled by
dominant alleles and recessive traits
were controlled by recessive alleles.
•Letters are used to represent the alleles that
carry the trait found on genes. A letter
represents a trait.
•CAPITAL if the gene that controls the trait is
dominant.
•Lowercase if the gene is recessive.
•
•
T- represents a dominant allele for
tallness
t – represents a recessive allele for lack
of tallness, or shortness
•Genotype- The genetic
combination, or pair of alleles
an organism inherits for a
certain trait.
•Phenotype- The appearance of
a trait in an organism.
•Ex: A plant's genotype is TT.
Its phenotype is tall.
Blue alleles
b
b
Phenotype
(blue eyes)
Genotype
(bb)
Like and Unlike Alleles
• HOMOZYGOUS- organism has 2 of the same alleles for a
trait.
• Homozygous Dominant-has 2 dominant alleles;
dominant trait is displayed
• Ex: TT = tall pea plant
• Homozygous Recessive-has 2 recessive alleles;
recessive trait is displayed
• Ex: tt = short pea plant
• HETEROZYGOUS-organism has 1 dominant and 1
recessive allele; the dominant trait is displayed.
• Ex: Tt = tall pea plant
Blue alleles
b
b
Homozygous –
alleles are the same
Blue Allele
b
Brown Allele
B
Heterozygous –
alleles are different
X
=
X
=
•If you crossbred homozygous tall plants (TT)
(purebred) with homozygous short plants (tt)
(purebred)…
All of the offspring are tall! WHY?
X
=
All of the offspring had received a tall gene (T) from one
parent and a short gene (t) from the other parent.
•Organisms with different
genotypes may have the
same phenotype.
tt
•For example, a homozygous
tall plant (TT) and a
heterozygous tall plant (Tt)
have different genotypes.
TT
Tt
TT
Tall
Tall
Tt
Tall
Tall
Tall
Tt
Tt
Short
tt
•However, they have the
same phenotype, which is
tall.
Remember, the combination of
alleles makes you look the way you do.
You get traits from your parents,
just like pea plants do.
For example, the trait of earlobe
shape in humans is inherited.
There is an allele for free earlobes
and an allele for attached earlobes.
The allele for free earlobes is
dominant, and the allele for attached
earlobes is recessive.
This is one of 9000 traits in humans
controlled by a single gene.
Mendel developed the following three laws to describe
how an organism's various traits are inherited and
expressed.
1. The Law of Dominance and
Recessiveness.
2. The Law of Segregation.
3. The Law of Independent Assortment.
2. The Law (Principle) of Segregation:
•Organisms have 2 copies of each gene (one from mom
and one from dad), but put only 1 copy in each sperm or
egg.
• During fertilization, the alleles come together and new
combinations of alleles are randomly formed.
•The trait observed in an individual depends on the two
copies of the gene it inherits from its parents.
3. The Law (Principle) of Independent
Assortment:
• The alleles for different genes, on different chromosomes, are not
connected.
• The chromosomes are distributed into gametes independently
(randomly) during meiosis, not based on the distribution of any
other chromosome.
• EX: in pea plants, a gamete that receives a dominant (yellow) allele
for pea color can receive either a dominant (round) or recessive
(wrinkled) allele for pea shape
•
However, if two genes are located on the same
chromosome, they are inherited together .
Principle of Independent
Assortment
3. The Law (Principle) of Independent
Assortment:
http://www.sumanasinc.com/webcontent/animations/con
tent/independentassortment.html
PART 2
Key Concepts
• Alleles and genes interact to produce
phenotypes.
• Observing and individual’s phenotype is
not sufficient for determining its
genotype.
• The outcome of a genetic cross can be
predicted.
AFTER MENDEL
• Today, Geneticists rely on Mendel’s work to
predict the likely outcome of genetic crosses.
•Probability is the likelihood that a chance event will
occur.
•The value of studying genetics is in understanding how
we can predict how likely it is to inherit a particular
trait.
•One of the easiest ways to calculate the
mathematical probability of inheriting a
specific trait was invented by an early
20th century English geneticist named
Reginald Punnett .
• MONOHYBRID CROSS-cross between 2 individuals
involving 1 pair of contrasting traits (2 alleles).
•His technique employs what we now call a Punnett
square.
•Punnett square-chart that shows the possible allele
combinations for potential offspring of two parents
whose genotypes are known.
•The results of Mendel’s experiments with two homozygous
plants can be shown using a Punnett Square…Let’s try it!
STEP 1 - Draw a chart with four boxes.
T
T
STEP 2 - Above the two boxes going across, write the
symbol for a the first parent’s alleles (in this
case…homozygous tall plant (TT)) with one
letter over each box.
STEP 3 Write the symbol for the other parent’s alleles (in this
case…a homozygous short plant (tt)) going down the left side
with one letter next to each box.
T
T
t
Tt
Tt
t
Tt
Tt
STEP 4 Fill in the left side of each box with the letter at the
top of the column (T).
STEP 5 Fill in the right side of each box with the letter at the
left of the row (t).
You should see that in all of the boxes are
the alleles for a heterozygous
T
T
tall (Tt) offspring.
t
Tt
Tt
t
Tt
Tt
The combination of genes from organisms with
heterozygous genes can also be shown in a Punnett
square.
Lets cross two heterozygous tall pea plants (Tt).
T
t
T
TT
T t
t
Tt
tt
So, two heterozygous tall plants could potentially
produce tall and short offspring!
A Punnett square can help you predict the chances
that a certain combination may occur.
This Punnett Square shows there is a 1/4 (25%)
chance that an offspring from this cross will be
homozygous tall (TT)
T
t
T
TT
T t
t
Tt
tt
It also shows a l out of 4 (1/4) chance that the
offspring will be homozygous short (tt).
T
t
T
TT
T t
t
Tt
tt
There is also a 2 out of 4 chance (2/4 or 1/2) that
each offspring will be heterozygous tall (Tt). The
probability ratio for the genotypes is 1:2:1
You can show probability as a percent by using this formula:
Number of times an event occurs
Number of total possible events
x 100 = Probability %
For example the probability for heterozygous tall (Tt) is:
2 x 100 = 2 x 100 = 200 = 50 %
4
4
1
4
The Punnett square also shows that there is a 25% chance (1/4 X
100) that the offspring could be homozygous tall (TT) and a 25%
chance the offspring could be homozygous short (tt).
EXAMPLE :HETEROZYGOUS X HOMOZYGOUS
T= tall plant
t = short plant
TALL X SHORT (Tt x tt)
Genotype = 2 Tt; 2 tt
Phenotype = 2 tall plants;
2 short plants
T
t
t Tt tt
t Tt tt
Probability: 2/4 tall plants; 50% tall plants
2/4 short plants; 50% short plants
TESTCROSS
What if we only know what the parents LOOK like, but we
aren’t sure of their genotypes?
• Testcross- Cross to determine the “unknown” genotype
of a parent with a dominant phenotype (trait displayed).
• Use to determine if the unknown parent’s genotype is
heterozygous or homozygous dominant.
• Ex: A plant with green seed pods could have a
genotype of GG or Gg. We don’t know yet!!
• To find out, we cross the unknown parent with a
homozygous recessive parent.
G? X
gg
The results of the crosswill
tell us the unknown genotype!
G
?
g Gg ?g
g Gg ?g
If only dominant offspring result- then
the ? must be dominant.
If both dominant and recessive offspring
result- then the ? is recessive.
PART 3
Key Concepts
• The effects of both alleles in a
genotype can show up in the phenotype.
Much of Mendel's success was due to the traits he
studied. The traits Mendel picked to study were each
determined by single genes (pieces of DNA). The
height of the pea plants is due to a single gene that
occurs in two different versions, or alleles.
But are people either tall or short?
Are there only two skin colors?
Do people have only blonde or brown
hair?
Of course not. In humans, and in most organisms,
almost all traits do not follow the patterns of
heredity that Mendel found.
All of these traits we have looked at so far have been
examples of COMPLETE DOMINANCE.
• COMPLETE DOMINANCE-one allele is totally dominant
over the other allele and adheres to the Principle of
Dominance.
EXAMPLE: PP and Pp = purple flower plants
The way genes control traits can be
complex and interact in different ways.
When one gene for a certain trait is not
completely dominant over the other gene,
a blending effect occurs.
Incomplete dominance- a type of
inheritance in which one allele for a
specific trait is not completely dominant
over the other allele. This results in a
combined phenotype (expressed physical
trait).
Incomplete Dominance can be seen in the
color of the flowers in four o'clock plants.
Four o'clock plants can have red flowers,
white flowers, or pink flowers.
Red flowers are homozygous dominant (RR). White flowers
are homozygous recessive (rr).
When four o'c1ocks that have red flowers are crossed with
four o'c1ocks that have white flowers, a blending effect
occurs. Heterozygous pink flowers (Rr) are produced.
What if we cross two pink four o’clock plants?
PINK ( Rr) X PINK (Rr)
Genotype = 1RR, 2Rr, 1rr
Phenotype = 1 red, 2 pink, 1 white
R r
R RR Rr
r Rr rr
Another pattern of
heredity can occur when
two equally dominant genes
are present for a certain
trait.
Co-dominance- both
variations of the gene
appearing at the same time
in a heterozygous
individual.
EXAMPLE: R = red
RR = red
R’ = white
R’R’ = white
RR’ = red and white
Example: a homozygous chicken with black
feathers (BB) and a homozygous chicken with
white feathers (WW) reproduce, the
offspring is heterozygous (BW).
It shows that offspring will have some black
feathers and some white feathers.
Roan Cow
Blood Type A (IAIA) X Blood Type B (IBIB)
IA IA
IBIAIB IAIB
IB IAIB IAIB
Genotype = 4 IAIB
Phenotype = 4 Blood Type AB
FYI
The A and B alleles are equally dominant.
A child who inherits and A allele from one parent and a B allele
from the other parent will have type AB blood.
What type of dominance is this?
co-dominance
PREDICTING THE RESULTS OF A
DIHYBRID CROSS
• DIHYBRID CROSS- a cross between two individuals that
involves two pairs of contrasting traits (looking at 2
genes).
• More complicated than monohybrid because more
possible combinations.
• MONOHYBRID CROSS
• 2 traits/4 possible offspring
• DIHYBRID CROSS
• 4 Traits/ 16 possible offspring
Dihybrid Cross Example:
Heterozygous x Homozygous
Tall, Round Plant (Tt Rr) X Short, Wrinkled Plant (TT RR)
• First, we need to determine what
alleles each parent could possibly give all possible combinations of the alleles
from each trait per gamete.
• TtRr
• ttrr
TR, Tr, tR, tr
tr, tr, tr, tr
The alleles are carried down into boxes and across into boxes like in the
smaller square. Take care to keep the alleles for each trait together
inside the boxes (Here, Ts come first and stay together, Rs are next)
Tall, Round Plant (Tt Rr) X Short, Wrinkled Plant (tt rr)
tr
tr
tr
tr
GENOTYPE: 4TtRr,
4 Tt rr,
4 ttRr
4 ttrr
TR TtRr TtRr
TtRr TtRr
Tr Ttrr
Ttrr
Ttrr
Ttrr
PHENOTYPE:
4 Tall, Round
4 Tall, Wrinkled
4 Short, Round
4 Short, Wrinkled
tR ttRr
ttRr
ttRr
ttRr
tr ttrr
ttrr
ttrr
ttrr
EXAMPLE: HETEROZYGOUS X HETEROZYGOUS
Tall, Round Plant (Tt Rr) X Tall, Round Plant (Tt Rr)
GENOTYPE: 1 TTRR,
2TTRr, 2TtRR, 4TtRr,
1 TTrr, 2 Ttrr, 1ttRR,
2ttRr, 1 ttrr
PHENOTYPE:
9 tall, round
3 tall, wrinkled
3 short, round
1 short, wrinkled
Phenotypic
Ratio= 9:3:3:1
TR
Tr
tR
tr
TR TTRR TTRr TtRR TtRr
Tr TTRr TTrr
TtRr
Ttrr
tR TtRR TtRr ttRR ttRr
tr TtRr
Ttrr
ttRr
ttrr