Download Punnett Squares

Georgia Standards:
1. Explain the relationship between changes in DNA
and the appearance of new traits.
2. Explain the relationship between changes in DNA
and the appearance of new traits.
Essential Questions:
•How do you predict the probability of various
genotypes inherited and the expressed phenotypes?
•How does meiosis generate variation in offspring?
Gregor Mendel: Father of Genetics
• Genetics is the scientific
study of heredity.
• Gregor Mendel (1860’s)
an Austrian Monk, was
interested in figuring out
how heredity was
determined in plants and
animals.
– used pea plants
– quantitative approach to
collect data.
• Mendel studied seven
different pea plant traits.
– Seed shape & color, pod
shape & color, flower
height & position and seed
coat color
• A trait is a specific
characteristic, such as
seed color or plant height,
that varies from one
individual to another.
Gregor Mendel’s Experiment:
• Mendel crossed plants with each of the seven
contrasting traits and studied their offspring.
• Mendel called each original pair of plants the P
(parental) generation.
– These peas were true-breeding (self-pollination),
meaning that if they were allowed to self-pollinate,
they would produce offspring identical to themselves.
Gregor Mendel’s Experiment:
• He called the offspring of the
P-generation, the F1, or “first
filial,” generation. Filius is the
Latin word for “son.”
• The offspring of crosses
between parents with different
traits are called hybrids.
– These pea plants were
cross pollinated.
• The F2 generation was
allowed to self-pollinate
– In cross-pollination, male
sex cells in pollen from the
flower on one plant fertilize
the egg cells of a flower on
another plant.
• Out of 929 F2 Generation
plants, 705 were purple and
224 were white.
– Ration of 3 purple to 1 white
How do we find the results of a genetic cross?
• The principles of probability can be used
to predict the outcomes of genetic
crosses.
• The gene combinations that might result
from a genetic cross can be determined by
drawing a diagram known as a Punnett
square. (See note sheet)
Gregor Mendel’s Conclusions:
• Genes and Dominance
– Mendel’s first conclusion was that biological
inheritance is determined by factors (genes)
that are passed from one generation to the
next.
• Each of the traits Mendel studied was controlled by
one gene that occurred in two contrasting forms.
• The different forms of a gene are called alleles
Punnett Squares
• Organisms that have two identical alleles for a
particular trait—TT or tt in this example—are
said to be homozygous
• Organisms that have two different alleles for the
same trait are heterozygous (Ex: Tt)
• Homozygous organisms are true-breeding for a
particular trait. Heterozygous organisms are
hybrid for a particular trait.
Punnett Squares
• All of the tall plants have the same
phenotype, or physical characteristics.
• They do not, however, have the same
genotype, or genetic makeup.
Gregor Mendel’s Conclusions:
• Mendel’s second conclusion is called the
principle of dominance, which states that
some alleles are dominant and others are
recessive.
– Alleles that are expressed are considered
DOMINANT.
• An organism with a dominant allele for a particular form of a
trait will always have that form.
– Alleles that are present in the genotype, but not
expressed in the phenotype are considered
RECESSIVE.
• An organism with a recessive allele for a particular form of a
trait will have that form only when the dominant allele for the
trait is not present.
Gregor Mendel’s Conclusions:
• Segregation of Alleles:
– When each F1 plant flowers, the two alleles are
segregated from each other so that each gamete
carries only a single copy of each gene.
– Therefore, each F1 plant produces two types of
gametes—those with the allele for tallness and those
with the allele for shortness.
Law of Segregation:
• During gamete
formation, alleles are
segregated from each
other so that each
gamete carries only a
single copy of each
allele.
– Each F1 plant produces
two types of gametes—
those with the allele for
tallness and those with the
allele for shortness.
– The alleles are paired up
again when gametes fuse
during fertilization.
Did this mean that the two dominant alleles would
always stay together?
• (Law of Independent
Assortment)
– Genes for different
traits can segregate
independently during
the formation of
gametes.
Principle of dominance
• Some alleles are dominant and others are
recessive.
• The importance of Mendel’s work on heredity
was not discovered until 30 years later.
• Scientist soon realized that CHROMOSOMES
are the carriers of heredity.
A Summary of Mendel’s Principles
• Genes determine
inheritance of biological
characteristics.
• Genes are passed from
parents to offspring
• In most sexually reproducing
organisms, each adult has
two copies of each gene—
one from each parent.
These genes are
segregated from each
during gamete formation.
• Some forms of the gene
may be dominant and
• The alleles for different
others may be
genes are assorted
recessive.
independently of one
another.
Pop Quiz: Use Book- Chp. 10.2
1. What are dominant and recessive alleles?
2. What happens to alleles during segregation?
3. What did Mendel conclude determines
biological inheritance?
4. Describe how Mendel cross-pollinated pea
plants.
5. Describe how Mendel self-pollinated pea
plants.
6. Give an example of the law of independent
assortment.
Probability and Punnett Squares
• The principles of probability can be used
to predict the outcomes of genetic
crosses.
• The gene combinations that might result
from a genetic cross can be determined by
drawing a diagram known as a Punnett
square. (See note sheet)
Punnett square
• This Punnett square
shows the probability of
each possible outcome of
a cross between hybrid
tall (Tt) pea plants.
• Genotype: 25% TT, 50%
Tt, 25%tt (1:2:1)
• Phenotype: 75% Tall,
25% short (3:1)
A Dihybrid Cross: F1
• Dihybrid Cross:
– Shows inheritences of two
traits at once
• F1 Mendel crossed plants that
were homozygous dominant
for round yellow peas with
plants that were homozygous
recessive for wrinkled green
peas. All of the F1 offspring
were heterozygous dominant
for round yellow peas.
• The cross did not indicate
whether genes assort, or
segregate, independently.
Did this mean that the two dominant alleles would
always stay together? Or would they “segregate
independently.”
• A combination of alleles
were produced that were
not found in either parent
– This means that genes for
different traits can
segregate independently
during the formation of
gametes. (Independent
Assortment)
Dihybrid Cross Problem
• Determine the genotype and phenotype
ratios and percents for a F2 generation
dihybrid cross of two plants; one that is
homozygous recessive for short, purple
leaves and the other that is homozygous
dominant for tall, orange leaves.
• Height (H-tall, h-short)
• Color (C-orange, c-purple)
Concept Map
Gregor
Mendel
experimented
with
concluded that
Pea
plants
genes for different
traits can segregate
independently during
the formation of
gametes
which is
called the
Go to
Section:
“Factors”
determine
traits
Law of
Independent
Assortment
Some alleles
are dominant,
and some alleles
are recessive
which is
called the
Law of
Dominance
Alleles are
separated during
gamete formation
which is
called the
Law of
Segregation
Exceptions to Simple Dominance
Does the segregation of one pair of alleles
affect the segregation of another pair of
alleles?
• For example, does the gene that
determines whether a seed is round or
wrinkled in shape have anything to do with
the gene for seed color? Must a round seed
also be yellow?
Exceptions to the Laws:
• Some alleles are neither dominant nor
recessive, and many traits are controlled by
multiple alleles or multiple genes.
• incomplete dominance occurs when one
allele is not completely dominant over
another
• In incomplete dominance, the heterozygous
phenotype is somewhere in between the two
homozygous phenotypes.
Incomplete Dominance
• Some alleles are neither
dominant nor recessive.
• In four o’clock plants, for
example, the alleles for
red and white flowers
show incomplete
dominance.
• Heterozygous (RW)
plants have pink
flowers—a mix of red and
white coloring
Incomplete Dominance Practice
• In Mendel's experiments, if the gene for tall (T)
plants was incompletely dominant over the gene
for short (t) plants, what would be the result of
crossing two Tt plants?
•
•
•
•
•
A. 1/4 would be tall; 1/2 intermediate height; 1/4 short
B. All the offspring would be tall.
C. 1/2 would be tall; 1/4 intermediate height; 1/4 short.
D. All the offspring would be intermediate.
E. 1/4 would be tall; 1/4 intermediate height; 1/2 short.
Incomplete Dominance Practice
•
Disappearance of parental
phenotypes in the F1 generation
•
A. pink flower color is epistatic to red
flower color.
• A genetic cross of inbred
snapdragons with red flowers
with inbred snapdragons with
white flowers resulted in F1hybrid offspring that all had
pink flowers. When the F1
plants were self-pollinated, the
resulting F2-generation plants
had a phenotypic ratio of 1 red:
2 pink: 1 white. The most likely
explanation is:
•
B. pink flowers are the result of a
blending of the red and white
genotypes.
•
C. flower color is due to 2 or more
complementary genes.
•
D. heterozygous plants have a
different phenotype than either inbred
parent because of incomplete
dominance of the dominant allele.
•
E. flower color inheritance in
snapdragons does not behave as a
Mendelian trait.
Codominance
• In codominace, both • In certain varieties of
alleles contribute to
chickens, the allele
the phenotype of the
for black feathers is
organism.
codominant with the
allele for white
• For example, in cattle
the allele for red hair
feathers.
is codominant with
– Heterozygous
the allele for white
chickens appear
hair.
speckled with black
– Cattle with both alleles
are roan, or pinkish
brown, because their
coats are a mixture of
both red and white hairs.
and white feathers.
Codominant alleles: The human
ABO markers
• Human blood type is
determined by codominant
alleles. There are three
different alleles, known as IA,
IB, and i. The IA and IB alleles
are co-dominant, and the i
allele is recessive.
• The possible human
phenotypes for blood group
are type A, type B, type AB,
and type O. Type A and B
individuals can be either
homozygous (IAIA or IBIB,
respectively), or heterozygous
(IAi or IBi, respectively).
• A woman with type A blood
and a man with type B blood
could potentially have offspring
with which of the following
blood types?
•
•
•
•
•
A. type A
B. type B
C. type AB
D. type O
E. all of the above
Codominant Practice: Predicting
human blood types
• What are the possible blood types of the
offspring of a cross between individuals that are
type AB and type O? (Hint: blood type O is
recessive)
•
•
•
•
•
A. AB or O
B. A, B, or O
C. A or B
D. A, B, AB, or O
E. A, B, or AB
Sickle Cell Disease
• Sickle cell disease is a
common genetic disorder
inherited in a
codominant fashion, and
mainly found in African
Americans.
• Sickle cell disease is
characterized by the bent
and twisted shape of the
red blood cells
• These sickle-shaped red blood
cells are more rigid than
normal cells and tend to get
stuck in the capillaries, the
narrowest blood vessels in the
body.
• As a result, blood stops
moving through these vessels,
damaging cells and tissues
beyond the blockage.
• Sickle cell disease produces
physical weakness and
damage to the brain, heart,
and spleen. In some cases, it
may be fatal.
Sickle Cell Disease
• Hemoglobin is the protein
that carries oxygen in the
blood.
• Mutation: the amino acid
valine in place of glutamic
acid.
• As a result, the abnormal
hemoglobin is somewhat
less soluble than normal
hemoglobin. Blood gets
stuck in cappillaries.
Why do so many African Americans carry
the sickle cell allele?
• Most African Americans can trace their ancestry
to west central Africa.
• Malaria, a serious parasitic disease that infects
red blood cells, is common in this region of
Africa.
• People who are heterozygous for the sickle
cell allele are generally healthy and are
resistant to malaria.
Multiple Alleles
• Many genes have
more than two alleles
and are therefore said
to have multiple
alleles.
• One of the best-known
examples is coat color
in rabbits.
• This does not mean
that an individual can
have more than two
alleles. It only means
that more than two
possible alleles exist
in a population.
• A rabbit’s coat color is
determined by a single
gene that has at least
four different alleles.
– Full color, albino,
himalayan, chinchilla
• ABO blood groups have
three forms of alleles.
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Coat Color of Rabbits
Chinchilla
Albino
Light gray
Dark gray
Himalayan
Blood Groups
Section 14-1
Phenotype
(Blood Type
Go to
Section:
Genotype
Antigen on
Red Blood Cell
Safe Transfusions
To
From
Epistasis:
• The result of one allele hiding the effects
of another allele.
• Ex: Labrador’s coat color can vary from
yellow to black
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Epistasis
 Variety is the result of one allele hiding the
effects of another allele.
eebb
eeB_
No dark pigment present in fur
E_bb
E_B_
Dark pigment present in fur
Polygenic Traits
• Many traits are produced
by the interaction of
several genes.
• Traits controlled by
two or more genes
are said to be
polygenic traits.
• Polygenic traits often
show a wide range of
phenotypes.
– Ex: skin color, eye
color, height, and
fingerprint pattern
Polygenic Traits:
• Most traits are controlled by
two or more genes and are,
therefore, called polygenic
traits.
• Each gene of a polygenic
trait often has two or more
alleles.
• As a result, one polygenic
trait can have many possible
genotypes and even more
possible phenotypes.
EX: height (A bell-shaped curve is
also called a normal distribution)
Chapter
11
Complex Inheritance and Human Heredity
11.2 Complex Patterns of Inheritance
Environmental Influences
 Environmental factors
 Diet and exercise
 Sunlight and water
 Temperature
Checkpoint Point Questions:
1. Explain what independent assortment means.
2. Describe two inheritance patterns besides
simple dominance.
3. What is the difference between incomplete
dominance and codominance?
4. Identify examples of polygenic traits and
multiples alleles. (one example for each)