Introduction
On a 3 x 5 card write down a description of
yourself using only your inherited traits.
For example, you can describe your hair
color or texture but not your hair style that
day. The description will aloud to the class.
The class will try to figure out if they could
identify the student based on the
description.
11-1 The Work of Mendel
• What does every living thing inherit from
their parents?
Look around at your classmates and make a
list of some of the traits that are inherited.
Genetics – the study of heredity
The Work of Mendel
• Genes – the chemical factors (DNA) that
determine traits
• Alleles – different forms of a gene
– Ex. From pea plants:
gene = plant height
alleles = tall and short
Read about Mendel on p. 263 – report some findings
about his life.
The Work of Mendel
Mendel wanted to see how trait in pea plants
were inherited.
What he already knew:
– Each flower produces pollen and egg cells
– Cross-pollination(sexual) – pollen from one
plant fertilizes the egg of another plant
– Self-pollination (asexual) – pollen fertilizes
eggs from same plant
– Mendel’s pea plants were true-breeding
• A tall plant with green seeds would produce a tall
plant with green seeds
How Mendel started his experiment
• Cross-pollinated the pea plants to produce hybrid
plants.
• Had to prevent self-pollination in order to do
this…HOW?
Cut away the male parts of the plant
• How did Mendel cross-pollinate the plants?
Dusted the female reproductive part with pollen
from another plant
Mendel crossed plants to study offspring
• Mendel wanted to see what would happen if he
crossed to true-breeding tall plant with a truebreeding short plant.
– The original plants = P generation
– The offspring = F1 generation
– What did Mendel expect to see in the P
generation cross?
Mendel crossed plants to study offspring
What did Mendel find after in the F1 generation after he
did the P generation cross?
• Principle of dominance: some alleles are dominant
and others are recessive
• Dominant allele:
– the trait is always shown when present
– Capital letter
• Recessive allele –
• the trait that will only show if there is no dominant
allele
– Lower case letter
Segregation
• Mendel’s next question…Where
had the recessive allele gone?
• He crossed F1 generation plant
with other F1 plant to get the F2
generation.
– F1 generation – (offspring of P)
tall x tall
– F2 generation – (offspring of
F1) 3 tall, 1 short
• F2 generation shows that recessive
alleles have reappeared.
Segregation
Why did the recessive allele disappear in
the F1 generation only to reappear in
the F2 generation.
• The tall and short alleles are present in
the F1 plants.
• The tall and short alleles must
separate, or segregate, during the
formation of gametes.
• What are gametes?
• Each gamete only carries one alleles
for each gene.
11-2 Genetics and Probability
•Mendel realized…the principles of
probability could be used to explain the
results of genetic crosses.
Genetics and Probability
• Probability – the likelihood a particular event will
occur.
– Ex: probability of flipping a coin to heads = ½ or 50%
– Probability of head 3 times in a row = ½ x ½ x ½ = 1/8
– The more number or trials, the closer to the expected
ratio
– Past outcomes do not affect future outcomes
• Alleles segregate randomly (like a coin)
Probabilities Predict Averages
• To get an accurate prediction of flipping a
coin – the coin should be flipped many
times and an average taken.
• In genetics …the more offspring you get,
the closer to the predicted ratio.
Punnett Squares
• Punnett square – diagram to predict and
compare outcomes of genetic crosses.
• Homozygous – organisms that have 2
identical alleles for a particular trait
– Ex: TT or tt
• Heterozygous – organisms that have 2
different alleles for the same trait
– Ex: Tt
Punnett Squares
• Phenotype – physical characteristics
– Ex: tall, short, yellow, green
• Genotype – genetic make-up
– Ex: TT, Tt, tt
• All tall plants have the same
phenotype (tall), but not the same
genotype (TT or Tt)
• Why are two genotypes for tallness,
but only one for shortness.
Punnett Squares
Make a Punnett to show Mendel’s
P generation where he crossed a
homozygous tall plant with
homozygous short plant.
Make a Punnett square to show
Mendel’s F1 cross with two
heterozygous tall plants.
Independent Assortment
Mendel questioned how alleles segregate if more that
one gene is involved
Ex: Does the seed shape gene influence the seed
color gene?
• Mendel needed to follow 2 diff. alleles from one
generation to the next.
• Genetic crosses involving two genes are known as
dihybrid crosses (single gene crosses are known as
monohybrid crosses)
Independent Assortment
• P generation: Crossed a homozygous RRYY
(round yellow peas) with a homozygous rryy
(wrinkled, green peas)
Yellow – Y
Green – y
Round – R
Wrinkled -r
What was the
genotype and
phenotype of the
F1 offspring?
Independent Assortment
• If genes do segregate independently, then he
would see every possible combination of alleles.
• Mendel could not tell if the two genes segregate
independently in the P generation cross.
• Why?
• Mendel crossed the two of the F1 plants which
where heterozygous for both traits.
Independent Assortment
The F1 cross: RrYy x RrYy
What is the phenotype ratio?
Independent Assortment
• Mendel found that his actual
results were vary close to the
9:3:3:1 ratio that he predicted.
• Every allele combination is
possible
• Principle of Independent
assortment – genes for different
traits can segregate
independently during the
formation of the gametes
– Seed shape & color gene do
not influence each other
Summary of Mendel’s Principles
• Law of Dominance: Some alleles are
dominant and others are recessive.
• Law of Segregation: Each adult has two
alleles for each gene but they can only pass
on one.
• Law of Independent Assortment: Each
gene is inherited independently of each
other.
11-3 Other Patterns of Inheritance
Genes can act in various ways:
1. Dominant vs. Recessive – one allele
completely covers another allele.
2. Incomplete Dominance
1. Definition : one allele is not completely
dominant over another
Ex: red flowers + white flowers = pink
flowers
Beyond Dominant and Recessive
• Codominance
– Definition: both alleles contribute to the phenotype
of the organism
Ex: Red cow + White Cow = Red and White Cow
Colors don’t blend like incomplete dominance
Multiple Alleles
• Definition: a gene with
more than two alleles
• More than 2 alleles can
exist in a population not
an individual.
Ex: rabbit’s coat color
Ex: human’s blood type
Polygenic Traits
• Definition: traits that are controlled by 2 or
more genes
Ex: fruit fly red eyes - 3 genes involved in making
pigment
– Diff. combo of genes produce different eye colors
Ex: Human skin color – more than 4 different genes
Gene Linkage
• Thomas Morgan discovered that many genes
in fruit flies were inherited together.
• Ex: Flies with orange eyes almost always
had small wings
• This seemed to violate the principle of
independent assortment.
• Morgan’s finding lead to two conclusions:
1. Each chromosome is a group of linked
genes
2. It’s the chromosomes that assort
independently, not the individual genes.
Gene Mapping
• Scientist realized that the further apart the
two genes were on a chromosome the more
likely that crossing over would occur
between them.
• They could used the frequency of crossing
over between genes to determine their
distance from each other.
– If genes were close then a crossover
between them would be rare.
– If genes were far apart then a crossover
between them would be more likely.
Gene Mapping
• Gene Maps: show the relative location of each gene on
a chromosome.
Looking at the parents chromosomes, explain why
short legs and purple eyes are more likely to be
inherited together than short legs and vestigial wings.
Genes and the Environment
• Genes provide a plan for development, but
how the plan unfolds depends on the
environment.
• Environmental conditions can influence
genetically determined traits.
• Examples of traits influenced by genes and
environment.
– Height, skin color, IQ
Short Answer Questions 5.0
• Identify and explain Mendel’s 3 principles
he developed while studying pea plants. (3
points)
• Explain why fruit flies are a much better
model organism for studying genetics when
compared to humans. (Relate your answer
to probability – 4 points)
• Explain why the cells in your would need to
undergo mitosis and meiosis. (4 points)
Dihybrid Practice
• T = tall
• R = round
t = short
r = wrinkled
If a short, heterozygous round plant is crossed
with a heterozygous tall and round plant,
what are the chances that they will produce
a short, wrinkled plant.
ttRr x TtRr
Punnett Square Practice
1. S = straight hair
s = curly hair
Draw a Punnett square for SS x ss. What is the
phenotype ratio for the offspring.
2. Straight hair is dominant to curly hair. A woman
with straight hair marries a man with curly hair.
They have three children, 2 with straight hair and one
with curly hair. What is the probability that their next
child will have curly hair.
3. Two parents who are heterozygous for curly hair
want to have a child. Draw a Punnett square, give the
genotype and phenotype ratio.