Chapter 14: Mendel and the
Gene Idea
Heredity
• Heredity= passing of traits from parent to
offspring
– Reproductive cells are only way traits are passed
to offspring
– Changes in somatic cells will not be passed to next
generation
• Genetics= scientific study of heredity
What genetic principles account for the
passing of traits from parents to offspring?
1. The “blending” hypothesis is the idea that genetic
material from the two parents blends together
– Kind of like blue and yellow paint blend to make green
2. The “particulate” hypothesis is the idea that
parents pass on discrete heritable units (genes)
– Heritable factors may be rearranged, but retain identity
from one generation to the next
– This hypothesis can explain the reappearance of traits
after several generations
Gregor Mendel
• Gregor Mendel, Austrian monk
– Documented a particulate
mechanism in the 1800s by
experiments with garden pea plants
• 1860s- Studied inheritance patterns
in pea plants
– Character= heritable feature with multiple variations
– Trait= each type of variation for a character
– At this time, chromosomes and genes had not been
discovered yet
• Used the term heritable factors to represent what we call a gene
Gregor Mendel
• Discovered the basic principles of heredity by
breeding garden peas in carefully planned
experiments
• Advantages of pea plants for genetic study
– Short-life cycle
– There are many varieties with distinct characters
Gregor Mendel
• Parents pass characters on to offspring
– Pea plants= one character is flower color
• Parents can pass on different traits
– Pea plants= purple or white flower color
Gregor Mendel
• Discovered the basic principles of heredity by
breeding garden peas in carefully planned
experiments
• Advantages of pea plants for genetic study
– Short-life cycle
– There are many varieties with distinct heritable
characters
– Mating can be controlled
• Each flower has sperm-producing organs (stamens) and
egg-producing organ (carpel)
• Cross-pollination (fertilization between different plants)
involves dusting one plant with pollen from another
Gregor Mendel
• Mendel chose to track
only those characters
that occurred in two
distinct alternative
forms
• He also used varieties
that were truebreeding in P
generation
• Produce offspring
of same variety
when they selfpollinate
In a typical experiment, Mendel
mated two different true-breeding
varieties= hybridization
Gregor Mendel
• The hybrid offspring of the P generation are called the F1
generation
• When F1 individuals self-pollinate or cross- pollinate with
other F1 hybrids, the F2 generation is produced
Gregor Mendel
• F1 Generation:
100% purple
flowers
• Mendel
discovered a
ratio of about
3:1, purple to
white flowers, in
the F2
generation
Gregor Mendel
• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1
hybrids
• The factor for white flowers was not diluted or
destroyed because it reappeared in the F2
generation
• Mendel called the purple flower color a
dominant trait and the white flower color a
recessive trait
Pea Plant
Experiments
•7 Characters in Pea Plants
•Crossed different varieties of
each character
•Hybrids
•Genetic Test Cross
•P generation
•F1 generation
•F2 generation
•Mendel developed a
hypothesis to explain the 3:1
inheritance pattern he
observed in F2 offspring
Mendel’s Hypotheses Based on 4
Concepts
1. There are alternate forms of genes (alleles) and
these account for the variation of inherited
characters.
2. Offspring inherit 2 alleles for each character, one
from each parent.
3. When alleles are different, one allele determines
appearance (dominant allele) and other has no
effect (recessive allele).
4. Mendel’s Law of Segregation: Allele pairs
separate during gamete production and sex cells
carry one allele for each inherited character.
1. Alternate forms of genes (alleles)
account for variation
• Alleles= alternative forms of genes
• Flower Color: 2 Forms
2. Offspring inherit 2 alleles for each
character, one from each parent.
• 1 chromosome with
alleles for genes are
inherited from mom
and one from dad
• Alleles can be the
same or different
– Homozygous- alleles
same
– Heterozygous- alleles
different
3. When alleles are different,
dominant allele masks recessive allele
• Complete Dominance
• Dominant Alleles (uppercase letters)
– Determines appearance of offspring if present
• Homozygous Dominant= PP
• Recessive Alleles (lowercase letters)
– Masked by dominant allele
– Only determines appearance if both alleles are
recessive
– Homozygous recessive= pp
• Heterozygous: 1 dominant, 1 recessive= Pp
Frequency of Dominant Alleles
• Dominant alleles are not necessarily more
common in populations than recessive alleles
– For example, one baby out of 400 in the United States
is born with extra fingers or toes
• The allele for this unusual trait is dominant to the
allele for the more common trait of five digits per
appendage
• In this example, the recessive allele is far more
prevalent in the population than the dominant
allele
Exceptions to the Rule
Red = RR
• Incomplete Dominance:
Expression of recessive allele not
completely masked in
heterozygote
– Snapdragon flower color
Pink = Rr
• Co-dominance: Two alleles are
dominant and expressed in
heterozygote
– Example: Blood Type: AB
White = rr
4. Mendel’s Law of Segregation
• Allele pairs separate during gamete production
and sex cells carry one allele for each inherited
character
• Meiosis- haploid number of chromosomes
• Fertilization- fusion of egg and sperm result in
diploid offspring
– Genotype= alleles present in offspring for character
• Example: PP, pp, Pp
– Phenotype= physical appearance as a result of alleles
• Purple flowers, white flowers
Homologous chromosomes can have
different alleles
G1
S phase
4. Mendel’s Law of Segregation
• Mendel derived the law of segregation by
following a single character
• The F1 offspring produced in this cross were
monohybrids, individuals that are
heterozygous for one character
• A cross between such heterozygotes is called a
monohybrid cross
Mendel’s Experimental Crosses
Cross homozygous dominant plant with homozygous
recessive plant
P Generation
PP
pp
Mendel’s Law of Segregation
Genotype and Phenotype
• Because of the different effects of dominant
and recessive alleles, an organism’s traits do
not always reveal its genetic composition
– Phenotype= physical appearance
– Genotype= genetic makeup
• Flower color in pea plants:
– PP and Pp plants have the same phenotype
(purple) but different genotypes
Heterozygotes
• Organisms with heterozygous genotype are
considered “carriers” of the recessive allele
– Not a true-breeding variety
– Pea Plants Flower Color= Pp
• Recessive allele is present, but not “visible” in
phenotype
• Can be passed on to next generation
Punnett Square
• Used to predict potential traits in offspring
– 4 possible combinations of alleles
– Each parent creates 2 potential gametes 2 x 2= 4
• Need to know type of gametes produced by
parent
– Genotype= alleles for character
– Each gamete has one allele
AA
Aa
AA
Aa
Pea Plants Experiment
•Cross F1 generation to
produce F2 generation
•Phenotype: Purple Flowers
•Genotype: Heterozygous
•Cross Bb X Bb
•F2 generation:
•1 BB (purple)
•2 Bb (purple)
•1 bb (white)
Research in Genetics
• Testcross
– Mating between unknown
genotype and homozygous
recessive genotype
– Use Punnett Squares to
predict offspring traits and
compare to actual outcomes
Mendel’s Law of Independent
Assortment
• Mendel identified his second law of inheritance
by following two characters at the same time
• Crossing two true-breeding parents differing in
two characters produces dihybrids in the F1
generation, heterozygous for both characters
• A dihybrid cross, a cross between F1 dihybrids,
can determine whether two characters are
transmitted to offspring as a package or
independently
Mendel’s Law of Independent
Assortment
• Mendel’s Law of Independent Assortment
– Genes inherited independently of each other
– Inheritance of one character independent from
another character
• This law applies only to genes on different
chromosomes or those far apart on the same
chromosome
– Genes located near each other on the same
chromosome tend to be inherited together
Mendel’s Law of Independent
Assortment
Genetic Inheritance
• Mendel’s laws of segregation and
independent assortment reflect the rules of
probability
• When tossing a coin, the outcome of one toss
has no impact on the outcome of the next toss
– In the same way, the alleles of one gene segregate
into gametes independently of another gene’s
alleles
Genetic Inheritance
• The multiplication rule states that the probability
that two or more independent events will occur
together is the product of their individual
probabilities
• Probability in an F1 monohybrid cross can be
determined using the multiplication rule
• Segregation in a heterozygous plant is like flipping
a coin:
– Each gamete has a ½ chance of carrying the dominant
allele and a ½ chance of carrying the recessive allele
Genetic Inheritance
The addition rule=
probability that any
one of two (or more)
exclusive events will
occur is calculated by
adding together their
individual
probabilities
Probability of
heterozygote:
¼+¼=½
Probability of
homozygote:
¼+¼=½
Dihybrid Crosses
• A dihybrid or other multicharacter cross is
equivalent to two or more independent
monohybrid crosses occurring simultaneously
• Crosses with 2 characters of interest
– RrYy x RrYy
• In gametes, alleles for one character can be paired with
either allele for other character
• For each parents gametes, R can be paired with Y or y
• 4 possible gamete types for each parent
–RY, Ry, rY, ry
Dihybrid Cross
Exceptions to the Rule
• The relationship between genotype and
phenotype is rarely as simple as in the pea
plant characters Mendel studied
• However, the basic principles of segregation
and independent assortment apply even to
more complex patterns of inheritance
Exceptions to the Rule
• Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in the
following situations:
1. When alleles are not completely dominant or
recessive
2. When a gene has more than two alleles
3. When a gene produces multiple phenotypes
Exceptions to the Rule
Red = RR
• Incomplete Dominance:
Expression of recessive allele not
completely masked in
heterozygote
– Snapdragon flower color
Pink = Rr
White = rr
Exceptions to the Rule
• Co-dominance: Two alleles are dominant and expressed
in heterozygote
• This occurs in blood types, which also has more than 2
alleles
– Four phenotypes: A, B, O, AB
– A and B codominant, both are dominant to O
• Determined by three alleles for the enzyme (I) that
attaches A or B carbohydrates to red blood cells: IA, IB,
and i
• The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by the IB
allele adds the B carbohydrate
• Enzyme encoded by the i allele adds neither
Pleiotropy
• Most genes have multiple phenotypic effects,
a property called pleiotropy
• For example, pleiotropic alleles are
responsible for the multiple symptoms of
some hereditary diseases
– Cystic fibrosis
– Sickle-cell disease
Sickle-cell disease
• The disease is caused by the substitution of a
single amino acid in the hemoglobin protein in
red blood cells
– Causes cell to collapse and form sickle shape
– Clogs blood vessels
• In homozygous individuals, all hemoglobin is
abnormal (sickle-cell)
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
Sickle-Cell Disease
• Heterozygotes= produce
both types of blood cells
Normal Blood Cell
– Heterozygotes have sickle-cell
trait
• Usually healthy but may suffer
some symptoms of disease
• Sickle-cell disease affects
one out of 400 AfricanAmericans
– Unusual to have high
frequency of an allele with
detrimental effects in
homozygotes
Sickle-Cell
Sickle-Cell Disease
• Reason: “Heterozygote
Advantage”
– Less susceptible to the
malaria parasite, so there is
an advantage to being
heterozygous in areas where
malaria is common
– Keeps allele in population
despite detrimental effects of
homozygous condition
Normal Blood Cell
Sickle-Cell
Epistasis
• In epistasis, a gene at one locus alters the
phenotypic expression of a gene at a second
locus
• In Labrador retrievers and many other
mammals, coat color depends on two genes
– One gene determines the pigment color (with
alleles for black or brown)
– The other gene (with alleles
for color or no color)
determines whether the
pigment will be deposited in
the hair
Polygenic Inheritance
• Quantitative characters are those that vary in
the population along a continuum
• Quantitative variation usually indicates
polygenic inheritance, an additive effect of
two or more genes on a single phenotype
• Skin color in humans is an example of
polygenic inheritance
Environmental Variation
in Phenotypes
• Another exception arises when
phenotype of a character depends on
environment as well as genotype
– Multifactorial characters: genetic and
environmental factors collectively
influence phenotype
• Norm of reaction: phenotypic range of
a genotype influenced by the
environment
– Usually broadest for polygenic
characters
• Hydrangea flowers of the same
genotype range from blue-violet to
pink, depending on soil acidity
Human Genetics
• Humans are not good subjects for genetic
research
– Generation time is too long
– Parents produce relatively few offspring
– Breeding experiments are unacceptable
• However, basic Mendelian genetics still serves
as the foundation of human genetics
Genetic Traits in Humans
Pedigree= Family tree of genetic traits for multiple
generations
Pedigrees
• Pedigrees can also be used to make predictions about
future offspring
• We can use the multiplication and addition rules to
predict the probability of specific phenotypes
• Many genetic disorders are inherited in a recessive
manner
– Recessively inherited disorders show up only in individuals
homozygous for the allele
• Carriers are heterozygous individuals with 1 copy of recessive allele
but are phenotypically normal
• Most individuals with recessive disorders are born to carrier parents
• Albinism is a recessive condition characterized by a lack
of pigmentation in skin and hair
Albinism in Humans
Human Genetics
• For a growing number of diseases, tests are
available that identify carriers and help define the
odds of offspring having disease more accurately
– Many diseases, such as heart disease, diabetes,
alcoholism, mental illnesses, and cancer have both
genetic and environmental components, making
predictions for multifactorial diseases difficult
• Now that the human genome has been
described, research is focusing on identifying
mutations in sequence and how they affect
health
– Advances in understanding the human genome will
potentially lead to new medicines, treatments, and
preventive care
Current Genetic Research
• Breast Cancer
– BRCA1 and BRCA2- human genes part of a group
of genes called tumor suppressors
• Stabilize DNA and prevent uncontrolled growth
– Mutations in these genes increase risk of
hereditary breast and ovarian cancers
– Mutations can be found in men and women and
may increase risk of other types of cancers
– Blood test can detect genetic mutation
• Not all mutations are harmful, many types are neutral
Current Genetic Research
• Using family histories, genetic counselors help
couples determine the odds that their children
will have genetic disorders
– Individuals can evaluate chances of development
of genetic disorders present in close family
members
– Potential parents with genetic disorders can
determine the chances of passing genetic disorder
on to offspring
Current Genetic Research
• Punnett Square: Predicting the chance of
passing genetic disorder on to offspring
– If a woman has a blood test and finds she has a
recessive mutation for a genetic disorder
(heterozygous), what are the chances of her
offspring having the mutation if her husband is
homozygous dominant?
A
A
A
a
AA
Aa
AA
Aa
Genotype Probability:
50% AA (no mutation)
50% Aa (mutation
carrier)
Inbreeding
• In a large population, chance of mating with a relative
is low because there are many choices
– Most societies and cultures have restrictions against
marriages between close relatives
• Sexual reproduction between non-related individuals
increases or maintains high genetic diversity
• In a small population, chance of mating with a relative
is high because there are few choices
– Increases frequency of deleterious recessive genotypes
– Exposes rare deleterious mutations
Example: White Tigers
• White coloration in Bengal
tigers
– 1 in 15,000 wild births
• Homozygous recessive
condition (aa)
• In 1951, one white male
tiger was captured in the
wild
• Most white tigers in zoos
are descendants of this
original tiger
Practice
• Use a Punnett Square to find probabilities of
offspring from the following matings:
1. White male (aa) x Orange female (AA)
2. White male (aa) x Orange daughter (Aa)
3. White male (aa) x White daughter (aa)
Example: White Tigers
• Over time, repeated matings between close
relatives to produce white tiger decreased
genetic diversity and evidence of inbreeding
depression was observed
–
–
–
–
Birth defects and low reproductive success
Weak immune system- more susceptible to disease
Poor eyesight and hearing
Weak bones
• In 2011, American Zoological Association
banned breeding of white tigers