Chapter 15 Parents to Offspring I- Patterns that follow Mendel's laws

Chapter 15 – Transmission of Genetic Information from
Parents to Offspring I: Patterns That Follow Mendel’s Laws
Chapter Outline
1. Mendel’s laws of
inheritance
2. Chromosome theory of
inheritance
3. Pedigree analysis of human
traits
4. Variations in inheritance
patterns and their
molecular basis
5. Sex chromosomes and X-
linked inheritance patterns

15.1 Mendel’s Laws of Inheritance
Section 15.1 Learning
Outcomes
1. Explain the advantages of
using the garden pea to study
inheritance
2. Describe the difference
between dominant and
recessive traits
3. Distinguish between
genotype and phenotype and
between genes and alleles
4. Determine the possible
gametes that a parent can
generate, when given
parental genotype or
phenotype

15.1 Mendel’s Laws of Inheritance
Section 15.1 Learning
Outcomes Cont’d
5. Predict the outcomes of both
single-factor and two-factor
genetic crosses using a
Punnett square
6. State the expected genotype
and phenotype ratios for a
simple monohybrid cross
(Aa x Aa)
7. State the expected phenotype
ratio for a simple dihybrid
cross (AaBb x AaBb)
8. Explain Mendel’s law of
segregation and law of
independent assortment

15.1 Mendel’s Laws of Inheritance
• Parents and offspring often show a striking resemblance to each
other; inheritance is the acquisition of traits by their transmission
from parent to offspring
• Observations of chromosome transmission during mitosis and
meiosis provided evidence for particulate inheritance
• Particulate inheritance is the idea that determinants of
hereditary traits (now called genes) are transmitted in discrete
units from one generation to the next
• A gene can be broadly defined
as a unit of heredity

15.1 Mendel’s Laws of Inheritance
• Many different patterns of inheritance exist

15.1 Mendel’s Laws of Inheritance
• In 1856 Gregor Mendel, an Austrian monk, began his studies on
pea plants; Mendel analyzed thousands of pea plants over 8 years
• He used quantitative analysis of his experimental results to arrive at
the concept of the gene
• In 1866 he published
his results; his paper
was largely ignored
at the time but was
rediscovered in 1900
Fig 12.1, Principles of Life, 2014 Sinauer Associates, Inc.

15.1 Mendel’s Laws of Inheritance
Mendel Chose the Garden Pea to Study Inheritance
• Advantageous properties of pea plants:
• Plants have many different characters (general features) each
found in discrete forms called variants
• A trait is an
identifiable
characteristic
of an organism;
the term trait
usually refers
to a variant for
a character

15.1 Mendel’s Laws of Inheritance
Mendel Chose the Garden Pea to Study Inheritance
• Advantageous properties of pea plants:
• Pea plants are normally self-fertilizing; a female gamete is
fertilized by a male gamete from the same plant
• The stamens form male gametes, and the ovules form female
gametes
• A line of plants that
continues to exhibit the
same trait after several
generations of self-
fertilization is called a
true-breeding line

15.1 Mendel’s Laws of Inheritance
Mendel Chose the Garden Pea to Study Inheritance
• Advantageous properties of pea plants:
• Although pea plants normally self-fertilize, large flowers make it
easy to perform crosses when desired
• When two individuals
of the same species
with different
characteristics are
bred, or crossed, to
each other, the
process is called
hybridization, and
the offspring are
referred to as hybrids

15.1 Mendel’s Laws of Inheritance
By Following the Inheritance Pattern of Single Traits,
Mendel’s Work Revealed the Law of Segregation
• Mendel began his work by studying the inheritance patterns of
plants that differed in a single character; he conducted single-
factor crosses (including monohybrid crosses)
• The adjacent cross follows tall and
dwarf varieties for height
• The P (parental) generation are
true-breeding, and their offspring
are the F1 (first filial) generation
• The F1 offspring are single-
character hybrids, or monohybrids
• The F1 monohybrids grow to
maturity, then self-fertilize (this is
the monohybrid cross Tt x Tt) to
generate the F2 generation

15.1 Mendel’s Laws of Inheritance
By Following the Inheritance Pattern of Single Traits,
Mendel’s Work Revealed the Law of Segregation
• Mendel documented a
pattern in the traits of
the F1 and F2 generations
• All the plants in the
F1 were tall
• Three-fourths of the
plants in the F2
generation were tall
and one-fourth were
dwarf

15.1 Mendel’s Laws of Inheritance
By Following the Inheritance Pattern of Single Traits,
Mendel’s Work Revealed the Law of Segregation
• Mendel obtained
similar results for each
of the seven characters
he studied
• Consistently, three-
fourths of the plants in
the F2 generation
displayed one trait and
one-fourth displayed
the other trait
• The ratio was
consistently 3:1

15.1 Mendel’s Laws of Inheritance
By Following the Inheritance Pattern of Single Traits,
Mendel’s Work Revealed the Law of Segregation
• Mendel named the trait that was displayed by the F1 generation the
dominant trait; the trait that was masked in the F1 generation and
reappeared in the F2 generation was called the recessive trait
• Mendel’s results were consistent
with particulate inheritance; the
determinants of traits were
inherited in unchanging, discrete
units
• Mendel called these determinants
“unit factors”; we call them genes
• The variant forms of a gene are
called alleles

15.1 Mendel’s Laws of Inheritance
By Following the Inheritance Pattern of Single Traits,
Mendel’s Work Revealed the Law of Segregation
• Based on quantitative analysis,
Mendel concluded that each
plant carried 2 versions (alleles)
of a gene and that each gamete
only carried 1 allele
• These ideas are formally stated in
Mendel’s law of segregation
• The two alleles of a gene
separate (segregate) from
each other during the process
that gives rise to gametes so
that every gamete receives
only one allele

15.1 Mendel’s Laws of Inheritance
Genotype Describes an Organism’s Genetic Makeup,
Whereas Phenotype Describes Its Characteristics
• The genotype is the genetic composition of an individual; it is often
represented with letters (numbers and symbols may also be used)
• TT is homozygous dominant
• Tt is heterozygous
• tt is homozygous recessive
• The phenotype is the physical or
behavioral characteristics that are
the result of gene expression
• TT and Tt have a tall phenotype
• tt has a dwarf phenotype

15.1 Mendel’s Laws of Inheritance
The Punnett Square Is Used to Predict the Outcome
of Crosses
A common way to predict the outcome of simple genetic crosses is to
make a Punnett Square
1. Write the genotypes of the parents (both Tt)
2. Write the possible gametes that each parent
can make (both T or t)
3. Create an empty Punnett square
• # of columns = # of male gametes
• # of rows = # of female gametes
4. Fill in the possible genotypes
5. Determine the relative proportions of
genotypes and phenotypes
• Genotype ratio is 1:2:1 (TT: Tt: tt)
• Phenotype ratio is 3:1 (tall: dwarf)

15.1 Mendel’s Laws of Inheritance
Analyzing the Inheritance Pattern of Two Characters
Demonstrated the Law of Independent Assortment
• To further investigate inheritance, Mendel conducted two-factor
crosses (including dihybrid crosses), using parents that differed for
two characters
• Parent plants were true-breeding
for both characters
• Possible patterns:
• Parental information is linked 
parental information for the 2
characters is inherited as a unit
• Parental information assorts
independently  information for
the 2 characters is randomly
assorted into gametes

15.1 Mendel’s Laws of Inheritance
Analyzing the Inheritance Pattern of Two Characters
Demonstrated the Law of Independent Assortment
• Results from the two-factor crosses gave rise to
Mendel’s law of independent assortment
• The alleles of different genes assort
independently of each other during the process
that gives rise to gametes
• Parents differed for seed shape and seed color
• F1 offspring were all heterozygous and displayed the
dominant traits: yellow and round
• F1 plants grew and reproduced through self-fertilization
(this is the dihybrid cross YyRr x YyRr), generating the F2
offspring
• F2 offspring displayed trait combinations
different than the parental generation
(ex: yellow, wrinkled seeds and green,
round seeds)

15.1 Mendel’s Laws of Inheritance
• When solving problems, it is important to remember the expected
patterns for the F2 generation for monohybrid (Aa x Aa) and
dihybrid (AaBb x AaBb) crosses
• Note: not all single-factor crosses are monohybrid crosses
• Note: not all two-factor crossed are dihybrid crosses
• Genotype and phenotype ratios for a
monohybrid cross:
• 1:2:1
• 3:1
• It is also important to recognize these
patterns when presented as fractions
or percentages
• 1/4, 2/4, 1/4 or 25%, 50%, 25%
Fig 8.2, Principles of Life, 2014 Sinauer Associates, Inc.

15.1 Mendel’s Laws of Inheritance
• Phenotype ratio for a
dihybrid (AaBb x AaBb) cross:
• 9:3:3:1
• It is also important to
recognize this pattern when
presented as fractions
• 9/16, 3/16, 3/16, 1/16
Fig 8.5, Principles of Life, 2014 Sinauer Associates, Inc.

15.1 Mendel’s Laws of Inheritance
• Phenotype ratio for a
dihybrid (AaBb x AaBb) cross:
• 9:3:3:1
• It is also important to
recognize this pattern when
presented as fractions
• 9/16, 3/16, 3/16, 1/16
Fig 8.5, Principles of Life, 2014 Sinauer Associates, Inc.

15.2 Chromosome Theory of Inheritance
Section 15.2 Learning
Outcomes
1. Explain the principles of the
chromosome theory of
inheritance
2. Relate the behavior of
chromosomes during
meiosis to Mendel’s two
laws of inheritance

15.2 Chromosome Theory of Inheritance
• At the time of Mendel’s work, the nature and location of “genes”
was unknown
• As time progressed, other scientists used microscopes to observe
dividing cells and suggested that chromosomes are the carriers of
hereditary information
• It was noted that Mendel’s
laws of segregation and
independent assortment
showed a striking parallel
with the segregation and
sorting of chromosomes
during meiosis
Fig 12.1, Principles of Life, 2014 Sinauer Associates, Inc.

15.2 Chromosome Theory of Inheritance
A modern view of the chromosome theory of inheritance:
1. Chromosomes contain DNA, which is the genetic material. Genes
are found within the chromosomes.
2. Chromosomes are replicated and passed from parent to offspring.
3. The nucleus of a diploid cell contains 2 sets of chromosomes,
which are found in homologous pairs. Maternal and paternal sets
of homologous chromosomes are functionally equivalent; each
set carries a full complement of genes.
4. At meiosis, one member of each homologous pair segregates into
one daughter nucleus, and its homolog segregates into the other
daughter nucleus. Members of different homologous pairs
segregate independently of one another.
5. Gametes are haploid cells that combine to form a diploid cell
during fertilization, with each gamete transmitting one set of
chromosomes to the offspring.

15.2 Chromosome Theory of Inheritance
Law of Segregation Is Explained by the Segregation of
Homologous Chromosomes During Meiosis
• A gene’s locus is its physical location on a
chromosome
• Each member of a homologous pair carries
an allele of the same gene at the same locus
• The pairing and segregation of homologous chromosomes during
meiosis explains the patterns that led to Mendel’s law of segregation

15.2 Chromosome Theory of Inheritance
Law of Segregation Is Explained by the Segregation of
Homologous Chromosomes During Meiosis
• The chromosomal basis of allele
segregation is depicted in the figure
• Mendel’s law of segregation
states that the two alleles of
a gene separate (segregate)
from each other during the
process that gives rise to
gametes so that every
gamete receives only one
allele

15.2 Chromosome Theory of Inheritance
Law of Independent Assortment Is Explained by the
Alignment of Different Chromosomes During Meiosis
• The law of independent assortment can also be explained by the
behavior of chromosomes during meiosis
y R Y r Y R y r
• Alleles for seed
color (Y or y) and
seed shape (R or r)
are on different
chromosomes
• Random alignment
can lead to one
dominant and one
recessive allele on the
same side (left part of
figure) or to both
dominant alleles on
the same side (right
part of figure)

15.3 Pedigree Analysis of Human Traits
Section 15.3 Learning
Outcomes
1. Apply pedigree analysis to
deduce inheritance
patterns in humans
2. Distinguish between
recessive disorders and
dominant disorders

15.3 Pedigree Analysis of Human Traits
• Human geneticists rely on information contained in pedigrees, or
family trees, to determine patterns of inheritance
• Pedigree analysis allows us to determine whether a mutant allele is
dominant or recessive and to predict the likelihood of an individual
being affected
• A wild-type allele is common, and a mutant allele is rare
• Most genes display autosomal
inheritance patterns
• Genes located on the sex
chromosomes display distinct
inheritance patterns

15.3 Pedigree Analysis of Human Traits
• The adjacent pedigree traces a
disease (cystic fibrosis) that
displays an autosomal recessive
pattern
• Individuals who are
homozygous recessive for
the mutant allele experience
the disease
• Important feature  two
unaffected individuals can
produce an affected offspring
• Presumed heterozygotes
• However, those same two
parents can also produce an
unaffected offspring

15.3 Pedigree Analysis of Human Traits
• The pedigree below traces a disease (Huntington disease) that
displays an autosomal dominant pattern
• Individuals who are heterozygous (or homozygous dominant) for
the mutant allele experience the disease
• Important feature  every affected individual has an affected
parent
• However, an affected parent can also produce an unaffected
offspring

15.4 Variations in Inheritance Patterns & Molecular Basis
Section 15.4 Learning
Outcomes
1. Relate dominant and
recessive alleles and
genotypes to protein
function
2. Describe pleiotropy, and
explain why it occurs
3. Predict the outcome of
crosses that exhibit
incomplete dominance
4. Discuss how the
environment plays a critical
role in determining the
outcome of traits

15.4 Variations in Inheritance Patterns & Molecular Basis
• The phrase Mendelian inheritance describes the inheritance
patterns of genes that segregate and assort independently
• In simple Mendelian inheritance the alleles are dominant or
recessive
• There are many other types of inheritance patterns
• Understanding
protein function
at the molecular
level explains
differences in
inheritance
patterns

15.4 Variations in Inheritance Patterns & Molecular Basis
Protein Function Explains the Phenomenon of Dominance
• A wild-type allele usually encodes a protein that is made in the
proper amount and functions normally
• Mutations that produce recessive alleles are often loss-of-
function alleles
• Sometimes a single copy of
the dominant allele is
sufficient; there is enough
functional protein to
provide a normal phenotype
 heterozygote displays
dominant phenotype
• Heterozygotes may also use
gene regulation to increase
the expression of the
functional allele

15.4 Variations in Inheritance Patterns & Molecular Basis
Protein Function Explains the Phenomenon of Dominance
• In many human genetic
diseases, a recessive allele
fails to produce a specific
functional protein
• Over 9,000 human
disorders are caused by
mutations in a single
protein-encoding gene

15.4 Variations in Inheritance Patterns & Molecular Basis
Recessive Alleles That Cause Diseases May Have
Multiple Effects on Phenotype
• Pleiotropy occurs when a mutation in a single gene can have
multiple effects on an individual’s phenotype
• A single gene can affect cell function in more than one way; ex:
microtubule proteins affect cell division and cell movement
• A single gene may be expressed in different cell types in a
multicellular organism
• A gene may be expressed at different stages of development
• Individuals with cystic fibrosis have a
poorly functioning Cl- transporter
• Compromised ion transport leads
to thick mucus in the lungs, thick
mucus in digestive system that can
block release of digestive enzymes,
and altered sweat gland function

15.4 Variations in Inheritance Patterns & Molecular Basis
Incomplete Dominance Results in an
Intermediate Phenotype
• Incomplete dominance occurs when
the heterozygote shows an
intermediate phenotype
• This pattern occurs for flower color of
the four-o’clock plant
• Alleles are designated with superscripts
rather than upper/lowercase because
neither is dominant
• CR for red and CW for white
• In this case, the heterozygote (CRCW)
does not produce enough pigment to
appear red; instead, it appears pink
• Genotype and phenotype ratios for
the F2 are both 1:2:1

15.4 Variations in Inheritance Patterns & Molecular Basis
The Environment Plays a Vital Role in the
Making of a Phenotype
• An organism’s genotype provides the plan to create the phenotype;
the environment provides nutrients and energy so the plan can be
executed
• The norm of reaction is the
phenotype range that
individuals with a particular
genotype exhibit under
differing environmental
conditions
• Genetically identical
plants grow to different
heights in different
temperatures

15.5 Sex Chromosomes and X-Linked Inheritance Patterns
Section 15.5 Learning
Outcomes
1. Describe the different
systems of sex
determination in animals
and plants
2. Predict the outcome of
crosses when genes are
located on sex
chromosomes
3. Explain why X-linked
recessive traits are more
likely to occur in males

15.5 Sex Chromosomes and X-Linked Inheritance Patterns
In Many Species, Sex Differences Are Due to the
Presence of Sex Chromosomes
• Sex chromosomes are different between
males and females and determine the
sex of individuals
• Sex chromosomes are found in many
(but not all) species with 2 sexes
• There are several mechanisms for sex
determination
• X-Y system in mammals
• X-O system in many insects
• Z-W system in some birds and fish
• In bees, the male is haploid, and the
female is diploid
• Sex is controlled by environment
(temperature) in some reptiles and fish

15.5 Sex Chromosomes and X-Linked Inheritance Patterns
In Many Species, Sex Differences Are Due to the
Presence of Sex Chromosomes
• Sex chromosomes are different between
males and females and determine the
sex of individuals
• Sex chromosomes are found in many
(but not all) species with 2 sexes
• There are several mechanisms for sex
determination
• X-Y system in mammals
• X-O system in many insects
• Z-W system in some birds and fish
• In bees, the male is haploid, and the
female is diploid
• Sex is controlled by environment
(temperature) in some reptiles and fish

15.5 Sex Chromosomes and X-Linked Inheritance Patterns
In Humans, X-Linked Recessive Traits Are More Likely
to Occur in Males
• Sex-linked genes are found on one sex chromosome but not the
other; X-linked genes are found on the X chromosome
• In humans the X chromosome is larger (~1,000 protein-
encoding genes) than the Y chromosomes (< 100 genes)
• Male mammals are hemizygous
for X-linked genes; they have
only one copy of genes on the X
chromosome
• X-linked recessive diseases
occur more frequently in males
than females
• Ex: hemophilia A

Chapter 15 Summary
15.1 Mendel’s laws of inheritance
• Mendel chose the garden pea to study inheritance
• By following the inheritance pattern of single traits, Mendel’s
work revealed the law of segregation
• Genotype describes an organism’s genetic makeup, whereas
phenotype describes its characteristics
• A Punnett square is used to predict the outcome of crosses
• Analyzing the inheritance pattern of two characters
demonstrated the law of independent assortment
15.2 Chromosome theory of inheritance
• Mendel’s law of segregation is explained by the segregation of
homologous chromosomes during meiosis
• Mendel’s law of independent assortment is explained by the
independent alignment of different chromosomes during
meiosis

Chapter 15 Summary
15.3 Pedigree analysis of human traits
• Pedigree analysis allows determination of whether a mutant allele
is dominant or recessive
15.4 Variations in inheritance patterns and their molecular basis
• Protein function explains the phenomenon of dominance
• Recessive alleles that cause diseases may have multiple effects on
phenotype (pleiotropy)
• Incomplete dominance results in an intermediate phenotype
• The environment plays a vital role in the making of a phenotype
15.5 Sex chromosomes and X-linked inheritance patterns
• In many species, sex differences are due to the presence of sex
chromosomes
• There are several mechanisms of sex determination: X-Y system, X-
O system, Z-W system, haplodiploid system, environmental systems
• In humans, X-linked recessive traits are more likely to occur in
males