Concepts of Genetics
Twelfth Edition
Chapter 3
Mendelian Genetics
Copyright © 2021, 2019, 2015 Pearson Education, Inc. All Rights Reserved
Learning Objectives
• Define these terms: gene, allele, locus, genotype and phenotype
• Define homozygous and heterozygous traits
• Differentiate between Genotype and Phenotype
• Identify and explain Mendel’s four postulates
• Use Punnett squares to predict outcomes of monohybrid and dihybrid
crosses
• Describe non-Mendelian inheritance patterns
• Use genetic crosses to predict outcomes for X-linked recessive disorders
• Apply Probability to Genetic Outcomes
• Determine if traits follow dominant, recessive, autosomal, or sex-linked
inheritance patterns
Popular misconception of Genetics
• Since no one else in my family has this disorder, it’s not genetic.
• He was born with that problem, so it must be genetic.
• Since the people in my family with the problem (e.g., breast cancer) are
all women, it can only be passed through women.
• I resemble my father, and he has a disorder, so I know I will get it too.
• All my children and siblings are healthy, so we can't have or be carriers
of that disorder.
• If the condition is autosomal recessive and my child has the condition,
then my next three children will be healthy.
• Genetic disorders are so rare, I won’t see people with them in my
everyday life.
Genetic Vocabulary – 1
Transmission of traits from one generation to the next
Study of heredity
• Gregor Mendel: Father of genetics
Genetic Vocabulary – 2
An inherited feature
that varies from one
individual to another
Two or more
variations in genetic
character
Genetic
Vocabulary – 3
• Individual unit of inheritance
• Contains specific nucleotide
sequence that codes for a
specific protein
• Each chromosome has one
DNA, and each DNA has
thousands of genes
Genetic
Vocabulary – 4
• The position of a gene
on a chromosome
• Alternate forms of a
particular gene
• Two alleles of a gene
are identical
• Two alleles of a gene
are different
Genetic Vocabulary – 5
• Organism’s appearance or
observable trait
• Organism’s genetic makeup
Mendel’s
Experiment – 1
• Mendel’s model organism: Peas
• Easy to grow
• True breeding strains
• Controlled matings
• Self or cross fertilization
• Grow to maturity in one season
• Observable characteristics with
two distinct forms
Mendel’s
Experiment – 2
• Involving a single pair of
contrasting traits
• Original true breeding parents
• First filial generation - offspring
• Second filial generation
• F1 generation was self fertilized
to produce F2 generation
Mendel’s Model (1 of 6)
• Mendel developed a model to explain the 3:1 inheritance
pattern he observed in F2 offspring
• Four related concepts make up this model
Mendel’s Model (2 of 6)
• First concept:
• Alleles of genes are responsible
for inherited characters
• There are two alleles for flower color
• one for purple flowers and
• the other for white flowers
Mendel’s Model (3 of 6)
• Second concept:
• For each character, an
organism inherits two
alleles, one from each
parent
Mendel’s Model (4 of 6)
• Third Concept:
• If the alleles at a locus differ, then,
the organism’s appearance
determines
has no noticeable effect on appearance
Mendel’s Model (5 of 6)
• Fourth Concept:
• the two alleles for a
heritable character separate
(segregate) during gamete
formation and end up in
different gametes
Mendel’s Model (6 of 6)
• This segregation of alleles corresponds to the distribution of homologous
chromosomes to different gametes in meiosis
• Thus, an egg or a sperm gets only one of the two alleles that are present
in the organism
Dominant vs Recessive
• One of the alleles that determines the organism’s appearance
• The allele has no noticeable effect, although it may be passed on to offspring
Genotype vs Phenotype
Genetic Cross
• If you breed a black Labrador retriever with a chocolate Lab,
what color can you expect the puppies in the litter to be?
The experiment is called
This can be represented using a diagram called
• A capital letter represents a dominant allele, and a lowercase letter
represents a recessive allele
Punnett Square – 1
Mendel’s Law – 1
Inheritance of one character has
no effect on the inheritance of another character
• When chromosomes separate during meiosis, each pair of alleles
separate independently.
Mendel’s Law - 2
Dihybrid
Cross
Human Genetic Characters
•
Dominant trait is not necessarily “normal” or more common than the
recessive trait
is the one most often seen in nature
is the less common form
Mendelian Inheritance Patterns – 1
• Many human traits follow Mendelian inheritance pattern
Mendelian Inheritance Pattern – 2
Mendelian Inheritance Pattern – 3
• Most human genetic
disorders are
• Person must have
two copies of the
disease allele to
express the disease
One copy of
the normal allele and
one copy of the
disease allele
Mendelian genetics
• Each character controlled by a single gene
• Each gene has only 2 alleles.
• F1 offspring all looked the same, as one allele was dominant
over the other.
Mendelian Inheritance
• The phenotype of the
heterozygote and the
dominant homozygote
are identical.
Non – Mendelian Inheritance – 1
• Heterozygous condition produces
an intermediate appearance
Non – Mendelian Inheritance – 2
X
• Joint expression of
both alleles in a
heterozygote.
• Both phenotypes
are expressed.
Non – Mendelian Inheritance – 3
• Most genes occur in
• Each individual carries
of genes are possible
• Human blood types are determined by a gene with
Non – Mendelian Inheritance – 4
Based on the surface
phenotype, what are the
dominance relationships
among the alleles?
Non – Mendelian Inheritance – 5
• In some cases, one gene influences many characters, a situation called
Non – Mendelian Inheritance – 6
• Many genes contribute to one character -
Polygenic Inheritance
• Eye color, hair color, and skin
pigmentation in humans are
polygenic traits
• 180 genes – height
• 378 genes – skin color
Non – Mendelian Inheritance – 7
• Expression of a gene at one locus alters the phenotypic expression of a
gene at a second locus • For example, in Labrador retrievers
and many other mammals, coat color
depends on two genes
• One gene determines the pigment
color (with alleles B for black and b
for brown)
• The second gene (with alleles E for
color and e for no color) determines
whether the pigment will be deposited
in the hair
The phenotypic ratio will be 9
black to 3 brown to 4 yellow labs
• The gene for pigment deposition E/e is said to be epistatic to the gene that
codes for Black or Brown (B/b) pigment
Environment vs Genetics
Non – Mendelian Inheritance – 7
Both alleles are inherited together
Do not follow the law of
independent assortment
Sex-Linked Genes
• A gene that is located on either sex chromosome is called a
• Genes on the Y chromosome are called
• Only 78 genes, coding for about 25 proteins, have been identified
on the human Y chromosome
• Genes on the X chromosome are called
The human X chromosome contains about 1,100 genes
Inheritance of X-Linked Genes – 1
• X chromosomes have genes for many characters unrelated to
sex
• Many Y-linked genes are related to sex determination
Inheritance of
X-Linked
Genes – 2
• X-linked genes follow a
specific pattern of inheritance
• Because males and
females inherit different
# of X chromosomes
Inheritance of XLinked Genes – 3
• For a recessive X-linked trait to
be expressed
• a female needs two copies of
the allele
• a male needs only one copy
of the allele
• X-linked recessive disorders are
much more common in males
than in females
Inheritance of X-Linked Genes – 4
• What happens when
the father carries the
allele?
• Fathers pass Xlinked alleles to all
the daughters but not
the sons
Inheritance of X-Linked Genes – 5
• What happens when
the mother carries
the allele?
• Mother passes the
X-linked alleles to
both sons and
daughters
X-Linked Recessive Disorders
• Some disorders caused by recessive alleles on the X
chromosome in humans:
• Color blindness (mostly X-linked)
• Duchenne muscular dystrophy
• Hemophilia
X-linked recessive traits – 1
X-linked recessive traits – 2
X-linked recessive traits – 3
Genetic Transmission Table
transmission
father-to-son
father-to-daughter
mother-to-son
mother-to-daughter
autosomal
yes, 50% chance
yes, 50% chance
yes, 50% chance son
yes, 50% chance
recessive
son is a carrier
daughter is a carrier
is a carrier
daughter is a carrier
inheritance
yes, 50% chance
yes, 50% chance
yes, 50% chance
yes, 50% chance
penetrant
X-linked recessive
cystic fibrosis
Huntington
autosomal
dominant, fully
common examples
Alzheimer
Neurofibromatosis 1
Marfan
no
yes, 100% chance
daughter is a carrier
yes, 50% chance.
Son may show an
Duchenne muscular
yes, 50% chance
abnormal phenotype.
dystrophy
Hemophilia
yes, 50% chance.
X-linked dominant,
fully penetrant
Mother may show a
no
yes, 100% chance
less severe
(incontinentia
yes, 50% chance
pigmenti)
Rett syndrome
phenotype. May be
fatal to sons.
Y-linked
yes, 100% chance
no
no
no
mitochondrial
no
no
yes
yes
no, neither father nor
no, neither father nor
no, neither mother nor
no, neither mother nor
Dominant and Y-
his direct ancestors
his direct ancestors
her direct ancestors
her direct ancestors
linked disorders are
have it
have it
have it
have it
often caused de novo
de novo mutation*
male infertility
Leber hereditary
optic neuropathy
* de novo mutations can be inheritable, but to the son or daughter of the person who first acquired the variation they are no longer de novo.
ex
Mendel’s Four Postulates, summarized
1. Unit factors (genes/alleles) exist in pairs
Genetic characters are controlled by unit factors existing in pairs in
individual organisms
Mendel’s Four Postulates, summarized
2. Dominance/recessiveness
When two unlike unit factors responsible for a single character
are present in a single individual, one unit factor is dominant to
the other, which is said to be recessive
Mendel’s Four Postulates, summarized
3. Random segregation
During the segregation of gametes, the paired unit factors
separate, or segregate, randomly so that each gamete receives
one or the other with equal likelihood
Mendel’s Four Postulates, summarized
4. Independent assortment
During gamete formation, segregating pairs of unit factors assort
independently of each other
yellow/green
round/wrinkled
How can you predict ratios when there are two
unlinked genes involved?
Monohybrid Cross
Dihybrid Cross
Dihybrid Cross – 2
Probability – Product Law
Trihybrid Cross
• Trihybrid cross
• Segregation and independent assortment applied to three pairs of
constraining traits
• Punnett square with 64 boxes
• Easier method:
Trihybrid Cross – 2
Forked Line Method
Probability and genetic analysis
is defined as the number of successful outcomes
expected divided by the number of attempts
• Probability is expressed as a value between 0 and 1
Laws of Probability
Calculates probability of outcomes occurring together
The “and” rule
Calculates probability of outcomes independent of each other
Simple probability
• Using a single six-sided die, what is the probability of rolling a 4?
• The die can land any of 6 ways, but only one of those will have a 4
facing upward. Thus, the probability of rolling a “4” is 1/6.
Mutually exclusive events
• Consider the probability of rolling a 4 and a 2 on one roll of a die.
• It can’t happen. If the die reads “4” it cannot read “2” at the same time.
• The events “4” and “2” are mutually exclusive events.
Independent events
• If a 4 is rolled on the first roll of a die, what is the probability that a 4
will be rolled on the second roll of the same die?
• The probability is still 1/6. The outcome of the first roll has had no
effect on the outcome of the second roll. The two events are
independent.
Mendel’s fourth postulate: (Independent
Assortment) The alleles of different genes
sort independently of each other into
gametes.
Two events at the same time
• Suppose we throw two dice. What is the probability that each of the
two dice lands on “6”?
• The
occurring at the same time is the
probabilities.
events
of their individual
The Additive (Sum) Rule
⮚ What is the probability of rolling either a 2 or a 5 on one roll of a single
die?
⮚ Since we will accept as a success EITHER a roll of a “2” OR a roll of
a “5”, we ADD the probability of a “2” to the probability of a “5” to get
the required probability.
Pedigrees
A genetic
family history
• Pedigree analysis
reveals patterns of
inheritance of human
traits
• For example, is it
due to a dominant or
recessive allele?
Pedigree Conventions (1 of 3)
• Pedigree conventions
• Circle = female
• Square = male
• Diamond = unknown sex
• Parents connected by single horizontal line
• Offspring stem off vertical line from parent
• Double line = related parents, such as two cousins
(“consanguineous”)
Pedigree Conventions (2 of 3)
• Twins
• Diagonal lines stemming from vertical line connected to
the sibship line
• Identical (monozygotic) twins
• Diagonal lines are linked by horizontal line
• Fraternal (dizygotic) twins
• Lack this connecting line
Pedigree Conventions (3 of 3)
• Proband
• The individual whose phenotype first brought attention to
the family
• Is indicated by an arrow connected to the designation p
Pedigree
Symbols
Example Pedigrees