Chapter 11: Mendelian Patterns of
Inheritance (Outline)
 Blending Inheritance
 Monohybrid Cross
 Law of Segregation
 Modern Genetics
 Genotype vs. Phenotype
 Punnett Square
 Dihybrid Cross
 Law of Independent Assortment
 Extending the Range of Mendelian Genetics
 Multiple Allelic Traits
 Incomplete Dominance & Incomplete Penetrance
 Polygenic Inheritance
Gregor Mendel
 Austrian monk
 Studied science and mathematics
at the University of Vienna
 Conducted breeding experiments
with the garden pea Pisum sativum
 Applied mathematics to biology; gathered
statistical data from his breeding experiments
 Formulated fundamental laws of heredity in
early 1860s
 Had no knowledge of cells or chromosomes
 Did not have a microscope
Blending Inheritance
 Theories of inheritance in Mendel’s time:
 Based on blending
 Parents of contrasting appearance produce offspring
of intermediate appearance
 Example – red and white flowered plants will produce
intermediate pink flowers
 Caused a major problem with Darwin's theory of
evolution by natural selection
 Mendel’s findings were in contrast with this
 He formulated the particulate theory of inheritance
 Inheritance involves reshuffling of genes from
generation to generation
Mendel’s Experimental Procedure
 Conducted breeding experiments with the
garden pea Pisum sativum (22 vaireites), why?
 Easy to cultivate
 Short generation time
 Although peas self-pollinate (true breeding),
they could be cross-pollinated by hand
 Mendel isolated true breeding strains of peas
with distinctive traits
 Mendel looked at only one trait at a time
 Example: Seed shape, flower color, height, etc.
Fruit and Flower of the
Garden Pea
Garden Pea Traits
Studied by Mendel
Monohybrid Cross
 Mendel's first experiments were monohybrid
crosses
 Monohybrid crosses have two parents that are
true-breeding for contrasting forms of a trait
 One form of the trait disappears in F1
generation, only to show up in F2 generation
 A 3:1 ratio among the F2 generation was
possible if the F1 parents contained 2 separate
copies of each hereditary factor
 These factors separate during gamete
formation, each containing only one copy
Mendel’s Monohybrid Crosses:
An Example
Mendel’s Monohybrid Crosses
Law of Segregation
 The law of segregation states that:
 Each individual has a two factors (alleles) for
each trait
 The factors (alleles) segregate (separate)
during the formation of gametes
 Each gamete contains only one factor (allele)
from each pair of factors
 Fertilization gives each new individual two
factors (alleles) for each trait
Modern Genetics View
 Each trait in a pea plant is controlled by two
alleles (i.e. alternate forms of a gene)
 Dominant allele (capital letter) masks the
expression of the recessive allele (lower-case)
 Alleles occur on a homologous pair of
chromosomes at a particular gene locus
 Homozygous = identical alleles
 Heterozygous = different alleles
 During meiosis I, bivalents separate; thus
explains the law of segregation and why one
allele for each trait is found in a gamete
Homologous Chromosomes
Genotype vs. Phenotype
 Genotype
 Refers to the two alleles an individual has for a
specific trait
 If identical, genotype is homozygous
 If different, genotype is heterozygous
 Phenotype
 Refers to the physical
appearance of the individual
 Example: TT or Tt (tall plant),
tt (short plant)
One-Trait Genetics Problems
 Basic steps to solving genetics problems
 Identify which allele is dominant & decide on
appropriate allele key (Use capital letters for
dominant traits, lower case for recessive
traits)
 Determine the genotype of both parents & the
various types of gametes for both parents
 Cross the male and female gametes
 Example: Unattached earlobes (E) are
dominant over attached earlobes (e)
Laws of Probability
 Mendel's laws are basically real-life applications of the
rules of probability that apply to a coin toss
 Rule of multiplication: segregation of the alleles into
gametes is like a coin toss (heads or tails = equal
probability)
 In the cross Ee X Ee, the child will inherit an allele from
each parent
 The probability of receiving these genotypes






1/2
1/2
1/2
1/2
x
x
x
x
1/2
1/2
1/2
1/2
=
=
=
=
1/4
1/4
1/4
1/4
chance
chance
chance
chance
of
of
of
of
having
having
having
having
child
child
child
child
with
with
with
with
EE
Ee
eE
ee
The chance of a child with unattached earlobes is ¾, or 75%
The chance of a child with unattached earlobes is 1/4, or 25%
Punnett Square
 Table listing all possible genotypes resulting
from a cross
 Introduced by geneticist R.C. Punnett as a simple
way to show probability of a genetic cross
 All possible sperm genotypes are lined up on one
side
 All possible egg genotypes are lined up on the
other side
 Every possible zygote genotypes are placed within
the squares
 In humans, phenotypic ratio is used to estimate the
chances any child has for a particular characteristic
Punnett Square Showing
Earlobe Inheritance Patterns
One-Trait Genetics Problems
 Set up a Punnett square using the possible
gametes of the P generation
 Complete the Punnett square and use this
information to answer the original question!!
 Always write ratios in the following format
 Genotypic ratio
 homozygous dominant: heterozygous: homozygous
recessive
 Phenotypic ratio
 dominant phenotype: recessive phenotype
 You may express probabilities in a percent (%)
or fraction
Monohybrid Testcross
 Individuals with recessive phenotype always have
the homozygous recessive genotype
 However, individuals with dominant phenotype
have indeterminate genotype
 May be homozygous dominant, or
 Heterozygous
 Test cross determines genotype of individual
having dominant phenotype
 Mendel crossed tall plant with true-breeding
short plant
 If heterozygous, the phenotypic ratio is 1:1
 If homozygous, all offspring are tall
One-Trait Testcross
Heterozygous or Homozygous
Monohybrid Cross Example
Problems

In pea plants, green pea color is dominant over yellow pea
color. Two plants that have heterozygous green peas are
crossed. What are the genotypic & phenotypic ratios of the
F1 generation offspring?

In mice, white color is dominant over black color. A white
mouse is crossed with a black mouse and 24 offspring are
produced, 18 of which are white and 6 black. What were the
genotypes of each of the parent mice?

The ability to roll the tongue is dominant over the inability
to do so in humans. A woman who can roll her tongue
marries a man who cannot. Their first child has his father's
phenotype. What are the genotypes of the mother, father
and child?
Mendel's Law of Independent
Assortment
 Mendel also performed testcrosses between plants
that differed in two traits (dihybrid)
 For example, a tall plant (TT) having green pods
(GG) color was cross-pollinated with a short plant
(tt) having yellow pods (gg)
 The progeny from the cross (the F1 generation)
were allowed to self-pollinate to generate the F2
generation

If the dominant factors (T and G) segregate during
meiosis together, progeny will all be tall with green pods

If the factors segregate separately, then four possible
phenotype could result
Two-Trait Testcross
Two-Trait Inheritance
 These results
demonstrated that
the two factors
segregated
independently
Two-Trait Inheritance
 Based upon these results, Mendel formulated
the law of independent assortment
 The pair of factors for one trait segregate
(assorts) independently of the factors for
other traits
 All possible combinations of factors can occur
in the gametes
 Example: Eye & hair color
Two-Trait Genetics Problems
 Because the fruit fly Drosophila melanogaster
has a variety of heritable mutations, this insect
has been used extensively in genetic research
 Allele that occurs most frequently in population
(normal) is known as wild type
 Two mutations displayed by this fly are wing
length and body color
 Wild type (normal) flies have long wings (L) and
gray bodies (G)
 Mutant flies can have short wings (l) and black
bodies (g)
Laws of Probability
 If two flies are heterozygous for both traits, what are the
probable results?
 Each trait is inherited separately from any other, thus it
is possible to use the rule of multiplication
 The F2 results for two separate monohybrid crosses are


Long wings = ¾, short wings = ¼
Gray body = ¾, black body = ¼
 The probability of receiving these genotypes




Chance
Chance
Chance
Chance
of
of
of
of
long wings & gray body = ¾ x ¾ = 9/16
long wings & black body = ¾ x ¼ = 3/16
short wings & gray body = ¼ x ¾ = 3/16
short wings & black body = ¼ x ¼ = 1/16
 The phenotypic ratio is 9:3:3:1
Two-Trait Testcross
 Fruit flies that have long wings and gray
bodies could be heterozygous or
homozygous for each trait
 When the genotype is in doubt, it can be
written as L__G__ to indicate that there is
one dominant allele but the other is
unknown
 A two-trait testcross can be used to
determine genotype of an L__G__
organism
Two-Trait Testcross
 In a two-trait testcross, a dominant L__G__ fly is
crossed with a true-breeding recessive fly of
known genotype (llgg)
 A heterozygous dominant fly would produce four
possible gametes
 LG
 Lg
 lG
 lg
 The homozygous recessive fly can only form
gametes containing lg
Two-Trait Testcross
 Since the homozygous
parent only contributes
lg, the other parent
determines the
phenotype of the
offspring
 Since the heterozygous
parent would provide
four gamete types, the
offspring should be
present in a 1:1:1:1
ratio
Two-Trait Testcross
 But if the parent is homozygous dominant
(LLGG), then the gametes would only
contain LG
 In this case, the testcross would produce
offspring that had only the dominant
phenotypes
 What would be the result if the test
individual was homozygous dominant for
one trait but heterozygous for the other?
Extending the Range of Mendelian
Genetics
 Mendel’s study of inheritance dealt with simple,
independently-segregating traits
 There are other complex patterns of
inheritance, such as
 Multiple Alleles
 Incomplete Dominance
 Polygenic Inheritence
Multiple Allelic Traits
 Some traits are controlled by multiple alleles
 The gene exists in several allelic forms
 For example, ABO blood types
 The alleles:
 IA = A antigen on red blood cells
 IB = B antigen on red blood cells
 i = Neither A nor B antigens on red blood cells
 Each person has only two of these possible
three alleles
Multiple Allelic Traits
 The combination of these alleles produce a
person’s blood type
 IA and IB alleles are dominant over i
 IAi and IAIA genotypes produce type A blood
 IBi and IBIB genotypes produce type B blood
 The ii genotype produces type O blood
 IA and IB alleles are codominant, meaning that
neither is dominant over the other
 The IAIB genotype produces type AB blood
Multiple Allelic Traits
Incomplete Dominance
 Heterozygote has phenotype intermediate
between that of either homozygote
 Phenotype reveals genotype without test cross
 Example: in four-o’clock flowers, a cross
between true-breeding parents with red or
white flowers yields F1 offspring with pink
flowers
 Homozygous red has red phenotype
 Homozygous white has white phenotype
 Heterozygote has pink (intermediate) phenotype
Incomplete Dominance (cont.)
 When F1 plants self-pollinate, the F2 generation
has a phenotypic ratio of 1 red-flowered: 2
pink-flowered: 1 white-flowered plant
 Each R1 allele produces pigment, but the R2
allele does not produce pigment
 In R1R1 individuals, a double dose of pigment result
in red flowers
 In R1R2 individuals, single dose of pigment result in
pink flowers
 R2R2 individuals do not produce pigment, thus flowers
are white
Incomplete Dominance (cont.)
 Incomplete
dominance – the
progeny show a
phenotype
intermediate to
the parents
Human Examples of Incomplete
Dominance
 In humans, familial hypercholesterolemia (FH)
is an example of incomplete dominance
 Individuals with two alleles (homozygous) for this
disorder develop fatty deposits in the skin and may
have heart attacks as children
 Individuals with one normal allele (heterozygous)
and one FH allele may suffer heart attacks as adults
 Individuals with two normal alleles (homozygous)
do not have this disorder
 Activity of the cystic fibrosis gene carrier
individuals
Incomplete Penetrance
 In some cases, a dominant allele may not
always lead to a dominant phenotype in a
heterozygote
 Example: polydactyly
 Polydactyly (extra fingers and/or toes) is an
autosomal dominant trait
 Expression of polydactyly may
require additional environmental
factors or be influenced by other
genes
Beyond Mendel’s Laws
 Mendel’s study of inheritance dealt with simple,
independently-segregating traits
 There are other patterns of inheritance other
than the dominance/recessive relationship
Mendel observed
 The environment can also influence the
phenotype of an organism
Mendelian Genetics Problems
 A rooster with grey feathers is mated with a hen of the
same phenotype. Among their offspring 15 chicks are
grey, 6 are black and 8 are white. What is the simplest
explanation for the inheritance of these colors in
chickens? What offspring would you expect from the
mating of a grey rooster and a black hen?
 The children of a man with Type A blood and a woman
with Type B blood were tested for their blood type. The
results were: one son Type A, one son Type B, one girl
Type AB and one girl Type O. What was the genotype of
the man and woman? Show your Punnett square.
Polygenic Inheritance
 Occurs when a trait is governed by two or
more sets of genes having different alleles
 Each dominant allele has a quantitative effect
on the phenotype; effects are additive
 Result in continuous variation of phenotypes
 Multifactorial traits are traits controlled by
polygenes (multiple genes) subject to
environmental influences
 Example: Melanin gene activity in Siamese cat
Human Examples of Multifactoral
Inheritance
 Human skin color is a polygenic trait affected
by the environment
 Skin color is determined by two or more genes
 The more dominant alleles a person has, the
darker their skin color
 Eye color is also a polygenic trait
 Amount of melanin deposited in the iris
increases the dark color of the eye
 The more dominant alleles a person has, the
darker their skin color
Frequency Distributions in
Polygenic Inheritance
Polygenic Human Disorders
 Genetic disorder resulting from the combined
action of alleles of more than one gene
 Often have major non-genetic influences (i.e.
environmental factors)
 There are several examples of polygenic
disorders
 Palate and lip disorders
 Hypertension and diabetes
 Allergies
 Some cancers
Environment and the Phenotype
 In some cases, the environment can affect
phenotype more than genetics
 In case of human height, nutrition is a major
factor
 For example, temperature can affect the color
of primroses and Himalayan rabbits