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Patterns of Inheritance Chapter 12 1
Patterns of Inheritance
Chapter 12
1
2
Gregor Mendel
Chose to study pea plants because:
1. Other research showed that pea hybrids
could be produced
2. Many pea varieties were available
3. Peas are small plants and easy to grow
4. Peas can self-fertilize or be cross-
fertilized
3
4
Mendel’s experimental method
• Usually 3 stages
1. Produce true-breeding strains for each trait he was studying
2. Cross-fertilize true-breeding strains having alternate forms of a trait
– Also perform reciprocal crosses
3. Allow the hybrid offspring to self-fertilize for several generations and count the number of offspring showing each form of the trait
5
Stigma
Style
Anthers (male) 1. The anthers are
cut away on the
purple flower.
Petals
Carpel (female)
4. All progeny
result in purple
lowers.
3. Pollen is
transferred to
the purple flower.
2. Pollen is obtained
from the white
flower.
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6
Monohybrid crosses
• Cross to study only 2 variations of a single
trait
• Mendel produced true-breeding pea
strains for 7 different traits
– Each trait had 2 variants
7
F1 generation
• First filial generation
• Offspring produced by crossing 2 true-
breeding strains
• For every trait Mendel studied, all F1
plants resembled only 1 parent
– Referred to this trait as dominant
– Alternative trait was recessive
• No plants with characteristics intermediate
between the 2 parents were produced
Learning Log
Entry #60
• A tall (Tt), purple flowered (Pp) plant is
crossed with a dwarf (tt), white (pp) plant.
What would the genotypic and phenotypic
ratio be of the F1 generation?
• What proportion of individuals be hybrids?
8
9
F2 generation
• Second filial generation
• Offspring resulting from the self-fertilization of F1 plants
• Although hidden in the F1 generation, the
recessive trait had reappeared among
some F2 individuals
• Counted proportions of traits
– Always found about 3:1 ratio
10
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Parent generation
Self-cross Self-cross Self-cross Self-cross
Cross-fertilize
Self-cross
True-
breeding
Purple
Parent
True-
breeding
White
Parent
Purple
Offspring
F1 generation
F2 generation
(3:1 phenotypic
ratio)
F3 generation
(1:2:1 genotypic
ratio)
Purple
Dominant
Purple
Dominant
Purple
Dominant
White
Recessive
True-
breeding
Non-true-
breeding
Non-true-
breeding
True-
breeding
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Five-element model
1. Parents transmit discrete factors (genes)
2. Each individual receives one copy of a
gene from each parent
3. Not all copies of a gene are identical
– Allele – alternative form of a gene
– Homozygous – 2 of the same allele
– Heterozygous – different alleles
4. Alleles remain discrete – no blending
5. Presence of allele does not guarantee
expression
– Dominant allele – expressed
– Recessive allele – hidden by dominant allele
• Genotype – total set of alleles an
individual contains
• Phenotype – physical appearance
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13
Principle of Segregation
• Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization
• Physical basis for allele segregation is the
behavior of chromosomes during meiosis
• Mendel had no knowledge of
chromosomes or meiosis – had not yet
been described
14
Human traits
• Some human traits are controlled by a single gene
– Some of these exhibit dominant and recessive inheritance
• Pedigree analysis is used to track inheritance patterns in families
• Dominant pedigree – juvenile glaucoma
– Disease causes degeneration of optic nerve leading to blindness
– Dominant trait appears in every generation
15
16
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21
2 3 4 51
21
Dominant Pedigree
Generation I
Generation II
Generation III
Key
affected female
affected male
unaffected female
unaffected male
3
• Recessive pedigree – albinism
– Condition in which the pigment melanin is not
produced
– Pedigree for form of albinism due to a
nonfunctional allele of the enzyme tyrosinase
– Males and females affected equally
– Most affected individuals have unaffected
parents
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18
1 2
1 2
1 2
3
3
1 2 3
4
4
5
5 6 7
Recessive Pedigree
Generation I
Generation II
Generation III
Generation IV
Heterozygous
Homozygous recessive
Key
male carrier
female carrieraffected female
affected male
unaffected female
unaffected male
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One of these persons
is heterozygous
Mating between
first cousins
Learning Log
Entry #60
Complete #4 on the pedigree practice that
we were working on yesterday.
19
Learning Log – 12/1/16
Entry #61
Hemophilia is a sex-linked trait. What does
it mean to be “sex-linked”? What is
hemophilia and who does it affect more
(males or females)? Explain.
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21
Sex Chromosomes
• Sex determination in Drosophila is based on the
number of X chromosomes
– 2 X chromosomes = female
– 1 X and 1 Y chromosome = male
• Sex determination in humans is based on the
presence of a Y chromosome
– 2 X chromosomes = female
– Having a Y chromosome (XY) = male
• Humans have 46 total chromosomes
– 22 pairs are autosomes
– 1 pair of sex chromosomes
– Y chromosome highly condensed• Recessive alleles on male’s X have no active
counterpart on Y
– “Default” for humans is female• Requires SRY gene on Y for “maleness”
22
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X chromosome
Y chromosome
© BioPhoto Associates/Photo Researchers, Inc.
Y chromosome
X chromosome
35,000 ×
Sex Linkage
• Certain genetic diseases affect males to a
greater degree than females
• X-linked recessive alleles
– Red-green color blindness
– Hemophilia
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Hemophilia
• Disease that affects a single protein in a
cascade of proteins involved in the formation of
blood clots
• Form of hemophilia is caused by an X-linked
recessive allele
– Heterozygous females are asymptomatic carriers
• Allele for hemophilia was introduced into a
number of different European royal families by
Queen Victoria of England
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25
The Royal Hemophilia Pedigree
Generation
I
II
III
IV
V
VI
VII
George III
Edward
Duke of Kent
Prince Albert Queen Victoria
Louis II
Grand Duke of Hesse
Prince
Henry
BeatriceLeopoldFrederick Victoria
No hemophilia
III
German
Royal
House
Duke of
Windsor
King
George VI
Earl of
Mountbatten
Waldemar
Queen
Elizabeth II
Princess
Diana
William Henry
British Royal House
Prince
Charles
Anne Andrew EdwardSpanish Royal House
No evidence
of hemophilia
No evidence
of hemophilia
KingJuan
Carlos
Alfonso
King of
Spain
Gonzalo
?
JuanJamie
?
Alfonso
Queen
Eugenie
Leopold
Russian
Royal
House
Prussian
Royal
House
Prince
Sigismond
Henry
?
Anastasia
??
Viscount
Tremation
??
Prince
Philip
Margaret
King
Edward VII
King
George V
Alice Duke of
Hesse
MauricePrincess
Alice
Earl of
Athlone
Alexis
Czar
Nicholas II
Irene Czarina
Alexandra
No hemophilia
ArthurHelenaAlfred
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26
Dihybrid crosses
• Examination of 2 separate traits in a single
cross
• Produced true-breeding lines for 2 traits
• RRYY x rryy
• The F1 generation of a dihybrid cross
(RrYy) shows only the dominant
phenotypes for each trait
• Allow F1 to self-fertilize to produce F2
27
F1 self-fertilizes
• RrYy x RrYy
• The F2 generation shows all four possible
phenotypes in a set ratio
– 9:3:3:1
– R_Y_:R_yy:rrY_:rryy
– Round yellow:round green:wrinkled
yellow:wrinkled green
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Principle of independent assortment
• In a dihybrid cross, the alleles of each
gene assort independently
• The segregation of different allele pairs is
independent
• Independent alignment of different
homologous chromosome pairs during
metaphase I leads to the independent
segregation of the different allele pairs
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Testcross
• Cross used to determine the genotype of an
individual with dominant phenotype
• Cross the individual with unknown genotype
(e.g. P_) with a homozygous recessive (pp)
• Phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent
30
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P
p
P P
p
p
Heterozygous
dominant
Homozygous
recessive
Alternative 2:
Half of the offspring are white and the unknown
flower is heterozygous (Pp)
PP or Pp
then
If Pp
Dominant
Phenotype
(unknown
genotype)
If PP
then
Alternative 1:
All offspring are purple and the unknown
flower is homozygous dominant (PP)
Homozygous
recessive
Homozygous
dominant
PpPp Pp pp
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Extensions to Mendel
• Mendel’s model of inheritance assumes
that
– Each trait is controlled by a single gene
– Each gene has only 2 alleles
– There is a clear dominant-recessive
relationship between the alleles
• Most genes do not meet these criteria
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Polygenic inheritance
• Occurs when multiple genes are involved in controlling the phenotype of a trait
• The phenotype is an accumulation of contributions by multiple genes
• These traits show continuous variation and are referred to as quantitative traits
– For example – human height
– Histogram shows normal distribution
33
30
20
10
00 5′6″ '6′0″5′0″
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Nu
mb
er
of
Ind
ivid
ua
ls
(top): From Albert F. Blakeslee, “CORN AND MEN: The Interacting Infl uence of Heredity and Environment—Movements for
Betterment of Men, or Corn, or Any Other Living Thing, One-sided Unless Th ey Take Both Factors into Account,” Journal of
Heredity, 1914, 5:511-8, by permission of Oxford University Press
Height
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Pleiotropy
• Refers to an allele which has more than
one effect on the phenotype
• Pleiotropic effects are difficult to predict,
because a gene that affects one trait often
performs other, unknown functions
• This can be seen in human diseases such
as cystic fibrosis or sickle cell anemia
– Multiple symptoms can be traced back to one
defective allele
35
Multiple alleles
• May be more than 2 alleles for a gene in a
population
• ABO blood types in humans
– 3 alleles
• Each individual can only have 2 alleles
• Number of alleles possible for any gene is
constrained, but usually more than two
alleles exist for any gene in an
outbreeding population
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• Incomplete dominance
– Heterozygote is intermediate in phenotype
between the 2 homozygotes
– Red flowers x white flowers = pink flowers
• Codominance
– Heterozygote shows some aspect of the
phenotypes of both homozygotes
– Type AB blood
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Parent generation
1 : 2 : 1
CR CW
Cross-fertilization
CWCWCRCR
F1 generation
CRCW
CRCWCRCR
CR
CW
CRCW CWCW
CRCR: CRCW: CWCW
F2 generation
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38
Human ABO blood group
• The system demonstrates both
– Multiple alleles
• 3 alleles of the I gene (IA, IB, and i)
– Codominance
• IA and IB are dominant to i but codominant to each
other
39
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Alleles
AB
NoneO
GalactosamineA
GalactoseB
Blood
Type
Sugars
Exhibited
Donates and
Receives
Receives A and O
Donates to A and AB
Receives B and O
Donates to B and AB
Universal receiver
Donates to AB
Receives O
Universal donor
Both galactose and
galactosamine
IAIA, IAi
(IA dominant to i)
IBIB, IBi
(IB dominant to i)
IAIB
(codominant)
ii
(i is recessive)
Environmental influence
• Coat color in
Himalayan
rabbits and
Siamese cats
– Allele
produces an
enzyme that
allows
pigment
production
only at
temperatures
below 30oC 40
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© DK Limited/Corbis
Temperaturebelow
33º C, tyrosinase
active, dark pigment
Temperature above
33º C, tyrosinase
inactive, no pigment
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Epistasis
• Behavior of gene products can change the
ratio expected by independent assortment,
even if the genes are on different
chromosomes that do exhibit independent
assortment
• R.A. Emerson crossed 2 white varieties of
corn
– F1 was all purple
– F2 was 9 purple:7 white – not expected
42
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AB Ab aB ab
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
9/16 Purple: 7/16 White
AB
Ab
aB
ab
Cross-fertilization
a.
b.
Parental
generation
F1 generation
F2 generation
Pigment
(purple)
Enzyme
B
Enzyme
APrecursor
(colorless)
Intermediate
(colorless)
White
(aaBB)
White
(AAbb)
All Purple
(AaBb)
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Gene Linkage
• Any two genes that are found on the same
chromosome are said to be linked
• A group of genes inherited together
because they are found on the same
chromosome are said to be a linkage
group
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Gene Linkage Notation
46
• In fruit flies –
G = allele for grey body
g = allele for black body
L = allele for long wings
l = allele for short wings
• Cross a grey, long wing fly (heterozygous)
with a black, short wing fly
– What is the expected vs. actual
