Download Biol 1408 : Chapter 9 Patterns of Inheritance

Biol 1408 : Chapter 9
Patterns of Inheritance
9.6 Geneticists can use the testcross to determine
unknown genotypes
§  A testcross can show whether the unknown
genotype includes a recessive allele.
§  A testcross is done by mating an individual of
unknown genotype and a homozygous recessive
individual.
§  Mendel used testcrosses to verify that he had truebreeding genotypes.
§  The following demonstrates how a testcross can
be performed to determine the genotype of a Lab
with normal eyes.
© 2012 Pearson Education, Inc.
9.6 Geneticists can use the testcross to determine
unknown genotypes
§  For example, Labrador’s have either a black coat
color or a chocolate coat color
§  With respect to coat color, we know that black is
dominant (B) and chocolate is recessive (b). So,
§  A black lab ,the phenotype, is genotypically BB or Bb
§  But a chocolate lab phenotype must be genotypically bb ( thus
not cc, since we have to use the same letters)
© 2012 Pearson Education, Inc.
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9.6 Geneticists can use the testcross to determine
unknown genotypes
§  If we want to know if a black lab is of the BB or Bb
variety, we can cross it with a homozygous
recessive trait ( bb , the brown lab)
§  If the black lab was BB and we cross it with a bb
lab, the outcome should be 100 % Bb labs ….. Or
100 % black labs
§  But if the black lab was of the Bb variety, it can
produce gametes with either B or b traits. Mating
with a bb individual would thus produce 50% Bb
(black) and 50 % bb (brown) labs
© 2012 Pearson Education, Inc.
9.6 Geneticists can use the testcross to determine
unknown genotypes
What is the genotype of the black dog?
Testcross
×
Genotypes
B?
bb
Two possibilities for the black dog:
BB
Gametes
Chocolate lab only
makes b gametes
Offspring
Bb
or
B
b
Bb
All black
b
B
b
Bb
bb
Black lab makes
either B or
B and b gametes
1 black : 1 chocolate
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
§  The inheritance of human traits follows Mendel’s
laws.
§  A pedigree
–  shows the inheritance of a trait in a family through
multiple generations,
–  demonstrates dominant or recessive inheritance, and
–  can also be used to deduce genotypes of family
members
© 2012 Pearson Education, Inc.
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Human genetics
§  Mendel’s law apply to the inheritance of human
traits as well.
§  In a simple dominant-recessive inheritance of
dominant allele A and recessive allele a,
–  a recessive phenotype always results from a
homozygous recessive genotype (aa) but
–  a dominant phenotype can result from either
–  the homozygous dominant genotype (AA) or
–  a heterozygous genotype (Aa).
© 2012 Pearson Education, Inc.
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
Kristin and Lori are sisters and have a different hairline
Kristin
Straight hairline
Lori
Widow s peak
Let s assign the allele letter for hairline characteristics H or h.
Which of these two traits is dominant and what are the
genotypes for this allele in these sisters ? HH, Hh or hh ?
Analyzing a pedigree can help us find the answer.
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
If two parents show the same characteristic, and one of their children
shows the opposite characteristic, then the parents characteristic must
be dominant and heterozygous !
Assume the opposite . If both parents have the same characteristic
and it is recessive ?
hh
x
hh
the offspring must all be hh
So the characteristic of the parents must be dominant. What is one is
homozygous dominant and the other heterozygous ?
HH
x
Hh
the offspring will be HH or Hh
Thus all off spring will show the dominant characteristic. Hence, the only
way the offspring shows a recessive trait if bot parents are heterozygous
for that trait.
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Figure 9.8-2
KEY Female Male
Mating
Al
Widow s peak hairline trait
Straight hairline trait
1ST
GENERATION
Beth
Charles
Debbie
Parents with same trait but with children having opposite trait
Children
Evelyn
Frank
Gary
2ND
GENERATION
Henry
Straight hairline
Isabel
Figure 9.8-3
Widow s peak
3RD
GENERATION
Kristin
Juliana
Lori
KEY Female Male
Widow s peak hairline trait
Straight hairline trait
H: widow s peak allele
h: straight allele
1ST
GENERATION
Al
Beth
Charles
Debbie
Hh
Hh
hh
Hh
2ND
GENERATION
Evelyn
Frank
Gary
Henry
hh
hh
Hh
Straight hairline
A parent with a widow s
peak who has a child
with a straight hairline
must be Hh.
Isabel
Juliana
Hh
hh
3RD
GENERATION
Kristin
hh
Widow s peak
Kristin has a straight
Lori
hairline but her parents
do not, so straight hairline
must be homozygous
recessive (hh).
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
Kristin has a straight hairline but
her parents do not, so straight
hairline must be homozygous
recessive (hh).
Not all genotypes can be determined.
Lori could be HH or Hh and there is no
way to know (unless she has some
children and the pedigree is extended).
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Human genetics
§  Although dominant traits rule over recessive
traits, dominant traits are not always the most
common trait in nature
§  Wild-type traits is the term used for traits
prevailing (common) in nature ( and are thus not
necessarily specified by dominant alleles).
© 2012 Pearson Education, Inc.
Human genetics
§  Freckles are an example that dominant alleles are not
always dominating in nature.
§  While freckles is a
governed by a
dominant allele, the
absence of freckles
is more common in
our societies
§  Thus the genotype FF and Ff, which produce the
phenotype of having freckles, is less common than
the genotype ff (not having freckles).
9.8 Genetic traits and human family pedigrees
§  Example : having freckles
§  Assume two parents that have freckles, and their child has
no freckles.
§  Let s also assume No freckles is dominant and designate
N for the trait of NOfreckles
§  The genotype of the child can thus be NN or Nn
§  Mom and Dad have freckles (assumed to be recessive).
That means, the only way that could be if they both are
nn.
The next slide shows this in diagram form.
© 2012 Pearson Education, Inc.
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9.8 Genetic traits and human family pedigrees
MOM (freckled) x DAD (freckled)
nn
x
nn
Child (Not freckled)
genotype =
NN or Nn
We assumed Not freckled is
dominant; that automatically
makes having freckles is
recessive
This diagram shows that the
assumption of having freckles
being recessive doesn t work.
The genotype of the child
cannot be explained using the
genotypes of the parents.
No way Mom and Dad can produce a child with NN or Nn
genotype if having freckles is recessive.
Thus having freckles is dominant and no freckles is recessive.
© 2012 Pearson Education, Inc.
9.8 Genetic traits and human family pedigrees
§  Thus freckles is dominant and the child with freckles means, the
child is ff
§  What are Mom and Dad in terms of genotype if they have freckles ?
§  Mom and Dad must both be Ff, because that is the only way we can
have a child with ff
MOM (freckled) x DAD (freckled)
Possible offspring ?
FF
x
FF
FF and Ff
or
FF
x
Ff
FF and Ff
or
Ff
x
Ff
FF , Ff and ff
Child (Not freckled)
genotype =
ff
© 2012 Pearson Education, Inc.
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
§  The use of human pedigree is important to trace
certain alleles within families.
–  Demonstration of the presence of dominant or
recessive alleles in plants and animals can be done by
using test-crosses
–  Not something that can be done with ease on animals
with small litters and long pregnancies
–  Not an option for humans to test-cross individuals just
for the sake of genetics.
© 2012 Pearson Education, Inc.
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Human Genetic traits
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow s peak
Straight hairline
Free earlobe
Attached earlobe
§  The following are some
examples of human traits with
dominant and recessive forms.
§  All of these are minor variations
with no major side effects, but do
obey Mendel s law of genetics
Human Genetic traits
Tongue rolling is dominant
Hitchkiker s thumb is as well
Bent pinky ( dominant) vs straight pinky
Dimples in cheek ( dominant) vs no dimples
Curly hair ( dominant) vs straight hair
Long eyelashes ( dominant) vs short eyelashes
Left thumb over right thumb when interlacing fingers is dominant
Hair on your middle digits of your fingers is dominant
9.8 Genetic traits and human family pedigrees
§  Due to the fact that recessive traits only show
up when the individual is homozygous for the
recessive allele, we can figure out who in the
family tree is the carrier.
§  For example , when two people, who have a
certain trait, have a child that does not have that
trait, one may assume that the trait of the child is
recessive.
§  Reason ? The only way to have gotten the trait IF
both parents have at least one copy of that
recessive allele.
© 2012 Pearson Education, Inc.
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9.8 Genetic traits and human family pedigrees
§  For Example : the characteristic of having attached
or free earlobes
§  Which trait is dominant ? Let’s look at a pedigree !
© 2012 Pearson Education, Inc.
9.8 Genetic traits and human family pedigrees :
Earlobes example
First generation
(grandparents)
Second generation
(parents, aunts,
FF
and uncles)
or
Ff
Third generation
(two sisters)
Female
Ff
Ff
Male
Attached
Free
ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Similar reasoning on this female child,
tells us that attached earlobes is
recessive (her parents had free
earlobes). Thus parents must be Ff.
Parents cannot be FF and Ff or both
be FF !
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
First generation
(grandparents)
One of the parents had
a grandparent and
sibling with attached
earlobes.
Ff
Second generation
(parents, aunts,
FF
and uncles)
or
Ff
Third generation
(two sisters)
Female
Male
Attached
Free
ff
Ff
ff
ff
Ff
Ff
ff
FF
or
Ff
Ff
Thus the other
grandparent must have
been Ff as well.
ff
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9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
§  Inherited human disorders show either
1.  recessive inheritance in which
–  two recessive alleles are needed to show disease,
–  heterozygous parents are the called carriers of the
disease-causing allele, and
–  the probability of inheritance increases with inbreeding,
mating between close relatives.
2.  dominant inheritance in which
–  one dominant allele is needed to show disease and
–  dominant lethal alleles are usually eliminated from the
population.
© 2012 Pearson Education, Inc.
Figure 9.9A
Normal
Dd
Normal
Dd
×
Parents
D
Sperm
This shows an example of a
recessive form of deafness.
Normal hearing is thus the
dominant allele, deafness due
to a mutation in the gene.
Only people with both mutated
genes become deaf (dd).
D
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Offspring
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
§  The most common fatal genetic disease in the United States
is cystic fibrosis (CF), resulting in excessive thick mucus
secretions. The CF allele is
–  recessive and an indiviudal must be homozygous recessive (aa) to
get the disorder
–  carried by about 1 in 31 Americans. ( they are heterozygous (Aa )
§  Dominant human disorders include (so, if an individual has
the dominant allele , they will get the disorder ( AA or Aa)
–  achondroplasia, resulting in dwarfism, and
–  Huntington’s disease, a degenerative disorder of the nervous
system.
© 2012 Pearson Education, Inc.
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9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
Dr. Michael C. Ain, a specialist in the repair of
bone defects caused by achondroplasia and
related disorders.
He himself has achondroplasia.
So his genotype is either AA or Aa for this trait. The only way he could have gotten this if either
both his parents had the disorder or one parent
did not and the other did
Parents : AA x AA Kids : all AA
Parents : AA x Aa Kids : 2 AA, 2 Aa
Parents : AA x aa Kids : all Aa
Parents : Aa x Aa Kids : 1 AA, 2 Aa, 1 aa
Parents : Aa x aa Kids : 2 Aa, 2 aa
Parents : aa x aa Kids : all aa
Those without the disorder in Blue
Table 9.9
9.10 CONNECTION: New technologies can
provide insight into one s genetic legacy
§  New technologies offer ways to obtain genetic
information
–  before conception,
–  during pregnancy, and
–  after birth.
§  Genetic testing can identify potential parents who
are heterozygous carriers for certain diseases.
© 2012 Pearson Education, Inc.
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9.10 CONNECTION: New technologies can
provide insight into one s genetic legacy
§  Several technologies can be used for detecting
genetic conditions in a fetus.
–  Amniocentesis extracts samples of amniotic fluid
containing fetal cells and permits
–  karyotyping and
–  biochemical tests on cultured fetal cells to detect other
conditions, such as Tay-Sachs disease.
–  Chorionic villus sampling removes a sample of
chorionic villus tissue from the placenta and permits
similar karyotyping and biochemical tests.
© 2012 Pearson Education, Inc.
Figure 9.10A
Amniocentesis
Chorionic Villus Sampling (CVS)
Amniotic fluid
extracted
Ultrasound
transducer
Fetus
Fetus
Placenta
Chorionic
villi
Placenta
Uterus
Cervix
Centrifugation
Amniotic fluid
Fetal cells
Several
hours
Cultured
cells
Tissue extracted
from the
chorionic villi
Ultrasound
transducer
Several
weeks
Several
weeks
Biochemical
and genetics
tests
Cervix
Uterus
Fetal cells
Several
hours
Several
hours
Karyotyping
9.10 CONNECTION: New technologies can
provide insight into one s genetic legacy
§  Blood tests on the mother at 14–20 weeks of
pregnancy can help identify fetuses at risk for
certain birth defects.
§  Fetal imaging enables a physician to examine a
fetus directly for anatomical deformities. The most
common procedure is ultrasound imaging, using
sound waves to produce a picture of the fetus.
§  Newborn screening can detect diseases that can
be prevented by special care and precautions.
© 2012 Pearson Education, Inc.
11
VARIATIONS ON MENDEL S LAWS
9.11 Incomplete
dominance results
in intermediate
phenotypes
§  Mendel’s pea crosses always
looked like one of the two
parental varieties, a situation
called complete dominance.
§  For some characters, the
appearance of F1 hybrids falls
between the phenotypes of
the two parental varieties.
This is called incomplete
dominance.
P generation
Red
RR
White
rr
Gametes
r
R
F1 generation
Pink hybrid
Rr
Gametes
1
2
1
2
R
F2 generation
r
Sperm
1
2
1
2
R
1
2
r
R
RR
rR
r
Rr
rr
Eggs
1
2
9.11 Incomplete dominance results in intermediate
phenotypes
•  One example of incomplete dominance in humans is
hypercholesterolemia, in which genes code for cholesterol (LDL)
receptors
•  dangerously high levels of cholesterol occur in the blood and
•  heterozygotes have intermediately high cholesterol levels.
HH
Homozygous
for ability to make
LDL receptors
Genotypes
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
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9.12 Many genes have more than two alleles in the
population
§  Although each individual carries, at most, two different alleles
for a particular gene, in cases of multiple alleles, more than
two possible alleles exist in a population.
§  For example, a certain trait has 3 flavors, A, B and C floating
around in a population
§  So, possible combinations for alleles in an individual could be
–  AB or AC or BC ( remember that only two alleles can
occur at same time)
–  Think like socks on your feet ( you have many pair but
only two socks fit on your feet at any time…. Many
combinations possible)
9.12 Many genes have more than two alleles in the
population
§  Human ABO blood group phenotypes involve three
alleles for a single gene.
§  The four human blood groups, A, B, AB, and O,
result from combinations of these three alleles.
§  The A and B alleles are both expressed in
heterozygous individuals, making both alleles
codominant.
§  The alleles code for carbohydrate groups on red
blood cells
9.12 Many genes have more than two alleles in the
population
§  The alleles are coded as follow
–  Allele IA codes for carbohydrates A on RBC
–  Allele IB codes for carbohydrates B on RBC
–  Allele IO codes for no A or B carbohydrates on RBC
( sometimes referred to as allele i )
§  So what are the possible combinations ?
13
Blood
Carbohydrates Present
Group
(Phenotype) Genotypes on Red Blood Cells
A
IAIA
or
IAi
Carbohydrate A
B
IBIB
or
IBi
Carbohydrate B
AB
IAIB
O
ii
Carbohydrate A
and
Carbohydrate B
Neither
9.12 Many genes have more than two alleles in the
population
§  Modern Paternal testing uses DNA analysis, but this
knowledge of blood groups makes for some quick
and easy determinations.
§  For example, a woman claims a man is the father of
her baby. She is AB blood type and the baby is
blood type A. He claims he is blood type O and thus
cannot be the Dad. True or False ?
9.12 Many genes have more than two alleles in the
population
§  MOM = AB = IAIB
§  DAD = O = ii
§  So possible genotypes (phenotypes) of children
IA
IB
i
IAi
IBi
i
IAi
IBi
Children will have either A or B blood type.
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9.13 A single gene may affect many phenotypic
characters
§  Pleiotropy occurs when one gene influences
multiple characters.
§  Sickle-cell disease is a human example of
pleiotropy.
–  This disease affects the type of hemoglobin produced
and the shape of red blood cells and causes anemia
and organ damage.
–  Sickle-cell and nonsickle alleles are codominant.
–  Carriers of sickle-cell disease have increased
resistance to malaria.
An individual homozygous for the sickle-cell allele
Produces sickle-cell (abnormal) hemoglobin
The abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Damage to organs
Kidney failure
Heart failure
Spleen damage
Brain damage (impaired
mental function, paralysis)
Other effects
Pain and fever
Joint problems
Physical weakness
Anemia
Pneumonia and other
infections
9.14 A single character may be influenced by many
genes
•  Many characters result from polygenic
inheritance, in which a single phenotypic
character results from the additive effects of two
or more genes on a single phenotypic character.
•  Human skin color is an example of polygenic
inheritance.
© 2015 Pearson Education, Inc.
15
P generation
aabbcc
(very light)
9.14 A single character
may be influenced by
many genes
AABBCC
(very dark)
F1 generation
AaBbCc
(medium
shade)
AaBbCc
(medium
shade)
Sperm
1
8
F2 generation
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
1
8
20
64
Eggs
Fraction of population
1
8
1
8
1
8
1
8
1
8
1
8
1
6
64
64
© 2015 Pearson Education, Inc.
15
64
20
64
15
64
6
64
1
64
15
64
6
64
1
64
Skin color
9.15 The environment affects many characters
§  Many characters result from a combination of heredity and the
environment. For example,
–  skin color is affected by exposure to sunlight and
–  heart disease and cancer are influenced by genes and the
environment.
§  Identical twins show that a
person s traits are the results of
–  genetics and
–  the environment.
9.20 Chromosomes determine sex in many species
§  Many animals have a pair of sex chromosomes,
designated X and Y, that determine an individual’s
sex.
§  Among humans and other mammals,
–  individuals with one X chromosome and one Y
chromosome are males, and
–  XX individuals are females.
§  In addition, human males and females both have
44 autosomes (nonsex chromosomes).
16
9.20 Chromosomes determine sex in many species
§  In mammals (including humans),
–  the Y chromosome has a crucial gene, SRY, for the
development of testes, and
–  an absence of the SRY gene directs ovaries to develop.
X
Y
9.20 Chromosomes determine sex in many species
§  In certain fishes, butterflies, and birds, the sex
chromosomes are Z and W,
–  males are ZZ, and females are ZW.
§  Some organisms lack sex chromosomes altogether.
§  In most ants and bees, sex is
determined by chromosome number.
–  Females develop from fertilized eggs and thus are diploid.
–  Males develop from unfertilized
Males are thus
–  fatherless and haploid.
eggs.
Chromosome
number
determines sex
9.20 Chromosomes determine sex in many species
§  In some animals, environmental temperature
determines the sex.
–  For some species of reptiles, the temperature at which
the eggs are incubated during a specific period of
embryonic development determines whether the
embryo will develop into a male or female.
–  Global climate change may therefore impact the sex
ratio of such species.
17
9.21 Sex-linked genes exhibit a unique pattern of
inheritance
§  Sex-linked genes are located on either of the sex
chromosomes.
§  The X chromosome carries many genes unrelated
to sex.
§  Most sex-linked human disorders are
–  due to recessive alleles and
–  seen mostly in males
9.22 CONNECTION: Human sex-linked disorders
affect mostly males
§  A male receiving a single X-linked recessive allele from his
mother will have the disorder.
§  A female must receive the allele from both parents to be
affected.
§  Recessive and sex-linked human disorders include
–  hemophilia, characterized by excessive bleeding because
hemophiliacs lack one or more of the proteins required for blood
clotting,
–  red-green colorblindness, a malfunction of light-sensitive cells in
the eyes, and
–  Duchenne muscular dystrophy, a condition characterized by a
progressive weakening of the muscles and loss of coordination.
9.22 CONNECTION: Human sex-linked disorders
affect mostly males
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Female Male
Hemophilia
Carrier
Alexis
Normal
Pedigree of Hemophelia in Royal Russian family
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9.23 EVOLUTION CONNECTION: The Y
chromosome provides clues about human male
evolution
§  The Y chromosome provides clues about human
male evolution because
–  Y chromosomes are passed intact from father to son
and
–  mutations in Y chromosomes can reveal data about
recent shared ancestry.
§  In 2003, geneticists discovered that about 8% of
males currently living in central Asia have Y
chromosomes of striking genetic similarity.
9.23 EVOLUTION CONNECTION: The Y
chromosome provides clues about human male
evolution
§  Further analysis traced their common genetic
heritage to a single man living about 1,000 years
ago.
§  In combination with historical
records, the data led to the
speculation that the Mongolian
ruler Genghis Kahn may be
responsible for the spread of the
telltale chromosome to nearly 16
million men living today
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