Chapter 9 ppt

Purebreds and Mutts–A Difference of Heredity
• Purebred dogs
– Variation?
– Selective breeding?

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings

• Mutts, or mixed breed dogs on the other hand

– Genetic variation? More? …less? Why?


• Modern Experimental Genetics

– Gregor Mendel’s quantitative experiments
with pea plants

Petal

Stamen
Carpel

Figure 9.2 A Figure 9.2 B


• Mendel crossed? ..bred? …pea plants that
differed in certain characteristics
• WHY?

– And traced traits from generation to
generation

WHY?


Flower color Purple White

• Mendel hypothesized that there are alternative
forms of genes
– The units that determine heritable traits
Flower position Axial Terminal

Seed color Yellow Green

Seed shape Round Wrinkled

Pod shape Inflated Constricted

Pod color Green Yellow

Stem length Tall Dwarf


Mendel’s Law’s:
P generation
(true-breeding
parents)

×

2)Dominance Purple flowers White flowers

3)Segregation From his experimental data
F1 generation All plants have
purple flowers

– Mendel deduced that an organism has two
genes (alleles) for each inherited characteristic
Fertilization
among F1 plants
(F1 × F1)

F2 generation

3 1
4 4
of plants of plants
have purple flowers have white flowers


• For each characteristic

– An organism inherits two alleles, one from
each parent
– Hmmm…does this remind you of anything
we studied? …what? Be Specific!


P plants Genetic makeup (alleles)

PP pp

• Mendel’s law of segregation
Gametes

– Predicts that allele pairs separate from
All P All p

each other during the production of
gametes
F1 plants
(hybrids) All Pp

1 1
2 P 2 p
Gametes

Sperm
P p

F2 plants Phenotypic ratio
3 purple : 1 white
P PP Pp

Eggs
Genotypic ratio
1 PP : 2 Pp: 1 pp
p Pp pp

Figure 9.3 B

Recal….Homologous chromosomes bear the two
alleles for each characteristic
• Alternative forms of a gene

– Reside at the same locus on homologous
chromosomes Gene loci
Dominant
allele

P a B

P a b
Recessive
allele
Genotype: PP aa Bb
Homozygous Homozygous Heterozygous
for the for the
dominant allele recessive allele


Hypothesis: Dependent assortment Hypothesis: Independent assortment

P generation RRYY rryy RRYY rryy
• Mendel’s law of independent assortment

– States that alleles of a pair segregate independently
Gametes RY ry Gametes RY × ry

of other allele pairs during gamete formation
RrYy RrYy
F1 generation

Sperm Sperm

1 1 1 1
RY ry RY ry
1 1 4 4 4 4
2 RY 2 ry

1
1 4 RY
F2 generation 2 RY RRYY RrYY RRYy RrYy

Eggs 1
4 ry
1
2 ry RrYY rrYY RrYy rrYy
Eggs
Yellow
1 9
round
4 Ry 16
RRYy RrYy RRyy Rryy Green
3
round
16
1
4 ry
Actual results Yellow
3
contradict hypothesis RrYy rrYy Rryy rryy wrinkled
16

Actual results 1 Green
support hypothesis 16 wrinkled


• An example of independent assortment

• Punnett Squares, Probablility & Predicting F1 & F2

Blind Blind

Phenotypes Black coat, normal vision Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA)
Genotypes B_N_ B_nn bbN_ bbnn

Mating of heterozygotes BbNn × BbNn
(black, normal vision)
Phenotypic ratio 9 black coat, 3 black coat, 3 chocolate coat, 1 chocolate coat,
normal vision blind (PRA) normal vision blind (PRA)
of offspring

Figure 9.5 B
PRA: Progressive Retinal Atrophy


Geneticists a testcross to determine unknown genotypes
• The offspring of a testcross, a mating between an individual
×
Testcross:
of unknown genotype and a homozygous recessive individual

Genotypes B_ bb

Two possibilities for the black dog:

BB or Bb

Gametes B B b

b Bb b Bb bb

Offspring All black 1 black : 1 chocolate


Mendel’s laws reflect the…..

RULES OF PROBABILITY
• Inheritance follows the rules of probability

Could you use a test cross to determine if an
organism was true breeding or pure breeding?
How?


F1 genotypes
• The Mule of Multiplication, OR …the Product Rule
Bb male

Formation of sperm
– Calculates the probability of two independent events
• The Rule of Addition
Bb female

– Calculateseggs probability of an event that can occur
Formation of
the
in alternate ways
1 1
2 B 2 b

1 B B B b
2 B
1 1
4 4
F2 genotypes

1 b B b b
2 b

1 1
4 4

Figure 9.7

• Family pedigrees

– Can be used to determine individual genotypes

Dd Dd D? D?
Joshua Abigail John Hepzibah
Lambert Linnell Eddy Daggett

D? dd Dd
Abigail Jonathan Elizabeth
Lambert Lambert Eddy

Dd Dd dd Dd Dd Dd dd

Female Male
Deaf
Hearing
Comet Hale Bopp seen from path to Lambert’s Cove Beach…Martha’s Vineyard
Figure 9.8 B

CONNECTION
9.9 Many inherited disorders in humans are
controlled by a single gene
• Some autosomal disorders in humans

Table 9.9


Recessive Disorders
• Most human genetic disorders are recessive
Parents Normal Normal
×
Dd Dd
Sperm
D d

Dd
D DD Normal
Normal (carrier)

Offspring Eggs

Dd dd
d Normal Deaf
(carrier)

Figure 9.9 A

Dominant Disorders
• Some human genetic disorders are dominant
• http://www.youtube.com/watch?v=zS7vCd8KQIA
• http://en.wikipedia.org/wiki/Human_genetics#Autosomal_dominant_inheritance

Figure 9.9 B


CONNECTION
New technologies can provide insight into one’s
genetic legacy
• New technologies

– Can provide insight for reproductive decisions

http://www.gaucherdisease.com/


Fetal Testing
• Amniocentesis and chorionic villus sampling (CVS)

– Allow doctors to remove fetal cells that can be tested
for genetic abnormalities
Amniocentesis Chorionic villus sampling (CVS)

Ultrasound Needle inserted
monitor through abdomen to Ultrasound Suction tube inserted
extract amniotic fluid monitor through cervix to extract
tissue from chorionic villi

Fetus
Fetus
Placenta
Placenta
Chorionic
Uterus villi
Cervix Cervix
Uterus
Amniotic
fluid Centrifugation
Fetal Fetal
cells cells

Biochemical
tests
Several Several
weeks hours

Karyotyping


Fetal Imaging
• Ultrasound imaging
– Uses sound waves to produce a picture of the fetus


NON-MENDELIAN INHERITANCE
Genotype = Phenotype?
2)What does this mean?
3)Mendel’s principles are valid for all sexually
reproducing species
4)D’OH…. genotype often does not dictate
phenotype in the simple way his laws describe


Genotypes:
Incomplete Dominance?
P generation

When an offspring’s phenotype × in between the phenotypes of its parents, it
Red HH is Hh
White hh
exhibits incompleteto make
RR dominance.
Homozygous Heterozygous
rr Homozygous
for ability for inability to make
LDL receptors LDL receptors
Gametes R r

F1 generation
Phenotypes:
Pink
Rr

LDL 1 1
Gametes
2
R 2 r
LDL
receptor
Sperm
1 1
2 R 2 r

1 Red Pink
Cell R rR
2 RR

F2 generation Eggs
1
Normalr Pink White Mild disease Severe disease
2 Rr rr

Figure 9.12 B

Multiple Alleles!!
• In a population

– Multiple alleles often exist for a
characteristic


• The ABO blood type in humans
– Involves three alleles of a single gene
• The alleles for A and B blood types are codominant
– And both are expressed in the phenotype
Blood Antibodies Reaction When Blood from Groups Below Is Mixed with
Group Present in Antibodies from Groups at Left
(Phenotype) Genotypes Blood O A B AB

O ii Anti-A
Anti-B

IAIA
A or Anti-B
IAi

IBIB
B or Anti-A
IBi

AB IAIB —

Figure 9.13


Pleiotropy: A single gene may affect many phenotypic
characteristics
•Pleiotropy describes the genetic effect of a single gene on multiple phenotypic
traits. The underlying mechanism is that the gene codes for a product that is, for
example, used by various cells, or has a signaling function on various targets.

PKU (phenylketonuria) Symptoms:
mental retardation
reduced hair
skin pigmentation,

…caused by any of a large number of mutations in a single gene that codes for
the enzyme (phenylalanine hydroxylase), which converts the amino
acid phenylalanine to tyrosine, another amino acid.

Depending on the mutation involved, this results in reduced or zero conversion
of phenylalanine to tyrosine, and phenylalanine concentrations increase to
toxic levels, causing damage at several locations in the body. PKU is totally
benign if a diet free from phenylalanine is maintained


A single characteristic may be influenced by many genes
• Polygenic inheritance: Creates a continuum of phenotypes
http://www.athro.com/evo/inherit.html
In humans three genes involved in eye color are known. They explain typical patterns of
inheritance of brown, green, and blue eye colors. However, they don't explain everything. Grey eye color,
Hazel eye color, and multiple shades of blue, brown, green, and grey are not explained. The molecular
basis of these genes is not known. What proteins they produce and how these proteins produce eye color
is not known. Eye color at birth is often blue, and later turns to a darker color. Why eye color can change
over time is not known. An additional gene for green is also postulated, and there are reports of blue eyed
parents producing brown eyed children (which the three known genes can't easily explain [mutations,
modifier genes that supress brown, and additional brown genes are all potential explanations]).

The known Human Eye color genes are: EYCL1 (also called gey), the Green/blue eye color
gene, located on chromosome 19 (though there is also evidence that another gene with similar activity
exists but is not on chromosome 19). EYCL2 (also called bey1), the central brown eye color gene,
possibly located on chromosome 15. EYCL3 (also called bey2), the Brown/blue eye color gene located
on chromosome 15. EYCL3 probably involves mutations in the regulatory region just before the OCA2
gene (which produces a protein that is expressed in melanocytes). A second gene for green has also
been postulated. Other eye colors including grey and hazel are not yet explained. We do not yet know
what these genes make, or how they produce eye colors. The two gene model (EYCL1 and EYCL3) used
above explains only a portion of human eye color inheritance. Both additional eye color genes and
modifier genes are almost certainly involved


The environmental affects many characteristics
• Many traits are affected, in varying degrees
– By both genetic and environmental factors


Designer Babies?
• Genetic testing can detect disease-causing
alleles
• Predictive genetic testing
– May inform people of their risk for developing genetic
diseases
– http://www.wired.com/wiredscience/2009/03/designerdebate/
– http://www.geneticsandsociety.org/article.php?id=4561


THE CHROMOSOMAL BASIS OF INHERITANCE
Chromosome behavior accounts for Mendel’s laws
• The structure and assembly of a eukaryotic chromosome:
http://www.youtube.com/watch?v=gbSIBhFwQ4s
• Genes are located on chromosomes
– Whose behavior during meiosis and fertilization accounts for
inheritance patterns


• The chromosomal basis of Mendel’s laws
F1 generation All round yellow seeds
(RrYy)
R
r y

Y
R r r R

Metaphase I
Y y of meiosis Y y
(alternative arrangements)
R r r R

Anaphase I
Y y of meiosis Y y
R r r R
Metaphase II
of meiosis
Y y Y y

Y y
Y Y Y y y
Gametes y
R R r r r r R R
1 1 1 1
RY ry rY Ry
4 4 4 4
Fertilization among the F1 plants

F2 generation 9 :3 :3 :1
(See Figure 9.5A)


Experiment

Genes on the same chromosome tend to be inherited
Purple flower

together PpLI × PpLI
Long pollen

Observed Prediction
Phenotypes offspring (9:3:3:1)
• Certain genes are linked Purple long
Purple round
284
21
215
71
Red long 21 71
Red round 55 24

– They tend to be inherited Explanation: linked genes

together because they Parental
PL

reside close together on diploid cell
PpLI PI

the same chromosome Meiosis

Most PL PI
gametes

Dominant or
Trait A Trait B Fertilization
References
Sperm
Recessive
Blond hair Blue eyes PL
both recessive
PI [5]
PL PL
PL A is recessive B is
Flexibility Anxiety disorderL
Most
P PI
[6]
offspring Eggs
PI dominant
PI
PI

Large ears broad nose PL both dominant
PI [7]
3 purple long : 1 red round
Not accounted for: purple round and red long
Figure 9.19

Crossing over produces new combinations of
alleles??? HOW?
• Crossing over can separate linked alleles
– Producing gametes with recombinant
chromosomes

A B a b
A B

a b A b a B

Tetrad Crossing over

Gametes

• Thomas Hunt Morgan
– Performed some of the early studies of
crossing over using the fruit fly Drosophila
melanogaster


Thomas Hunt Morgan
• Morgan began working seriously with Drosophila in 1907.
• But despite much effort and the breeding of successive generations, Morgan
initially failed to detect a single mutation. "Two years work wasted," he
lamented to one visitor to his laboratory. "I have been breeding those flies for
all that time and I've got nothing out of it."(Harrison, R.G., "Embryology and Its
Relations")
• April 1910 he suddenly had a breakthrough…one male fly with white : How
did this white eye color originate? What determines eye color?
• Morgan bred this white-eyed (mutant) male to a red-eyed (wild-type) virgin
sister and found that white-colored eyes are inherited in a special way. In the
first generation of brother-sister mating, labeled F1, there were only red-eyed
offspring, suggesting that red eye color is dominant and that white eye color is
recessive. To prove this idea Morgan carried out brother-sister matings with
the next generation (F2) and found that the offspring followed the expected
Mendelian ratio for a recessive trait: three red-eyed flies to every one white-
eyed fly. With these experiments Morgan started a tradition, which continues
to this day, whereby he named the gene "white" by the result of its mutation.
But then came a surprise. He had expected there would be an equal number
of males and females with white eyes, but it turned out that all the female flies
had red eyes; only males had white eyes, and, even more, only some of them
displayed the trait. Morgan realized that white eye color is not only recessive
but is also linked in some way to sex.

Morgan…continued
• By 1910, it was already known that chromosomes occur in pairs and that Drosophila had
four pairs of chromosomes. Several decades earlier, these thread-shaped structures had
been seen under a microscope to be located in the nucleus, but nobody knew their function.
Morgan later was to describe them in the following terms:
•"The egg of every species of animal or plant carries a definite number of bodies called
chromosomes. The sperm carries the same number. Consequently, when the sperm unites
with the egg, the fertilized egg will contain the double number of chromosomes. For each
chromosome contributed by the sperm there is a corresponding chromosome contributed by
the egg, i.e., there are two chromosomes of each kind, which together constitute a pair."
(Morgan, T.H. et al., The Mechanism of Mendelian Heredity) When Morgan turned to
examining the fruit fly's chromosomes under the microscope, he immediately appreciated
that not all four pairs of chromosomes were always identical. In particular, whereas female
flies had two identical-looking X chromosomes, in the male the X chromosome was paired
with a Y chromosome, which looks different and is never present in the female.
• Morgan deduced that a male must inherit the X chromosome from his mother and Y from
his father, and he immediately spotted a correlation between these sex-linked chromosomes
and the segregation of the factors determining eye color. When the mother was homozygous
and had two copies of the gene for red eyes, the male offspring invariably had red eyes,
even if the father had white eyes. But when the mother had white eyes, the male offspring
did too, even if the father's eyes were red. In contrast, a female fly gets one X chromosome
from each parent, and if one passed along an X chromosome with a gene for red eyes, the
offspring had red eyes because the color is dominant over white. Only when both parents
gave her an X chromosome with a gene for white eyes did she display the recessive trait.
From these observations, Morgan concluded that the allele-producing eye color must lie on
the X chromosome that governs sex. This provided the first correlation between a specific
trait and a specific chromosome.


• Morgan’s experiments Experiment

Gray body,
Black body,
vestigial
long wings wings

– Demonstrated the role ×
(wild type)

of crossing over in GgLI

Female
ggll
Male
inheritance

Offspring
Gray long Black vestigial Gray vestigial Black long

965 944 206 185

Parental Recombinant
phenotypes phenotypes
391 recombinants
Recombination frequency = = 0.17 or 17%
2,300 total offspring

Explanation
G L g l
GgLI ggll
(female) (male)
g l g l

G L g l G l g L g l

Eggs Sperm

G L g l G l g L
g l g l g l g l

Offspring

Figure 9.20 C

Geneticists use crossover data to map genes
• Morgan and his students
– Used crossover data to map genes in
Drosophila


• Recombination frequencies
– Can be used to map the relative positions
of genes on chromosomes.
Chromosome
g c l

Mutant phenotypes
17%

9% 9.5% Short Black Cinnabar Vestigial Brown
aristae body eyes wings eyes
Recombination (g) (c) (l)
frequencies

Long aristae Gray Red Normal Red
(appendages body eyes wings eyes
on head) (G) (C) (L)

Wild-type phenotypes


SEX CHROMOSOMES AND SEX-LINKED GENES
Chromosomes determine sex in many species
• In mammals, a male has one X chromosome
and one Y chromosome
– And a female has two X chromosomes


• The Y chromosome
– Has genes for the development of testes
• The absence of a Y chromosome
– Allows ovaries to develop


• Other systems of sex determination exist in
other animals and plants

22 22
+ +
XX X

Figure 9.22 B

76 76
+ +
ZW ZZ

Figure 9.22 C

32 16

Figure 9.22 D

Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes
– Are said to be sex-linked
• In many organisms
– The X chromosome carries many genes
unrelated to sex


• In Drosophila
– White eye color is a sex-linked trait

Figure 9.23 A

• The inheritance pattern of sex-linked genes
– Is reflected in females and males

Female Male Female Male Female Male

XR X R × Xr Y X R Xr × XR Y XR X r × Xr Y

Sperm Sperm Sperm
Xr Y XR Y Xr Y

Eggs XR X R Xr XR Y XR XR X R XR Y XR X R Xr XR Y
Eggs Eggs

R = red-eye allele Xr Xr X R Xr Y Xr Xr Xr Xr Y
r = white-eye allele

Figure 9.23 B Figure 9.23 C Figure 9.23 D


Genetic Determination of Sex Creates Dosage Problems

In mammals females have 2 X chromosomes, the males only 1
If nothing were done to compensate the females would get a double dose of any gene products
from the X chromosome, compared to the dose that males get
Nature solves this problem by shutting down one whole X chromosome in mammalian females
• X chromosome inactivation is called Lyonization after Mary Lyon who discovered it
• Inactive X chromosome appears in condensed state as a Barr body (p. 272, text)
• Inactivation of X chromosomes in different cells is somewhat random
The calico cat is a product of X chromosome inactivation
• Genes for coat color of the cat are on the X chromosome
• One gene produces a black color; its allele produces orange
• To get a calico coat a cat must be heterozygous, with genes for both the orange and the black
color
• If the X chromosome with the black gene is inactivated that cell will produce orange
• If the X chromosome with the orange gene is inactivated the cell will produce black
• Inactivation occurs in patches, giving the orange and black coat of the calico


CONNECTION
Sex-linked disorders affect mostly males
• Most sex-linked human disorders
– Are due to recessive alleles
– Are mostly seen in males

Queen Albert
victoria

Alice Louis

Alexandra Czar
Nicholas II
of Russia

Figure 9.24 A Figure 9.24 B Alexis


There are about 1,098 human X-linked genes.

Most of them code for something other than female anatomical traits. Many of
the non-sex determining X-linked genes are responsible for abnormal
conditions such as…
Hemophilia
Duchenne Muscular Dystrophy
Fragile-X Syndrome
Some High Blood Pressure
Congenital Night Blindness
G6PD Deficiency
Red-Green Color Blindness.
Male Pattern Baldness

Mechanism of PRO051 in the restoration of Dystrophin Expression through Exon
Skipping.

Normal muscle produces dystrophin, a critical protein, in response to signals
encoded in a precise lockstep manner into mRNA. The mRNA is then
translated into dystrophin protein. In the muscle of patients with Duchenne
muscular dystrophy, mutations in the dystrophin gene lead to the loss of one or

Mechanism of PRO051 in the Restoration of
Dystrophin Expression through Exon Skipping.

Normal muscle produces dystrophin, a critical protein, in response to signals encoded in a precise
lockstep manner into mRNA. The mRNA is then translated into dystrophin protein. In the muscle of
patients with Duchenne muscular dystrophy, mutations in the dystrophin gene lead to the loss of one
or more exons. The mRNA splices together the remaining exons; however, the missing pieces lead
to errors in translation (frame shift) and loss of production of the dystrophin protein. Intramuscular
injection of a small modified DNA molecule can enter Duchenne-affected muscle through abnormal
muscle membranes; then enters the nucleus and binds to the dystrophin mRNA. The modified DNA
molecule allows the mRNA to skip over the affected exons, and restores the reading frame of the
mRNA, for new production of dystrophin. The dystrophin that is produced is not normal but probably
retains considerable function.

• A male receiving a single X-linked allele from
his mother
– Will have the disorder
• A female
– Has to receive the allele from both parents
to be affected


Chapter 9 ppt

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