Transcript genetics ppt

Chapter 9
Patterns of Inheritance
https://www.youtube.com/watch?v=B_PQ8qYtUL0
PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Copyright © 2009 Pearson Education, Inc.
Analyzing Inheritance
Offspring resemble their parents. Offspring inherit genes
for characteristics from their parents. To learn about
inheritance, scientists have experimented by breeding
various plants and animals.
In each experiment shown in the table on the next slide,
two pea plants with different characteristics were bred to
produce a first generation. Then, the offspring produced
were bred to produce a second generation of offspring.
Consider the data and answer the questions that follow.
Go to
Section:
Parents First Generation
Second Generation
Section
Long11-1
stems  short stems
All long
787 long: 277 short
Red flowers  white flowers
All red
705 red: 224 white
Green pods  yellow pods
All green
428 green: 152 yellow
Round seeds  wrinkled seeds
All round
5474 round: 1850 wrinkled
Yellow seeds  green seeds
All yellow
6022 yellow: 2001 green
1. In the first generation of each experiment,
how do the characteristics of the offspring
compare to the parents’ characteristics?
2. How do the characteristics of the second
generation compare to the characteristics of
the first generation?
Go to
Section:
Early Advances in Genetics
 Hippocrates (460-370 BC) – pangenesis
– “pangenes” travel from each part of organism’s body to
egg/sperm and then passed to the next generation
Aristotle (384-322 BC) – rejected pangensis
– What is inherited is the potential to produce body
features rather than particles of the features themselves
Blending Hypothesis
Mendel (1860s) – see next slides
MENDEL’S LAWS
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Experimental genetics began in an abbey
garden
 Mendel experimented pea plants
– showed that parents pass heritable
factors to offspring
– Advantages of using pea plants
1. Controlled matings (self vs. cross
fertilization
2. Observable traits with two distinct
forms
3. True-breeding strains (completely
dominant/recessive for a trait)
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White
1
Removed
stamens from
purple flower
Stamens
Carpel
Parents
(P)
2
Purple
3
Transferred
pollen from stamens of white
flower to carpel of purple flower
Pollinated carpel
matured into pod
4
Offspring
(F1)
Planted seeds
from pod
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
P generation
(true-breeding
parents)
looked at
Purple flowers
Mendel
each trait individually
F1 generation
All plants have
purple flowers
Called monohybrid
cross
Shown in punnett
squares
Fertilization
among F1 plants
(F1 x F1)
Mendel needed to
explain:
F2 generation
1. Why did one trait
“disappear” in the F1
generation?
2. Why did that trait
reappear in the F2
offspring?
White flowers
3
–
4
1
–
4
of plants
of plants
have purple flowers have white flowers
Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
1. Genes are found in alternative versions called
alleles; a genotype is the listing of alleles an
individual carries for a specific gene
2. For each characteristic, an organism inherits two
alleles, one from each parent; the alleles can be
the same or different
– A homozygous genotype has identical alleles
– A heterozygous genotype has two different alleles
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Mendel’s law of segregation describes the
inheritance of a single character
 Four Hypotheses
3. If alleles differ, the dominant allele determines
the organism’s appearance, and the recessive
allele has no noticeable effect
– The phenotype is the appearance or expression of a
trait
– The same phenotype can have more than one
genotype
4. Law of segregation: Allele pairs separate from
each other during the production of gametes so
that a sperm or egg carries only one allele for
each gene
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Homologous chromosomes bear the alleles for each
character
 For a pair of homologous chromosomes, alleles of a
gene reside at the same locus
– Homozygous individuals have the same allele on both
homologues
– Heterozygous individuals have a different allele on each
homologue
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Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Bb
PP
aa
Homozygous
Heterozygous
Homozygous
for the
for the
dominant allele recessive allele
The law of independent assortment is revealed
by tracking two characters at once
 Dihybrid cross (2 traits)
– Parental generation: round, yellow seeds  wrinkled,
green seeds
– F1 generation: all plants with round yellow seeds
– F2 generation: 9/16 of
3/16 of
3/16 of
1/16 of
plants
plants
plants
plants
with
with
with
with
round, yellow seeds
round, green seeds
wrinkled, yellow seeds
wrinkled, green seeds
 Mendel needed to explain
– Why nonparental combinations were observed?
– Why a 9:3:3:1 ratio was observed among the F2
offspring?
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9.5 The law of independent assortment is
revealed by tracking two characters at once
 Law of independent assortment
– Each pair of alleles segregates independently of
the other pairs of alleles during gamete formation
– For genotype RrYy, four gamete types are
possible: RY, Ry, rY, and ry
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Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
1
–
4
ry
1
–
4
RY
Eggs
1
–
2
RY
1
–
4
ry
Hypothesized
(not actually seen)
1
–
4
rY
1
–
4
Ry
1
–
4
ry
RY
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
rY
Eggs
1
–
4
1
–
4
9
––
16
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
9.Geneticists use the testcross to determine
unknown genotypes
 Testcross
– Mating between an individual of unknown genotype and a
homozygous recessive individual
– Shows if unknown genotype includes a recessive allele
–
If offspring contain recessive traits then the
unknown genotype must be heterozygous
– Used by Mendel to confirm true-breeding genotypes
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Testcross:
B_
Genotypes
bb
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
F
Mendel’s laws
reflect the rules of
probability
1
The probability
of a specific
event is the
number of ways
that event can
occur out of the
total possible
outcomes.
genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
–
2
1
–
2
1
–
2
B
B
B
b
B
B
1
–
4
1
–
4
1
–
2
b
b
B
1
–
4
F2 genotypes
b
b
b
1
–
4
Genetic traits in humans can be tracked through
family pedigrees
 A pedigree
– Shows the inheritance of a trait in a family through
multiple generations
– Demonstrates dominant or recessive inheritance
– Can also be used to deduce genotypes of family members
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Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
First generation
(grandparents)
Ff
Second generation
(parents, aunts,
and uncles)
FF
or
Ff
Third generation
(two sisters)
Female Male
Affected
Unaffected
Ff
ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Many inherited disorders in humans are controlled
by a single gene
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New technologies can provide insight into one’s
genetic legacy
 Genetic testing of parents
 Fetal testing: biochemical and karyotype analyses
– Amniocentesis
– Chorionic villus sampling
 Maternal blood test
 Fetal imaging
– Ultrasound
– Fetoscopy
 Newborn screening
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Chorionic villus sampling (CVS)
Amniocentesis
Needle inserted
Ultrasound
through abdomen to monitor
extract amniotic fluid
Ultrasound
monitor
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Fetus
Placenta
Fetus
Placenta
Uterus
Chorionic
villi
Cervix
Cervix
Uterus
Amniotic
fluid
Fetal
cells
Centrifugation
Fetal
cells
Several
weeks
Biochemical
tests
Karyotyping
Several
hours
VARIATIONS ON MENDEL’S
LAWS
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Incomplete dominance results in intermediate
phenotypes
 Incomplete dominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as an
intermediate phenotype in the heterozygous
individual
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P generation
Red
RR
White
rr
r
R
Gametes
F1 generation
Pink
Rr
Gametes
1
–
2
R
1
–
2
r
Sperm
1
–
2
F2 generation
R
1
–
2
r
1
–
2
R
RR
rR
1
–
2
r
Rr
rr
Eggs
Many genes have more than two alleles in the
population
 Multiple alleles
– More than two alleles are found in the population
– A diploid individual can carry any two of these alleles
– The ABO blood group has three alleles, leading to four
phenotypes: type A, type B, type AB, and type O blood
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9.12 Many genes have more than two alleles in
the population
 Codominance
– Neither allele is dominant over the other
– Expression of both alleles is observed as a distinct
phenotype in the heterozygous individual
– Observed for type AB blood
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Blood
Group
(Phenotype) Genotypes
Red Blood Cells
Antibodies
Present in
Blood
Anti-A
Anti-B
O
ii
A
I AI A
or
I Ai
Carbohydrate A
Anti-B
B
IBIB
or
IBi
Carbohydrate B
Anti-A
AB
IAIB
—
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
A single gene may affect many phenotypic
characters
 Pleiotropy
– One gene influencing many characteristics
– The gene for sickle cell disease
– Affects the type of hemoglobin produced
– Affects the shape of red blood cells
– Causes anemia
– Causes organ damage
– Is related to susceptibility to malaria
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Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
 Polygenic inheritance
P generation
aabbcc
AABBCC
(very light) (very dark)
– Many genes influence one
trait
– Skin color is affected by at
least three genes
F1 generation
AaBbCc
AaBbCc
Sperm
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
F2 generation
1
–
8
1
–
8
1
–
8
1
–
8
Fraction of population
Eggs
20
––
64
1
–
8
1
–
8
1
–
8
1
–
8
15
––
64
6
––
64
1
––
64
1
––
64
6
––
64
15
––
64
20
––
64
15
––
64
6
––
64
1
––
64
Skin color
Genes on the same chromosome tend to be
inherited together
 Linked Genes
– Are located close together on the same chromosome
– Tend to be inherited together
 Example studied by Bateson and Punnett
– Parental generation: plants with purple flowers, long
pollen crossed to plants with red flowers, round
pollen
– The F2 generation did not show a 9:3:3:1 ratio
– Most F2 individuals had purple flowers, long pollen or
red flowers, round pollen
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Experiment
Purple flower
PpLl
Phenotypes
Purple long
Purple round
Red long
Red round
PpLl
Observed
offspring
284
21
21
55
Long pollen
Prediction
(9:3:3:1)
215
71
71
24
Geneticists use crossover data to map genes
 Genetic maps
– Show the order of genes on chromosomes
– Arrange genes into linkage groups representing
individual chromosomes
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Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae Gray
(appendages body
on head)
(G)
Cinnabar Vestigial
eyes
wings
(c)
(l)
Red
eyes
(C)
Normal
wings
(L)
Wild-type phenotypes
Brown
eyes
Red
eyes
SEX CHROMOSOMES AND
SEX-LINKED GENES
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Sex-linked genes exhibit a unique pattern of
inheritance
 Sex-linked genes - located on either of sex
chromosomes
– X-linked genes are passed from mother to son and
mother to daughter
– X-linked genes are passed from father to daughter
– Y-linked genes are passed from father to son
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Sex-linked disorders affect mostly males
 Males express X-linked disorders such as the
following when recessive alleles are present in one
copy
– Hemophilia
– Colorblindness
– Duchenne muscular dystrophy
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Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
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