Transcript Chapter 9

What happens to all those
genes?
Genetic inheritance
Copyright © 2009 Pearson Education, Inc.
Let’s look at an interesting example… the
hybrid
 Hybrid: the offspring of two different
varieties
 Cross a female tigress (Panthera tigress)
with a male lion (Panthera leo) and get a
LIGER
 Males are sterile, females “can” reproduce
 VERY large; have lived to 24 years but
often die shortly after birth
Copyright © 2009 Pearson Education, Inc.
http://www.bing.com/images/search?q=liger&view=detail&id=73F46F59A3487
133F45EC7D338EC493CF0A7EA0D&first=1&FORM=IDFRIR&qpvt=liger
What determines what the
liger will look like, if it will
live, if it can reproduce…?
Copyright © 2009 Pearson Education, Inc.
What determines what the
liger will look like, if it will
live, if it can reproduce…?
Genes
Copyright © 2009 Pearson Education, Inc.
 Gregor Mendel discovered genetics using the
garden pea
 Genes: Heritable factors passed from parent to
offpsring
 Advantages of using pea plants

Controlled matings

Self-fertilization or cross-fertilization

Observable characteristics with two distinct traits
Copyright © 2009 Pearson Education, Inc.
Petal
Stamen
Carpel
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
Character
Flower color
Traits
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)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 ´ F1)
F2 generation
3
–
4
Copyright © 2009 Pearson Education, Inc.
of plants
have purple flowers
1
–
4
of plants
have white flowers
 A monohybrid cross
 Track ONE character
 Parental (P) generation: purple flowers  white
flowers
 F1 generation: all plants with purple flowers
 F2 generation: ¾ of plants with purple flowers
¼ of plants with white flowers
 Questions?
 Why did one trait seemed to disappear in the F1
generation?
 Why that trait reappeared in one quarter of the
F2 offspring?
Copyright © 2009 Pearson Education, Inc.
 Here is why (4 hypotheses):
1. Genes are found in alternative versions called alleles
(all alleles/genes found in an organism are called
the genotype)
2. For each characteristic, an organism inherits two
alleles; the alleles can be the same or different

A homozygous genotype has identical alleles

A heterozygous genotype has two different alleles
Copyright © 2009 Pearson Education, Inc.
 Here is why (4 hypotheses):
3. If alleles are different:
- The dominant allele determines the organism’s
appearance (phenotype)
- The recessive allele has no noticeable effect
4. Law of segregation: Allele pairs separate (segregate)
from each other during the production of gametes
so that a sperm or egg carries only one allele for
each gene
How do we use these to explain Mendel’s
experiment??
Copyright © 2009 Pearson Education, Inc.
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1
–
2
1
–
2
P
p
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
Punnett square
 On 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
Gene loci
P
a
B
P
a
b
PP
Homozygous
for the
allele
Copyright © 2009 Pearson Education,dominant
Inc.
Genotype:
Dominant
allele
aa
Homozygous
for the
recessive allele
Recessive
allele
Bb
Heterozygous
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
Gametes RY
RrYy
RrYy
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
Eggs
1
–
2
1
–
4
ry
RY
ry
1
–
4
RY
1
–
4
rY
Eggs
1
–
4
Hypothesized
(not actually seen)
ry
1
–
4
RY
1
–
4
Sperm
1
– Ry
rY
4
1
–
4
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
9
––
16
Ry
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
(support hypothesis)
Copyright © 2009 Pearson Education, Inc.
ry
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
 A dihybrid cross
 Track TWO characters
 Parental generation: round yellow seeds x wrinkled
green seeds
 F1 generation: all plants with round yellow seeds
 F2 generation: 9/16 of plants with round yellow seeds
3/16 of plants with round green seeds
3/16 of plants with wrinkled yellow seeds
1/16 of plants with wrinkled green seeds
 Questions?
 Why nonparental combinations were observed
 Why a 9:3:3:1 ratio was observed among the F2 offspring
Copyright © 2009 Pearson Education, Inc.
 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
Copyright © 2009 Pearson Education, Inc.
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
Let’s try a dihybrid cross….
 Characters:
1. Earlobes- Free is dominant (F)
Attached is recessive (f)
2. Tongue rolling- Rollers are dominant (R)
Nonrollers are recessive (r)
Cross a true breeding unattached roller with an
attached nonroller to the F2 generation
Copyright © 2009 Pearson Education, Inc.
 What if we did NOT know the genotype of our
unattached tongue roller??
 Testcross:
 Mating between an individual of unknown genotype
and a homozygous recessive individual
 Will show whether the unknown genotype includes a
recessive allele
 Used by Mendel to confirm true-breeding genotypes
Copyright © 2009 Pearson Education, Inc.
Remember: Mendel’s laws reflect
probability and statistics
What is the probability that the Olsen sisters have such
similar genes but are NOT identical twins?
Copyright © 2009 Pearson Education, Inc.
 The probability of a specific event is the number
of ways that event can occur out of the total
possible outcomes.
1. Rule of multiplication
 Multiply the probabilities of events that must occur
together
 Example: Cross two AaBbCc parents
What is probability that offspring will be
aabbcc?
2. Rule of addition
 Add probabilities of events that can happen in
alternate ways
Copyright © 2009 Pearson Education, Inc.
F1 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
Law of segregation: Allele pairs separate
(segregate) from each other during the
production of gametes so that a sperm or
egg carries only one allele for each gene
Law of independent assortment: Each pair of
alleles segregates independently of the other
pairs of alleles during gamete formation
VARIATIONS ON MENDEL’S
LAWS
Copyright © 2009 Pearson Education, Inc.
 Incomplete
dominance
P generation
Red
RR
White
rr
r
Gametes R
 Neither allele is
dominant over the
other
F1 generation
Pink
Rr
1
 Heterozygous
individual showsan
intermediate
phenotype
Gametes–2 R
1
–
2
F2 generation
1
–
2
1
–
2
r
Sperm
1
– r
R
2
R
RR
rR
r
Rr
rr
Eggs
1
–
2
Copyright © 2009 Pearson Education, Inc.
 Multiple alleles
 A diploid individual can carry any two of these
alleles
 Example: ABO blood group
 Three alleles: IA, IB, and i
 Four phenotypes: Type A, Type B, Type AB and
Type O blood
Copyright © 2009 Pearson Education, Inc.
 Codominance
 Neither allele is dominant over the other
 Expression of both alleles is observed as a
distinct phenotype in the heterozygous
individual
Copyright © 2009 Pearson Education, Inc.
Blood
Group
(Phenotype) Genotypes
Red Blood Cells
O
ii
A
IAIA
or
IAi
Carbohydrate A
B
IBIB
or
IBi
Carbohydrate B
AB
IAIB
 Pleiotropy
 One gene influencing many characteristics
 Example: 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
Copyright © 2009 Pearson Education, Inc.
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
 Many genes
influence one trait
 Skin color is
affected by at
least three genes
Copyright © 2009 Pearson Education, Inc.
P generation
aabbcc
(very light)
AABBCC
(very dark)
F1 generation
AaBbCc
F2 generation
1
–
8
1
–
8
1
–
8
1
–
Eggs 1
8
–
8
1
–
8
1
–
8
1
–
8
1
––
64
1
–
8
6
––
64
1
–
8
15
––
64
AaBbCc
Sperm
1
– 1
– 1
–
8 8 8
20
––
64
15
––
64
1
–
8
1
–
8
6
––
64
1
–
8
1
––
64
Summary of variations on Mendel’s laws
Incomplete
dominance
White
rr
Red
RR
Pink
Rr
Pleiotropy
Single
gene
Multiple
genes
Multiple characters
Polygenic
inheritance
Single characters
(such as skin color)
INHERITANCE OF GENES
Genetic traits in humans can be tracked
through family pedigrees
 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
Free earlobe
Copyright © 2009 Pearson Education, Inc.
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
AUTOSOMAL DISORDERS
1. Autosomal recessive inheritance
 Two recessive alleles are needed to show disease
 Heterozygous parents are carriers
 Probability of inheritance increases with inbreeding
Parents
Normal
Dd
Normal
Dd
´
Sperm
D
Offspring
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Copyright © 2009 Pearson Education, Inc.
D
2. Autosomal dominant inheritance
 One dominant allele is needed to show disease
 Dominant lethal alleles are usually eliminated from
the population
Copyright © 2009 Pearson Education, Inc.
SEX CHROMOSOMES AND
SEX-LINKED GENES
Copyright © 2009 Pearson Education, Inc.
Chromosomes determine sex in many species
(male)
44
+
XY
 X-Y system in
mammals, fruit flies
 XX = female; XY
= male
 X-O system in
grasshoppers and
roaches
 XX = female; XO
= male
 Chromosome number
in ants and bees
 Diploid = female;
haploid = male
Copyright © 2009 Pearson Education, Inc.
22
+
X
Parents’
diploid
cells
(female)
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
44
+
XY
Egg
Offspring
(diploid)
22
+
XX
22
+
X
32
16
Sex-linked genes exhibit a unique pattern of
inheritence
 Sex-linked genes are located on either of the
sex chromosomes
 X-linked genes (Xg) can be passed from:
____________ to _______ and __________
____________ to _______
 Y-linked genes (Yg) can be passed from:
____________ to _______
Copyright © 2009 Pearson Education, Inc.
Example of sex-linked genes: fruit fly eye
color
Female
Male
Xr Y
XR XR
Sperm
Eggs XR
Xr
Y
XR Xr
XR Y
R = red-eye allele
r = white-eye allele
Female
Male
XR Xr
Xr Y
Sperm
Xr
Y
XR
XR XR
XR Y
Xr
Xr Xr
Xr Y
Eggs
Sex-linked disorders affect mostly males
 Males express X-linked recessive disorders
because they only have ONE copy
 Hemophilia
 Colorblindness
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Copyright © 2009 Pearson Education, Inc.
THE CHROMOSOMAL BASIS OF
INHERITANCE
Copyright © 2009 Pearson Education, Inc.
 Mendel’s Laws rely on chromosome
separation in meiosis
 The law of segregation depends on separation
of homologous chromosomes in anaphase I
 The law of independent assortment depends on
alternative orientations of chromosomes in
metaphase I
Copyright © 2009 Pearson Education, Inc.
F1 generation
All round yellow seeds
(RrYy)
R
r
y
Y
r
R
y
Y
R
Y
y
Y
y
R
R
Y
y
Anaphase I
of meiosis
r
Y
R
r
R
Y
Metaphase I
of meiosis
(alternative
arrangements)
r
Metaphase II
of meiosis
r
Y
y
r
R
Y
y
y
Y
Y
r
r
r
1
– ry
4
1
– rY
4
Fertilization among the F1 plants
F2 generation
R
Gametes
y
1
– RY
4
r
9
:3
:3
:1
y
y
R
R
1
–
4
Ry