Download Ch 9 Patterns of Inheritance

Patterns of Inheritance
Chapter 9
Overview
• Definitions
• Patterns of Mendelian
Inheritance
• Non-Mendelian Inheritance
Genes:
Info in chromosomal DNA
Heritable traits passed to offspring
Diploid (2n):
Pairs of genes on pairs of homologous
chromosomes
Alleles:
• Alternative forms of a gene
• One form usually dominant over other
• If pair is identical over many generations
= true-breeding lineage
Hybrid:
• Cross between 2 true-breeding individuals that
have non-identical alleles for trait
e.g. AA x aa = hybrid offspring
Homozygous:
Pair of identical alleles on pair of
homologous chromosomes
e.g. A & A
Heterozygous:
Pair of non-identical alleles on pair of
homologous chromosomes
e.g. A & a
chromosome 1
from tomato
pair of
homologous
chromosomes
M
M locus: leaf colour
Both alleles are the same =
homozygous
D
D locus: plant height
Both alleles are the same =
homozygous
Bk
Bk locus: fruit shape
Alleles are different =
heterozygous
Dominant allele (e.g. A):
Effect on trait masks effect of recessive
allele (e.g. a)
Note: dominant alleles are not necessarily
more common or “better”
Homozygous dominant genotype = AA
Homozygous recessive genotype = aa
Heterozygous genotype = Aa
Genotype:
“Genes”
Individual’s alleles e.g. Aa
Phenotype:
“How genes are expressed”
Individual’s observable traits e.g. green
eyes
P = true-breeding parents
F1 = 1st-generation offspring
F2 = 2nd-generation offspring of self-fertilized
or crossed (mated) F1 individuals
Old Inheritance Theory
Hereditary material from both parents mixed at
fertilization
e.g. red flowers + white flowers = pink flower
offspring
+
Couldn’t explain obvious variation in traits
Gregor Mendel & His Peas
Viennese monk who studied
botany & math
Pisum sativum: garden pea
Self-fertilizing
(flowers produce male & female
gametes that fuse to form new plant so
that parent & offspring = same traits)
Can also be cross-fertilized
Mendel tracked 7 traits over 2 generations
Mendel’s Theory of
Segregation
Monohybrid cross:
2 homozygous parents that differ in trait
dictated by alleles of 1 gene
P
F1
AA x aa  Aa
After Mendel tracked 7
traits for 2
generations, he
found that:
F2 : ¼ recessive forms
& ¾ dominant forms
of trait
Genetics is a science of
probability
Fertilization is chance event with # of
possible outcomes
Can calculate probabilities of possible
outcomes of genetic crosses
Can determine all types of genetically
different gametes that can be produced by
male & female parents
A
A
homozygous parent
A
A
gametes
A
a
heterozygous parent
A
a
gametes
The Punnett Square Method
Allows prediction of both genotypes & phenotypes
of genetic crosses
A
A
a
a
Draw Punnett square with each row &
column labelled with one of possible
gametes of sperm & eggs respectively
A
a
A
a
Fill in genotype of offspring in each box by
combining male & female gametes
A
a
A
AA Aa
a
aA
aa
Count # offspring with each genotype &
convert to fraction of total # offspring
A
a
AA = ¼
A
AA Aa
Aa = aA = 2/4 = ½
a
aA
aa = ¼
aa
To determine phenotype proportions, add
fractions of genotypes that would produce
given phenotype
Phenotype I (dominant; AA & Aa) = ¼ + 2/4 = ¾
Phenotype II (recessive; aa) = ¼
So, for Mendel’s cross of F1 offspring from
monohybrid cross, he predicted:
A
a
A
AA Aa
a
aA
aa
F2 = ¼ AA, ½ Aa, ¼ aa
Phenotypic ratio = 3:1
– ¼ AA + ½ Aa = ¾ dominant phenotype
– ¼ aa = ¼ recessive phenotype
Since each gamete is
equally likely, each of
these offspring is
equally likely
Due to dominance we
see a ratio of
3 purple:1 white
An Example
Imagine you are crossing a true breeding
plant with yellow peas & a true breeding
plant with green peas. If yellow color is
dominant:
What would the F1 generation look like?
What would the F2 look like?
sperm
eggs
Dominance creates
some problems for
1 scientists
1
2
P
2
P
offspring
genotypes
1
4
PP
genotypic
ratio
(1:2:1)
1
4
PP
These three
all look the
same!
For example:
1
2
P
1
2
p
How can I know
which genotype I
have
all I 1canPsee
1 if
P
2
is2 phenotype?
1
2
P
1
2
p
1
4
Pp
2
4
1
4
pP
1
4
pp
phenotypic
ratio
(3:1)
1
4
Pp
pp
1
4
white
Test cross:
Individual shows dominance for trait but
genotype is unknown
Cross with homozygous recessive
individual to see if homozygous dominant or
heterozygous
If homozygous dominant:
If heterozygous:
pollen
• Test crosses supported
Mendel’s predictions
PP or Pp
sperm unknown
 F1 purple flowers were
heterozygous
if Pp
p
egg
p
1
P
2
P
all Pp
sperm
½ F2 = purple (Aa), ½ F2 =
white (aa)
if PP
sperm
Mendel found that crossing F1
purple flowers with truebreeding white flowers:
pp
all eggs p
1 Pp
2
1
p
2
1
2 pp
egg
An Example
Imagine you have a plant with yellow peas but you
don’t know its genotype. Remember that yellow
is dominant to green.
What type of pea would you mate it with?
Why?
If the offspring are all yellow what does this tell
you?
Does it matter how many offspring there are?
Mendel’s Big Ideas
Genes have alternate versions (alleles)
Organisms have two “particles” for each gene = diploid
Some alleles are “dominant” to others
(in organisms with two different alleles (heterozygous),
the dominant allele masks the recessive allele)
Alleles separate during gamete formation
= the law of segregation
Heterozygotes produce two different types of gametes
Mendel’s Theory of Segregation
2n cells have pairs of genes on pairs of
homologous chromosomes
Members of each gene pair separate during
meiosis & end up in different gametes
Applying Mendel’s Ideas
Imagine you have mated a black guinea pig with an
albino guinea pig. They have 12 offspring & all
are black.
What alleles are dominant in this case? How do
you know?
What are the parents’ phenotypes?
Genotypes?
Now imagine a cross between a different pair of
guinea pigs, one black & one albino. If they have
7 black & 5 albino offspring:
What are the parents’ genotypes? How do you
know?
Mendel performed a lot of crosses &
sometimes he was tracking more than one
trait at a time
This let him develop one more “Big Idea”
Mendel’s Theory of Independent
Assortment
Dihybrid cross:
True-breeding homozygous parents that
differ in 2 traits dictated by alleles of 2 genes
P
F1
AABB x aabb  AaBb
F1 heterozygous for alleles of both genes
For P (AABB), gametes = AB
For P (aabb), gametes = ab
AB
ab
AaBb
F1 = 100% AaBb
With independent assortment, alleles for one
trait are independent of alleles for another
e.g. if you have A you are equally likely to
have B or b
This means that each of the four gametes are
equally likely
During meiosis of F1 cells, there are 4 possible
combos of alleles in sperm or eggs:
1/4 AB, Ab, aB, ab
With 4 different sperm & egg types, F2 offspring
of hybrid cross = 16 possible combos of
gametes
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
e.g. with A = purple, a = white
B = tall, b = dwarf
9/16 tall, purple
3/16 dwarf, purple
3/16 tall, white
1/16 dwarf, white
Phenotypic ratio = 9:3:3:1
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
ab
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
An Example
A true breeding plant with wrinkled green
seeds was mated to a true breeding plant
with smooth yellow seeds. In the first
generation all the plants had smooth yellow
seeds.
What alleles are dominant in this case?
How do you know?
Taking these (dihybrid) F1 plants, Mendel
allowed them to self-fertilize
We could write the F1 genotypes like this:
SsYy x SsYy
What would their gametes look like?
• SY
• Sy
• sY
• sy
What would the zygotes look like? Use a Punnett Square.
SsYy
self-fertilize
eggs
1
4 SY
1 Sy
4
1 sY
4
1
4
sy
9/16 smooth yellow
sperm
1 SY
4
1 Sy
4
1 sY
4
1 sy
4
1
16 SSYY
1
16 SSYy
1
16 SsYY
1
16 SsYy
1
16 SSyY
1
16 SSyy
1
16 SsyY
1
16 Ssyy
1
16 sSYY
1
16 sSYy
1
16 ssYY
1
16 ssYy
1
16 sSyY
1
16 sSyy
1
16 ssyY
1
16 ssyy
3/16 smooth green
3/16 wrinkled yellow
1/16 wrinkled green
Phenotypic ratio:
9:3:3:1
Pp
self-fertilize
1
2
P
eggs
1
2
p
Remember that a
monohybrid cross will
give you a 3:1 ratio
sperm
1
P
2
1
4 PP
1
4 Pp
1
4 pP
1
4 pp
1
p
2
The 9:3:3:1 ratio is
actually just two 3:1
ratios “stacked” on top
of each other
seed shape
seed color
(3:1)
(3:1)
3
4 smooth
3
4 smooth
x
3
4
yellow
1
4 wrinkled x
1
4 green
3
4 yellow
1
4 wrinkled x
1
4
x
green
phenotypic ratio
(9:3:3:1)
=
9
16 smooth yellow
=
3
16 smooth green
3
16 wrinkled yellow
=
1
16 wrinkled green
=
Independent Assortment
Alleles for one trait are independent of alleles
for another
This happens because of events in
metaphase of meiosis I
Remember that chromosomes line up
independently of non-homologous
chromosomes
S
pairs of alleles on homologous
chromosomes in diploid cells
s
Y
y
S
Y
s
y
chromosomes
replicate
replicated homologues
pair during metaphase
of meiosis I,
orienting like this
or like this
S
y
s
Y
meiosis I
S
Y
s
y
S
y
s
Y
S
Y
s
y
S
y
s
Y
meiosis II
S
S
Y
s
Y
SY
S
s
y
y
sy
S
y
s
y
Sy
s
Y
Y
sY
Independent assortment produces four equally likely allele combinations during meiosis
Mendel’s Big Ideas
Genes have alternate versions (alleles)
Organisms have two “particles” for each gene = diploid
Some alleles are dominant to others
(In organisms with two different alleles (heterozygous) the
dominant allele masks the recessive allele)
Alleles separate during gamete formation (the law of
segregation)
(heterozygotes produce two different types of gametes)
Alleles for one trait are independent of alleles for another
= the law of independent assortment
Mendel’s Theory of Independent
Assortment
After meiosis, genes on each pair of
homologous chromosomes are sorted out, but
independently of how genes on other pairs of
homologous chromosomes are sorted out
Independent assortment + segregation
= genetic variation
# genotypes = 3n where n = # gene pairs
More pairs = more genotypes
3n = 32 = 9 different genotypes
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
AaBb
Aabb
aaBb
aabb
ab
Using Mendel’s Big Ideas
In horses grey coat colour is dominant to
chestnut. Imagine you own a grey horse & a
chestnut horse & over the years they have
several offspring, 2 chestnut & 1 grey.
Given what you know about genetics, what is the
genotype of each parent & of each offspring?
How do you know?
Applying Mendel’s Ideas
Imagine you have mated a true-breeding tall plant
with round seeds to a true-breeding dwarf plant with
wrinkled seeds. In the F1 generation, the plants are
all tall with round seeds.
What alleles are dominant in this case?
Now imagine you have mistakenly mixed these
F1 plants with some true-breeding tall round
plants. What kind of cross do you need to do to
tell the plants apart?
A Test Cross!
You need to do a test cross on your tall round
plants.
What kind of plant will you mate your tall round plants
with?
Genotype? Phenotype?
Now predict the two possible outcomes of your
cross using a Punnett square.
Crossing Over & Inheritance
During meiosis, crossing-over occurs between
non-sister chromatids on homologous
chromosomes
Get combos of alleles not seen in parents
Some genes stay together more often than
others because closer together
 chance that crossing over will separate
A
B C
D
2 genes are closely linked when distance
between them is small
= combos of alleles usually end up in same
gamete
A
B C
D
When far apart, crossing over is very frequent
= genes independently assort into different
gametes
Dependent Assortment
Genes on the same chromosome are linked
Their alleles tend to assort dependently
flower color gene
pollen shape gene
sister
chromatids
purple allele, P
long allele, L homologous
chromosomes
(duplicated)
at meiosis I
sister
chromatids
red allele, p
round allele, l
Copyright © 2005 Pearson Prentice Hall, Inc.
Alleles for genes on the same chromosome
assort dependently
= alleles tend to stay together during meiosis
The 4 types of gametes are not equally likely:
Two (called the parental type) are common
Two (called the recombinant type) are rare
Dependent Assortment
How can you ever get recombinant gametes?
Remember the events of Prophase I?
Crossing-over generates recombinant gametes
Detecting Linkage
Imagine you have mated a true breeding black guinea
pig with smooth hair to a true breeding white one
with rough hair. All the offspring are black with
rough hair.
What are the dominant alleles here?
What is the genotype of the F1?
You now mate one of your black rough F1 guinea pigs
to a white smooth one.
What are the four types of offspring that should
be produced?
What ratio would you expect them to be in if:
There isn’t linkage?
There is linkage?
What are the four types of offspring that should be
produced?
Black rough
Black smooth
White rough
White smooth
Without linkage, all are equally likely:
Black rough
Black smooth
White rough
White smooth
With linkage:
Black smooth: more common (>25%)
White rough: more common (>25%)
Why are these the parental types?
White smooth: less common (<25%)
Black rough: less common (<25%)
Why are these the recombinant types?
Remember the lineage of the offspring:
We mated a true-breeding black smooth guinea pig
to a true-breeding white rough to get the F1 so ...
Black smooth & white rough are together from the
parents
The lineage of the offspring determines the
parental type
Linkage is between gene loci, not alleles
Recombination constantly shuffles the alleles so
that it is only if you know the lineage of an
organism that you can predict the parental type
But you can just observe the parental type...
An example
from fruit
flies
A grey bodied, normal
winged fly in a test
cross produces all four
offspring types but...
Four offspring are not in
equal proportions:
The rare offspring type
represent the
recombinant type
The more common
offspring represent the
parental type
You don’t really need to
know lineage to figure out
which is which
Mendel’s Big (Modified) Ideas
Genes have alternate versions (alleles)
Organisms have two “particles” for each gene: diploid
Some alleles are dominant to others (recessive)
Alleles segregate during gamete formation
Law of independent assortment
= isn’t true for linked genes (on same chromosome)
Exceptions to the Rule
Mendel looked at traits that were either
dominant or recessive
Some traits do not follow these clear patterns
Codominance
Pair of non-identical alleles expressed at
same time in heterozygotes
e.g. The ABO Blood System
RBCs have membrane glycolipid that
differentiate between types
Structure of glycolipid determined by
enzyme
3 alleles code for enzyme: IA, IB, i
= multiple allele system
IA & IB are codominant when paired
i is recessive when paired with IA & IB
IA & IB have different forms of enzyme that
attaches last sugar to glycolipid
IAIA or IAi = 1 type of sugar = blood type A
IBIB or IBi = other type of sugar = blood type B
IAIB = both sugars = blood type AB
ii = no sugar = blood type O
Using Multiple Alleles
If a boy’s father has blood type AB & his mother has
type O, what blood types could the boy have? How
likely is each?
Imagine a young woman has type B blood & her
mother has type AB. What blood type can you rule
out for her father?
Incomplete Dominance
1 allele not fully dominant, so both expressed in
heterozygotes
Phenotype is somewhere between 2
homozygotes
e.g. snapdragons
True-breeding P red flowers & white flowers
produce F1 pink flowers
+
Red flowers (AA) have 2
alleles & produce red
pigment
White flowers (aa) have 2
mutant alleles so produce
no pigment
Pink F1 (Aa) have 1 red &
1 white allele
= enough red pigment to
make pink colour, but red
allele not dominant enough
to make flowers red
Modifying Mendel’s Big Ideas
Sometimes alleles are not dominant
= heterozygote has a different phenotype
In snapdragons heterozygotes for flower color are
intermediate in phenotype (pink) to either parent
This does not mean there is a pink allele!
Epistasis
More than 1 gene affects 1 given trait
e.g. coat colour in labs determined by 2
genes (E/e & B/b)
Pleiotropy
1 gene affects more than 1 trait
Can have positive or negative effects
e.g. many genetic disorders, aging
e.g. SRY gene: codes for protein that
activates other genes, that code for proteins
that control male development
e.g. sickle cell anemia
Normal RBCs
Environmental Influence
Phenotype is not just a result of genotype
Environment plays a key role in many traits
e.g. skin colour, body size, intelligence,
personality
For many traits, genes & environment play a
roughly equal role in determining phenotype
BUT: Effects of the environment are not
heritable
Genes & the Environment
Environmental conditions can affect how
genes are expressed (i.e. variation in traits)
e.g. soil acidity (aluminum availability) &
hydrangea colour
e.g. the Himalayan rabbit
Gene for black fur expressed in cool areas of body
(has genotype for black fur all over but pigment
only produced if < 34°C)
What was the main idea about
inheritance prior to Mendelian
inheritance?
Blending inheritance
= offspring are a “blend” of parents
= offspring phenotype is usually in between the
phenotype of parents
We now explain this in terms of polygenic
inheritance
Polygenic Inheritance
Individuals in population show range of small
differences in most traits
e.g. eye colour, human height, etc.
Multiple genes influence a single trait
Polygenic inheritance
mimics blending
inheritance because of
the large number of
genes each with an
additive effect (plus
environmental effects)
Polygenic Inheritance
Alleles for different genes act additively to build a
phenotype
Several genes influence phenotype each with a
+1 or +0 allele
So traits have a characteristic distribution pattern in
a population & offspring are often intermediate
between parents
An Example: Wheat Grain Colour
2 genes with incompletely dominant alleles determine
wheat grain colour
= R1 & R1’ & R2 & R2’
(R alleles = 1 unit of red pigment)
(R’ alleles = no pigment)
2 heterozygous wheat plants will produce 5 colours of
offspring
Because 2 genes, are 5 possible combos of alleles:
(4 R), (3 R & 1 R’), (2 R & 2 R’), (1 R & 3 R’), (4 R’)
R alleles = +1 to colour
R’ alleles = +0 to colour
R1R1R2R2
eggs
R1R2
R1R2
R1R2
R1R2
sperm
R1R2
R1R1R2R2
R1R1R2R2
R1R1R2R2 
R1R1R2R2
R1R1R2R2
R1R1R2R2 
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2 
R1R1R2R2
R1R1R2R2
R1R1R1R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R2
R1R2
R1R2
Imagine a couple, one with very light skin, and
one with very dark skin, have children.
What will their children’s skin colour be?
(remember this is a bit of an oversimplification
of skin color inheritance)
In polygenic
inheritance,
alleles are
influenced
by
environment
so traits
blend even
more
Distribution of all forms of trait is more continuous
when  genes & environmental factors are
involved
= bell curve
Example
A rooster with grey feathers is mated to a hen who
also has grey feathers. Among their offspring 15
chicks are grey, 6 are black & 8 are white.
What is the simplest explanation for this inheritance
pattern?
What phenotypes would you expect in the offspring
resulting from a cross between a grey rooster & a
black hen?
Example
Imagine two organisms with the genotypes AABB &
aabb are bred to make a heterozygous offspring
(AaBb).
What will the offsprings’ gametes look like?
If these two genes (A & B) are linked, what would this
do to the gametes that are produced? Why?