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 traite.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
M locus: leaf colourBoth alleles are the same =
homozygous
D locus: plant heightBoth alleles are the same =
homozygous
Bk locus: fruit shapeAlleles are different =
heterozygous
chromosome 1from tomato
pair of homologous
chromosomes
M
D
Bk
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
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
Genetics is a science of probability
homozygous parent
A A AA
gametes
heterozygous parent
A a aA
gametes
The Punnett Square Method
Allows prediction of both genotypes & phenotypes of genetic crosses
A
a
aA
Draw Punnett square with each row &
column labelled with one of possible
gametes of sperm & eggs respectively
Fill in genotype of offspring in each box by combining male & female gametes
A
A
A
A
a
a
a
a
AA Aa
aA aa
Count # offspring with each genotype & convert to fraction of total # offspring
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
AA = ¼
Aa = aA = 2/4 = ½
aa = ¼
A
A
a
a
AA Aa
aA aa
So, for Mendel’s cross of F1 offspring from monohybrid cross, he predicted:
F2 = ¼ AA, ½ Aa, ¼ aa
Phenotypic ratio = 3:1– ¼ AA + ½ Aa = ¾ dominant phenotype
– ¼ aa = ¼ recessive phenotype
A
A
a
a
AA Aa
aA aa
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?
P P PP PP
P p Pp
These three all look the
same!
P P pP
Pp
P p pp pp white
sperm eggsoffspring
genotypesgenotypic
ratio(1:2:1)
phenotypicratio(3:1)
12
12
12
12
12
12
12
12
14
14
14
14
14
24
14
14
Dominance creates some problems for
scientists
For example:
How can I know which genotype I
have if all I can see is phenotype?
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:
• Test crosses supported Mendel’s predictions
Mendel found that crossing F1 purple flowers with true-breeding white flowers:
½ F2 = purple (Aa), ½ F2 = white (aa)
F1 purple flowers were heterozygous
sper
m
pp
P
pp
Pp
ppall eggs
PP or Ppsperm unknown
if PP if Pp
egg egg
pollen
p
12
12
12
P
p
12
all Pp
sper
m
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
ab
AB
F1 = 100% AaBbAaBb
For P (AABB), gametes = AB
For P (aabb), gametes = ab
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
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
AB
Ab
aB
ab
abaBAbAB
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
abaBAbAB
AB
Ab
aB
ab
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
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
• syWhat would the zygotes look like? Use a Punnett Square.
SY
SY
SSYY SsYY
ssYY
ssyY
SsyY
SSYy SsYy
SsYy
ssyy
SsyySSyy
sSyY sSyy
sSYY sSYy
SSyY
sY
sY
sy
sy
Sy
Sy
eggs
self-fertilize
ssYy
14
14
14
14
14
14
14
14
sper
m
116
116
116
116
116
116
116
116
116
116
116
116
116
116
116
116
9/16 smooth yellow
3/16 smooth green
3/16 wrinkled yellow
1/16 wrinkled green
Phenotypic ratio:
9:3:3:1
P p
PP
P
p
pp
Pp
eggs
Ppself-fertilize
pP
12
12
12
12
14
sper
m
14
14
14
Remember that a monohybrid cross will
give you a 3:1 ratio
The 9:3:3:1 ratio is actually just two 3:1
ratios “stacked” on top of each other
seed shape (3:1)
seed color(3:1)
phenotypic ratio(9:3:3:1)
smooth yellow smooth yellow
smooth green
wrinkled yellow
wrinkled green
yellow
green
green
smooth
wrinkled
wrinkled
34
34
34
34
14
14
14
14
316
316
916
116
x
x
x
x =
=
=
=
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
SS s ss s
Y
S
Y Y Yy y y
S
y
Independent assortment produces four equally likely allele combinations during meiosis
SY sy Sy sY
meiosis II
meiosis I
S
S
S
S
s
s
s
s
Y
Y
Y
Y
y
y
y
y
chromosomesreplicate
SS
ss
Y
Yy
y
replicated homologuespair during metaphaseof meiosis I,orienting like this
or like this
pairs of alleles on homologouschromosomes in diploid cells
Ss
Yy
Mendel’s Big IdeasGenes 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
AABB AABb AaBB AaBb
AABb AAbb AaBb Aabb
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
AB
Ab
aB
ab
abaBAbAB
3n = 32 = 9 different genotypes
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?
Using Mendel’s Big Ideas
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 C DB
2 genes are closely linked when distance between them is small
= combos of alleles usually end up in same gamete
When far apart, crossing over is very frequent= genes independently assort into different
gametes
A C DB
Dependent Assortment
Genes on the same chromosome are linked
Their alleles tend to assort dependently
flower color gene
purple allele, P long allele, L
red allele, p round allele, l
pollen shape gene
sister chromatids
homologouschromosomes(duplicated)at meiosis I
sister chromatids
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
How can you ever get recombinant gametes?
Remember the events of Prophase I?
Crossing-over generates recombinant gametes
Dependent Assortment
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
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)
Mendel’s Big (Modified) Ideas
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
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
IA & IB have different forms of enzyme that attaches last sugar to glycolipid
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?
Using Multiple Alleles
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
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!
Modifying Mendel’s Big Ideas
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 anemiaNormal RBCs
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
Environmental Influence
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)
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
Polygenic Inheritance
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’)
R1R1R2R2
eggs
R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R2R2
R1R1R1R2
R1R1R2R2
sper
m
R1R2
R1R2
R1R2
R1R2
R1R2
R1R2R1R2R1R2
R1R1R2R2
R alleles = +1 to colourR’ alleles = +0 to colour
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
Becomes harder to classify phenotypes reliably
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?
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