Chapter 9

• 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
D
Bk
M locus: leaf colour
Both alleles are the same = homozygous
D locus: plant height
Both alleles are the same = homozygous
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
F
1
= 1 st -generation offspring
F
2
= 2 nd -generation offspring of self-fertilized or crossed (mated) F
1 individuals

Hereditary material from both parents mixed at fertilization e.g. red flowers + white flowers = pink flower offspring
+
Couldn’t explain obvious variation in traits

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

Monohybrid cross:
2 homozygous parents that differ in trait dictated by alleles of 1 gene
P F
1
AA x aa

Aa

After Mendel tracked 7 traits for 2 generations, he found that:
F
2
: 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

A A homozygous parent
A gametes
A

A a heterozygous parent
A gametes a

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 a
AA Aa 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 aa = ¼
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 F
1 offspring from monohybrid cross, he predicted:
A a
A a
AA Aa aA aa
F
2
= 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

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 F
1 generation look like?
What would the F
2 look like?

sperm eggs
Dominance creates some problems for
1
2 scientists
P
1
2
P offspring genotypes
1
4
PP
For example:
1 1
2
P
2 p
How can I know which genotype I have if all I can see
P P
2
1
4
1
4
Pp pP genotypic ratio
(1:2:1) phenotypic ratio
(3:1)
1
4
PP
2
4
Pp
These three all look the same!
1
2
P
1
2 p
1
4 pp
1
4 pp
1
4 white

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
Mendel found that crossing F
1 purple flowers with truebreeding white flowers: if PP
PP or Pp sperm unknown p egg if Pp pp all eggs p p egg
½ F
2
= purple (Aa), ½ F
2 white (aa)
=
P

F
1 purple flowers were heterozygous all Pp
1
2
P
1
2 p
1
2
Pp
1
2 pp

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?

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

2n cells have pairs of genes on pairs of homologous chromosomes
Members of each gene pair separate during meiosis & end up in different gametes

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”

Dihybrid cross:
True-breeding homozygous parents that differ in 2 traits dictated by alleles of 2 genes
P F
1
AABB x aabb

AaBb
F
1 heterozygous for alleles of both genes

For P (AABB), gametes = AB
For P (aabb), gametes = ab
AB ab AaBb
F
1
= 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 F
1 cells, there are 4 possible combos of alleles in sperm or eggs:
1/4 AB, Ab, aB, ab
With 4 different sperm & egg types, F
2 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
AB Ab aB ab
AB
AABB AABb AaBB AaBb
9/16 tall, purple
AABb AAbb AaBb Aabb
3/16 dwarf, purple
3/16 tall, white
1/16 dwarf, white
Ab aB ab
AaBB AaBb aaBB aaBb
AaBb Aabb aaBb aabb
Phenotypic ratio = 9:3:3:1

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) F
1 plants, Mendel allowed them to self-fertilize
We could write the F
1 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 .

1
4
SY
SsYy self-fertilize
1
4
Sy eggs
1
4 sY
1
4 sy
1
4
SY
1
16 SSYY
1
4
Sy
1
16 SSyY
1
4 sY
1
16 sSYY
1
4 sy
1
16 sSyY
1
16 SSYy
1
16 SSyy
1
16 sSYy
1
16 sSyy
1
16 SsYY
1
16
1
16
1
16
SsyY ssYY ssyY
1
16
1
16 Ssyy
1
16 ssYy
1
16
SsYy ssyy
9/16 smooth yellow
3/16 smooth green
3/16 wrinkled yellow
1/16 wrinkled green
Phenotypic ratio:
9:3:3:1

1
2
P
Pp self-fertilize eggs
1
2 p
1
2
P
1
4 PP
1
2 p
1
4 pP
1
4
1
4
Pp pp
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)
3
4 smooth x
3
4
1
4 smooth wrinkled
1
4 wrinkled x x x
1
4
3
4
1
4
3
4 seed color
(3:1) yellow = green yellow green
=
=
= phenotypic ratio
(9:3:3:1)
9
16 smooth yellow
3
16
3
16 smooth green wrinkled yellow
1
16 wrinkled green

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 s
Y y pairs of alleles on homologous chromosomes in diploid cells
S s
Y y chromosomes replicate replicated homologues pair during metaphase of meiosis I, orienting like this or like this
S s meiosis I y
Y
S
S
Y
Y s s y y
S
S y y s s
Y
Y meiosis II
S
Y
S
Y s y s y
S y
S y s
Y s
Y
SY sy Sy sY
Independent assortment produces four equally likely allele combinations during meiosis

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

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 = 3 n where n = # gene pairs
More pairs = more genotypes

3 n = 3 2 = 9 different genotypes
AB Ab aB ab
AB AABB AABb AaBB AaBb
Ab
AABb AAbb AaBb Aabb aB
AaBB AaBb aaBB aaBb ab
AaBb Aabb aaBb aabb

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?

Imagine you have mated a true-breeding tall plant with round seeds to a true-breeding dwarf plant with wrinkled seeds. In the F
1 generation, the plants are all tall with round seeds.
What alleles are dominant in this case?
Now imagine you have mistakenly mixed these
F
1 plants with some true-breeding tall round plants. What kind of cross do you need to do to tell the plants apart?

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.

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

Genes on the same chromosome are linked
Their alleles tend to assort dependently

sister chromatids flower color gene pollen shape gene purple allele, P long allele, L homologous chromosomes
(duplicated) at meiosis I sister chromatids red allele, p round allele,
Copyright © 2005 Pearson Prentice Hall, Inc.
l

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

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 F
1
?

You now mate one of your black rough F
1 to a white smooth one.
guinea pigs
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 F
1 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...

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 looked at traits that were either dominant or recessive
Some traits do not follow these clear patterns

Pair of non-identical alleles expressed at same time in heterozygotes

RBCs have membrane glycolipid that differentiate between types
Structure of glycolipid determined by enzyme
3 alleles code for enzyme: I A , I B , i
= multiple allele system

I A & I B are codominant when paired i is recessive when paired with I A & I B

I A & I B have different forms of enzyme that attaches last sugar to glycolipid
I A I A or I A i = 1 type of sugar = blood type A
I B I B or I B i = other type of sugar = blood type B
I A I B = both sugars = blood type AB ii = no sugar = blood type O

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?

1 allele not fully dominant, so both expressed in heterozygotes
Phenotype is somewhere between 2 homozygotes

True-breeding P red flowers & white flowers produce F
1 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!

More than 1 gene affects 1 given trait e.g. coat colour in labs determined by 2 genes (E/e & B/b)

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

Normal 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 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)

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

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

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
R
1
R
2
R
1
R
2

R
1
R
1

R
2
R
2
 eggs
R
1

R
2
R
1

R
2

R
1
R
2
R
1
R
1
R
2
R
2
R
1
R
1
R
2
R
2

R
1
R
1

R
2
R
2
R
1
R
1

R
2
R
2

R
1
R
2

R
1
R
1
R
2
R
2

R
1
R
1
R
2

R
2

R
1
R
1

R
2
R
2

R
1
R
1

R
2

R
2

R
1
R
1

R
2
R
2

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1

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2
R
1
R
1

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2
R
2
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1
R
1

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2
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2

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1
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1

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2
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2
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1

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1
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2
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2
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1
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2
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1
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1
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1
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2
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1
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1
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2
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2
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1
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1
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2
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2
 R
1

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1

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2

R
2


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

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?

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?