Patterns of Inheritance Chapter 9. Overview Definitions Patterns of Mendelian 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 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

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

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

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

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

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

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

P P pP

P p pp pp white

sperm eggsoffspring

genotypesgenotypic

ratio(1:2:1)

phenotypicratio(3:1)

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

ppall eggs

PP or Ppsperm unknown

if PP if Pp

egg egg

pollen

all Pp

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

AABB x aabb AaBb

F1 heterozygous for alleles of both genes

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

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

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.

SSYY SsYY

SSYy SsYy

SsyySSyy

sSyY sSyy

sSYY sSYy

self-fertilize

9/16 smooth yellow

3/16 smooth green

3/16 wrinkled yellow

1/16 wrinkled green

Phenotypic ratio:

9:3:3:1

Ppself-fertilize

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

smooth

wrinkled

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 Y Yy y y

Independent assortment produces four equally likely allele combinations during meiosis

SY sy Sy sY

meiosis II

meiosis I

chromosomesreplicate

replicated homologuespair during metaphaseof meiosis I,orienting like this

or like this

pairs of alleles on homologouschromosomes in diploid cells

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

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

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

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

R1R1R2R2R1R1R2R2

R1R1R2R2

R1R1R1R2

R1R1R2R2

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

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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