genetics mendel

7/27/2019 Genetics Mendel

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Genetics

Mendel and the Gene Idea


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Heredity

What genetic principles account for the transmission of

traits from parents to offspring?

One possible explanation of heredity is a blending

hypothesis - The idea that genetic material contributed by

two parents mixes in a manner analogous to the way blue

and yellow paints blend to make green

An alternative to the blending model is the particulate

hypothesis of inheritance: the gene idea - Parents pass on

discrete heritable units, genes


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Gregor Mendel

Fig. 2.2

Gregor Mendels

monastery garden.

Documented a particulate mechanism of inheritance

through his experiments with garden peas


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Mendelian Genetics

Gregor Johann Mendel (1822-1884)

Augustinian monk, Czech Republic

Foundation of modern genetics

Studied segregation of traits in the garden pea (Pisumsativum) beginning in 1854

Published his theory of inheritance in 1865.Experiments in Plant Hybridization

Mendel was rediscovered in 1902

Ideas of inheritance in Mendels time were vague. One

general idea was that traits from parents came togetherand blended in offspring. Thus, inherited informationwas predicted to change in the offspring, an idea thatMendel showed was wrong. Characters, or what wenow call alleles, were inherited unchanged. Thisobservation and the pattern of inheritance of thesecharacters gave us the first definition of a gene


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Themes of Mendels work

Variation is widespread in nature

Observable variation is essential for following

genes

Variation is inherited according to genetic laws

and not solely by chance

Mendels laws apply to all sexually reproducing

organisms


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Mendels Experimental, Quantitative Approach

Mendel used the scientific approach to identify two laws

of inheritance

Mendel discovered the basic principles of heredity by

breeding garden peas in carefully planned experiments

Mendel chose to work with the garden pea (Pisum

sativum)

Because they are available in many varieties, easy to

grow, easy to get large numbers

Because he could strictly control which plants mated

with which


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Crossing Pea Plants

1

5

4

3

2

Removed stamens

from purple flower

Transferred sperm-

bearing pollen from

stamens of white

flower to egg-

bearing carpel of

purple flower

Parental

generation(P)

Pollinated carpel

matured into pod

Carpel

(female)

Stamens

(male)

Planted seeds

from pod

Examined

offspring:

all purple

flowers

First

generation

offspring

(F1)


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Mendels experimental design

Statistical analyses:

Worked with large numbers of plants

counted all offspring

made predictions and tested them

Excellent experimentalist

controlled growth conditions

focused on traits that were easy to score

chose to track only those characters that varied

in an either-or manner


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Mendels Studied Discrete Traits


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Genetic Vocabulary

Character: a heritable feature, such as flower color

Trait: a variant of a character, such as purple or

white flowers

Each trait carries two copies of a unit of

inheritance, one inherited from the mother and theother from the father

Alternative forms of traits are called alleles


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Dominant

Recessive

Antagonistic traits


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Mendels experimental design

Mendel also made sure that he started his experiments with

varieties that were true-breeding

X

X

X X

X

X


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Genetic Vocabulary

Phenotypeobservable characteristic of an

organism

Genotypepair of alleles present in and individual

Homozygoustwo alleles of trait are the same

(YY or yy)

Heterozygoustwo alleles of trait are different

(Yy)

Capitalized traits = dominant phenotypes Lowercase traits= recessive phenotypes


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Genetic Vocabulary

Generations:

P = parental generation

F1 = 1st filial generation, progeny of the P generation

F2 = 2nd filial generation, progeny of the F1 generation

(F3 and so on)

Crosses:

Monohybrid cross = cross of two different true-breeding

strains (homozygotes) that differ in a single trait.

Dihybrid cross = cross of two different true-breeding

strains (homozygotes) that differ in two traits.


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Phenotype vs Genotype

Figure 14.6

3

1 1

2

1

Phenotype

Purple

Purple

Purple

White

Genotype

PP

(homozygous)

Pp

(heterozygous)

Pp

(heterozygous)

pp

(homozygous)

Ratio 3:1 Ratio 1:2:1


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Dominant & recessive alleles (Fig. 10.7):Phenotype vs Genotype


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Mendels Experimental Design

In a typical breeding experiment Mendel mated two

contrasting, true-breeding varieties, a process

called hybridization

The true-breeding parents are called the P

generation The hybrid offspring of the P generation are called

the F1 generation

When F1 individuals self-pollinate the F2generation is produced


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Mendels Observations When Mendel crossed contrasting, true-breeding white and purple flowered pea

plants all of the offspring were purple

When Mendel crossed the F1 plants, many of the plants had purple flowers, butsome had white flowers

A ratio of about three to one, purple to white flowers, in the F2 generation

P Generation

(true-breeding

parents) Purpleflowers

Whiteflowers

F1 Generation

(hybrids)

All plants had

purple flowers

F2 Generation

EXPERIMENT True-breeding purple-flowered pea plants and

white-flowered pea plants were crossed (symbolized by ). The

resulting F1hybrids were allowed to self-pollinate or were cross-

pollinated with other F1hybrids. Flower color was then observed

in the F2generation.

RESULTS Both purple-flowered plants and white-

flowered plants appeared in the F2generation. In Mendels

experiment, 705 plants had purple flowers, and 224 had white

flowers, a ratio of about 3 purple : 1 white.


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Mendels Rationale

In the F1 plants, only the purple trait was affecting

flower color in these hybrids Purple flower color was dominant, and white flower

color was recessive

Mendel developed a hypothesis to explain the 3:1inheritance pattern that he observed among the F2

offspring

There are four related concepts that are integral to

this hypothesis


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Heredity Concepts1. Alternative versions of genes account for variations in inherited

characters, which are now called alleles

2. For each character an organism inherits two alleles, one from eachparent, A genetic locus is actually represented twice

3. If the two alleles at a locus differ, the dominant allele determines theorganisms appearance

4. The law of segregation - the two alleles for a heritable characterseparate (segregate) during gamete formation and end up in different

gametes

Allele for purple flowers

Locus for flower-color gene

Homologous

pair of

chromosomes

Allele for white flowers


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Law of Segregation

Mechanism of gene transmission

Fertilization:

alleles unite

Gametogenesis:

alleles segregate


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Parental P0cross

F1cross

P P

p

p

P p

P

p

Determine the genotype and phenotype

Mendelian Genetics

Classical Punett's Square is a way to determine ways traitscan segregate


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Parental P0cross

F1cross

P P

p Pp Pp

p Pp Pp

P p

P

p


Mendelian Genetics Classical Punett's Square is a way to determine ways traits

can segregate


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Parental P0cross

F1cross


Mendelian Genetics

Classical Punett's Square is a way to determine ways traits can

segregate

P P

p Pp Ppp Pp Pp

P pP PP Pp

p Pp pp


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Mendels Law Of Segregation, Probability And The

Punnett SquareP Generation

F1Generation

F2Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:Genetic makeup:

Purple flowersPP

White flowerspp

Purple flowers

Pp

Appearance:

Genetic makeup:

Gametes:

Gametes:

F1sperm

F1eggs

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Each true-breeding plant of the

parental generation has identical

alleles, PPorpp.

Gametes (circles) each contain only

one allele for the flower-color gene.

In this case, every gamete produced

by one parent has the same allele.

Union of the parental gametes

produces F1hybrids having a Pp

combination. Because the purple-

flower allele is dominant, all

these hybrids have purple flowers.

When the hybrid plants produce

gametes, the two alleles segregate,

half the gametes receiving the P

allele and the other half thepallele.

3 : 1

Random combination of the gametes

results in the 3:1 ratio that Mendelobserved in the F2generation.

This box, a Punnett square, shows

all possible combinations of allelesin offspring that result from an

F1F1(PpPp) cross. Each square

represents an equally probable product

of fertilization. For example, the bottom

left box shows the genetic combination

resulting from a p egg fertilized by

a P sperm.


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Purple(Pp)Purple(PP)

Pp p p

P

P

P

p

F1generationAll purple

White(pp)

Purple(Pp)

Pp PpPPGametes

F2generation purple, white

GametesGametes

GametesPp

Pp PpPp

pp

Mendels Monohybrid Cross


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Smooth and wrinkled

parental seed strains

crossed.

Punnett square

F1 genotypes: 4/4 Ss

F1 phenotypes: 4/4

smooth


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Mendel Observed The Same Pattern In Characters


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The Testcross

In pea plants with purple flowers the genotype is

not immediately obvious

A testcross

Allows us to determine the genotype of an

organism with the dominant phenotype, butunknown genotype

Crosses an individual with the dominant

phenotype with an individual that is homozygous

recessive for a trait


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Test Cross

To determine whether an individual with a dominant phenotype is

homozygous for the dominant allele or heterozygous, Mendel crossedthe individual in question with an individual that had the recessive

phenotype:

PP

P P

pp p

p

Recessivephenotype

Dominant

Phenotype

Gametes

Gametes

Alternative 1Plant with

dominant phenotype is

homozygous

?Pp

P p

p

ppp

Recessivephenotype

?Alternative 2Plant with


heterozygousDominant

Phenotype

Gametes

Gametes


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Test Cross

To determine whether an individual with a dominant phenotype is

homozygous for the dominant allele or heterozygous, Mendel crossedthe individual in question with an individual that had the recessive

phenotype:

PP

If all offspring are purple;unknown plant ishomozygous.

P P

pp p

p

Pp

PpPp

Pp

Recessivephenotype

Dominant

Phenotype

Gametes

Gametes

Alternative 1Plant with


homozygous

?Pp

If half of offspring arewhite; unknown plantis heterozygous.

P p

p

ppp

pp

ppPp

PpRecessivephenotype

?Alternative 2Plant with


heterozygousDominant

Phenotype

Gametes

Gametes


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The Testcross

Dominant phenotype,

unknown genotype:

PP or Pp?

Recessive phenotype,

known genotype:

pp

If PP,

then all offspring

purple:

If Pp,

then 12offspring purple

and 12offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous for

the dominant allele or heterozygous. To determine the organisms

genotype, geneticists can perform a testcross.

TECHNIQUE In a testcross, the individual with the

unknown genotype is crossed with a homozygous individual

expressing the recessive trait (white flowers in this example).

By observing the phenotypes of the offspring resulting from thiscross, we can deduce the genotype of the purple-flowered

parent.

RESULTS


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The Law of Independent Assortment

Mendel derived the law of segregation by following a single trait

2 alleles at a single gene locus segregate when the gametes areformed

The F1 offspring produced in this cross were monohybrids,heterozygous for one character

Mendel identified his second law of inheritance by following twocharacters at the same time

Mendel was interested in determining whether alleles at 2 differentgene loci segregate dependently or independently

Crossing two, true-breeding parents differing in two charactersproduces dihybrids in the F1 generation, heterozygous for bothcharacters


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Dihybrid Cross

With his monohybrid crosses, Mendel determined

that the 2 alleles at a single gene locus segregate

when the gametes are formed.

With his dihybrid crosses, Mendel was interested in

determining whether alleles at 2 different gene locisegregate dependently or independently.


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Dihybrid Cross

For example, in pea plants seed shape is

controlled by one gene locus where round (R) is

dominant to wrinkled (r) while seed color is

controlled by a different gene locus where yellow

(Y) is dominant to green (y). Mendel crossed 2 pure-breeding plants: one with

round yellow seeds and the other with green

wrinkled seeds.


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For example, in pea plants seed shape is

controlled by one gene locus where round (R) is

dominant to wrinkled (r) while seed color is

controlled by a different gene locus where yellow

(Y) is dominant to green (y). Mendel crossed 2 pure-breeding plants: one with

round yellow seeds and the other with green

wrinkled seeds.

Dihybrid Cross


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R RY Y

R rY y

R r

Y y

r r

y y

R

Y

RY

F2With Dependent Assortment:

ry

r

y

Ratio is 3 round, yellow : 1 wrinkled, green


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Mendelian Genetics

Dihybrid cross - parental generation differs in two traits

example-- cross round/yellow peas with wrinkled/green ones

Round/yellow is dominant

RY Ry rY ry

RY

Ry

rY

ry

What are the expected phenotype ratios in the F2generation?

round, yellow = round, green =

wrinkled, yellow = wrinkled, green =


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RR

YY

RR

Yy

Rr

YY

Rr

Yy

RR

Yy

RR

yy

Rr

Yy

Rr

yy

Rr

YY

Rr

Yy

rr

YY

rr

Yy

Rr

Yy

Rr

yy

rr

Yy

rr

yy

RY

RY

ry

ry

Phenotypic ratio is 9 : 3 : 3 : 1

F2with independent assortment:

Ry

rY

rYRy

A Dih brid Cross


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YYRRP Generation

Gametes YR yr

yyrr

YyRrHypothesis of

dependent

assortment

Hypothesis of

independent

assortment

F2Generation(predicted

offspring)

12YR

YR

yr

1

2

12

12 yr

YYRR YyRr

yyrrYyRr

34 14

Sperm

Eggs

Phenotypic ratio 3:1

YR14

Yr14

yR14

yr14

916316

316116

YYRR YYRr YyRR YyRr

YyrrYyRrYYrrYYrr

YyRR YyRr yyRR yyRr

yyrryyRrYyrrYyRr

Phenotypic ratio 9:3:3:1

315 108 101 32 Phenotypic ratio approximately 9:3:3:1

F1Generation

Eggs

YR Yr yR yr14 1414 14

Sperm

CONCLUSION The results support the hypothesis

ofindependent assortment. The alleles for seed color

and seed shape sort into gametes independently of

each other.

EXPERIMENT Two true-breeding pea plants

one with yellow-round seeds and the other with

green-wrinkled seedswere crossed, producingdihybrid F1plants. Self-pollination of the F1dihybrids,

which are heterozygous for both characters,

produced the F2generation. The two hypotheses

predict different phenotypic ratios. Note that yellow

color (Y) and round shape (R) are dominant.

A Dihybrid Cross How are two characters transmitted from parents to offspring?

As a package?

Independently?

A dihybrid cross Illustrates the inheritance of two characters

Produces four phenotypes in the F2 generation


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Law of Independent Assortment

Mendels dihybrid crosses showed a 9:3:3:1

phenotypic ratio for the F2 generation.

Based on these data, he proposed the Law of

Independent Assortment, which states that when

gametes form, each pair of hereditary factors(alleles) segregates independently of the other

pairs.


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Laws Of Probability Govern Mendelian Inheritance

Mendels laws of segregation and independent

assortment reflect the rules of probability The multiplication rule

States that the probability that two or moreindependent events will occur together is the product

of their individual probabilities The rule of addition

States that the probability that any one of two or moreexclusive events will occur is calculated by adding

together their individual probabilities

Of


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Laws Of Probability - Multiplication Rule

The probability of two or more independent events

occurring together is the product of the probabilities thateach event will occur by itself

Following the self-hybridization of a heterozygous purplepea plants (Pp), what is the probability that a given

offspring will be homozygous for the production of whiteflowers (pp)?

Probability that a pollen seed will carry p:

Probability that an egg will carry p:

Probability that the offspring will be pp:

1/2 X 1/2 = 1/4

L Of P b bilit Additi R l


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Laws Of Probability - Addition Rule

The probability of either of two mutually exclusive events

occurring is the sum of their individual probabilities

Following the self-hybridization of a heterozygous purplepea plant (Pp), what is the probability that a given offspring

will be purple?

Probability of maternal Puniting with paternal P: 1/4

Probability of maternal p uniting with paternal P: 1/4

Probability of maternal Puniting with paternal p: 1/4

Probability that the offspring will be purple:

1/4 + 1/4 + 1/4 = 3/4


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M d l l i


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Mendels conclusions

Genes are distinct entities that remain

unchanged during crosses

Each plant has two alleles of a gene

Alleles segregated into gametes in equal

proportions, each gamete got only one allele

During gamete fusion, the number of alleles

was restored to two


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E ti T M d l O i i l P i i l


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Exceptions To Mendels Original Principles

Incomplete dominance

Codominance

Multiple alleles

Polygenic traits

Epistasis

Pleiotropy

Environmental effects on

gene expression

Linkage

Sex linkage


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P l i T it


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Polygenic Traits

Most traits are not controlled by a single gene locus, but by

the combined interaction of many gene loci. These arecalled polygenic traits.

Polygenic traits often show continuous variation, rather thena few discrete forms:

Epistasis


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Epistasis

Type of polygenic inheritance where the alleles at one gene

locus can hide or prevent the expression of alleles at asecond gene locus.

Labrador retrievers one gene locus affects coat color by

controlling how densely the pigment eumelanin is deposited

in the fur. A dominant allele (B) produces a black coat while the

recessive allele (b) produces a brown coat

However, a second gene locus controls whether any

eumelanin at all is deposited in the fur. Dogs that arehomozygous recessive at this locus (ee) will have yellow fur

no matter which alleles are at the first locus:

E i t i


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58Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ee

No dark p igment in fur

eebb eeB_

Yellow fur Yellow fur

E_Dark pigment in fur

E_bb E_B_

Brown fur Black fur

Epistasis


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Environmental Effects on Gene Expression


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Environmental Effects on Gene Expression

The phenotype of an organism depends not only on which

genes it has (genotype), but also on the environment underwhich it develops.

Although scientists agree that phenotype depends on a complex interaction betweengenotype and environment, there is a lot of debate and controversy about the relativeimportance of these 2 factors, particularly for complex human traits.


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