BASIC GENETICS

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BASIC GENETICS

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Gregor MendelGregor Mendel

Gregor MendelGregor Mendel

Austrian monk Austrian monk

Studied science and mathStudied science and math

High school teacher and High school teacher and gardenergardener

Experimented on pea plantsExperimented on pea plants

Father of Father of GeneticsGenetics

(1822-1884)(1822-1884)

We credit Mendel for forming the basis of genetics.We credit Mendel for forming the basis of genetics.

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A zygote inherits traits from bothboth parents.

HeredityHeredity is the passing of traits or is the passing of traits or characteristics from parents to offspring.characteristics from parents to offspring.

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Sexual Reproduction

Diploid zygote

2n++ ==1n

Haploid sperm

(gamete)

Haploid egg

(gamete)

1n

http://www.christiananswers.net/q-sum/zygote-human.jpg

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So how can we predict what the offspring will look like?

?

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Through Probability!!!

is the likelihoodlikelihood that a specific event will happen.

helps understand past events and the possibilitypossibility of future events.

ProbabilityProbability:

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What are the possible outcomes when you flip a coin?

compare the number of times a certain outcome can occur to the total number of possible outcomes.

write it as a fraction.

Probability =Probability = number of times one outcome is likely to occurnumber of times one outcome is likely to occur

total number of all possible outcomestotal number of all possible outcomes

To find the probability of an event… To find the probability of an event…

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Equal chance of 2 possible outcomes:

“Heads” is one possible outcome out of a total of 2 possible outcomes.

oror

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22Probability =Probability =

What is the probability of flipping a coin and What is the probability of flipping a coin and it landing heads up?it landing heads up?

What is the likelihood that a flipped What is the likelihood that a flipped coin will land heads up?coin will land heads up?

So…So…

What does this have to do with genetics?What does this have to do with genetics?

““tails”tails”““heads”heads”

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We use probability to predict possible We use probability to predict possible outcomes of genetic crosses.outcomes of genetic crosses.

Genetic crosses involve 2 Genetic crosses involve 2 independentindependent events events

Alleles contributed by one parentAlleles contributed by one parent

Because:Because:

PpPp

PP

PP

pppp

pp

Do not depend onDo not depend on

Alleles contributed by the otherAlleles contributed by the other

pp

So…So…

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We We multiplymultiply the separate probabilities the separate probabilities of the two events.of the two events.

=X 1

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PpPp pp

pp

We combine both probabilities. We combine both probabilities.

PpPp

pp

12 2

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What is the probability of two heterozygous What is the probability of two heterozygous purple individuals (Pp x Pp) producing a purple individuals (Pp x Pp) producing a white offspring (pp)?white offspring (pp)?

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12

1

4

““p” from one parent = p” from one parent = 12

““p” from other parent = p” from other parent = 12

The probability of these parents producing The probability of these parents producing a white offspring ? a white offspring ?

One chance in fourOne chance in four

??

??

??pp

1

4

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predict possible outcomes from genetic crosses.predict possible outcomes from genetic crosses.

Probability using Pedigrees Probability using Pedigrees and Punnett Squaresand Punnett Squares

simplify analysis of genetic simplify analysis of genetic probabilities.probabilities.

Pedigrees and Punnett squares are scientific tools used to…

P

P

p

p

pP

pP

P

p

P

p

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PedigrePedigreee A chart of a family's history

showing relationships and how a trait or disease has been inherited over many generations

Unknown Gender

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Squares Squares represent represent Males.Males.

Horizontal lines show Horizontal lines show mating relationships.mating relationships.

Circles Circles represent represent Females.Females.

Vertical lines and Vertical lines and brackets show brackets show

birth relationships.birth relationships.

Half-shaded circle or Half-shaded circle or square represents a square represents a carrier of the trait.carrier of the trait.

Shaded circles or squares Shaded circles or squares show a person show a person

expressing the trait.expressing the trait.

Unshaded circles or squares Unshaded circles or squares show a person that does not show a person that does not

express the trait.express the trait.

Pedigree Pedigree AnalysisAnalysis

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Punnett SquaresPunnett Squares

A diagram that shows the possible A diagram that shows the possible gene combinations that might gene combinations that might

result from a genetic crossresult from a genetic cross

P

P

p

p

PpPP

ppPp

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Punnett SquaresPunnett Squares

The letters in a Punnett square The letters in a Punnett square stand for stand for allelesalleles (one of a number (one of a number

of different forms of a gene).of different forms of a gene).

P

P

p

p

PpPP

ppPp

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We call the actual genetic make-up of an organism its genotype.

Genotype: PP

Genotype: Pp

Genotype: pp

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We call the appearance of an organism its phenotype.

Genotype: PPPhenotype: Purple

Genotype: PpPhenotype: Purple

Genotype: ppPhenotype: White

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The The genotypegenotype (the (the genesgenes))

is represented by letters is represented by letters such as Pp.such as Pp.

P

P

p

p

PpPP

ppPp

The The phenotypephenotype (like a (like a photographphotograph))is represented by a is represented by a

description such as description such as purplepurple..

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Offspring can be:Offspring can be:

An organism that has An organism that has two different alleles two different alleles for the same traitfor the same trait

HomozygousHomozygousfor a traitfor a trait

oror

An organism that has An organism that has two identical alleles for two identical alleles for a particular traita particular trait

HeterozygousHeterozygousfor a traitfor a trait

PpPP

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Homozygous DominantHomozygous Dominant

Genotype: PP

Phenotype: Purple flowers

HeterozygousHeterozygous

Genotype: Pp


Homozygous RecessiveHomozygous Recessive

Genotype: pp

Phenotype: White flowers

More Examples…More Examples…

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DominantDominant and and RecessiveRecessive AllelesAlleles

When 2 different alleles for the same trait occur together, When 2 different alleles for the same trait occur together, one may be expressed while the other is not expressed.one may be expressed while the other is not expressed.

Some traits are…Some traits are…

dominantdominantwhile others are

recessiverecessive

Homozygous DominantHomozygous Dominant

Genotype: PP


Homozygous RecessiveHomozygous Recessive

Genotype: pp

Phenotype: White flowers

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The dominant trait will be expressed if a The dominant trait will be expressed if a dominant allele is present.dominant allele is present.

Dominant alleles “overpower” recessive Dominant alleles “overpower” recessive alleles.alleles.

DominantDominant Alleles Alleles

If there is a If there is a dominantdominant allele and a allele and a recessiverecessive allele, the dominant trait will allele, the dominant trait will be the one that is expressed.be the one that is expressed.

Example: A genotype of Pp (Purple/white) Example: A genotype of Pp (Purple/white) shows a phenotype of Purple.shows a phenotype of Purple.

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The recessive trait is expressed only when The recessive trait is expressed only when the dominant allele is not present.the dominant allele is not present.

The allele that is overshadowed The allele that is overshadowed by a dominant alleleby a dominant allele

RecessiveRecessive Alleles Alleles

Expressed only if both alleles Expressed only if both alleles are recessiveare recessive

Example: A genotype of pp (white/white) Example: A genotype of pp (white/white) shows a phenotype of white.shows a phenotype of white.

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Pea plants have purple and white alleles for Pea plants have purple and white alleles for flower color.flower color.

The allele for The allele for purplepurple flowers is flowers is dominantdominant and the allele for and the allele for whitewhite flowers is flowers is recessive.recessive.

If the allele for purple flowers is present, the If the allele for purple flowers is present, the plant will produce purple flowers.plant will produce purple flowers.

DominantDominant and and RecessiveRecessive AllelesAlleles

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Complete DominanceComplete Dominance

P

P

p

p

PpPP

ppPp

1/4 (25%) HomozygousOffspring

(Genotype PP)(Phenotype Purple)

2/4 (50%) HeterozygousOffspring

(Genotype Pp)(Phenotype Purple)


(Genotype pp)(Phenotype White)

2 Heterozygous Parents


would produce

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P

P

p

p

PpPP

ppPp









would produce


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P

P

p

p

PpPP

ppPp









would produce


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P

P

p

p

PpPP

ppPp


Genotype PpPhenotype Purple

would produce








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Let’s try some examples with Mendel’s pea plants.Let’s try some examples with Mendel’s pea plants.

gg

GG

Gg

Green

Green

Yellow

GenotypePea pod color Phenotype

This is This is complete dominancecomplete dominance because yellow is because yellow is hidden in a heterozygous genotype.hidden in a heterozygous genotype.

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A cross of a homozygous green pea plant and a A cross of a homozygous green pea plant and a heterozygous green pea plant would yield:heterozygous green pea plant would yield:

Gg

GG

G g

G

G GG

Gg

all offspring with a phenotype of green, but all offspring with a phenotype of green, but ½ (50%) heterozygous (Gg) and ½ (50%) heterozygous (Gg) and

½ (50%) homozygous (GG).½ (50%) homozygous (GG).

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A cross of a homozygous yellow pea plant and a A cross of a homozygous yellow pea plant and a heterozygous green pea plant would yield:heterozygous green pea plant would yield:

gg

Gg

G g

g

g Gg

gg

½ (50%) offspring with a phenotype of green, genotype Gg ½ (50%) offspring with a phenotype of green, genotype Gg and ½ (50%) with a phenotype of yellow, genotype gg.and ½ (50%) with a phenotype of yellow, genotype gg.

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Let’s add another trait.Let’s add another trait.

rr

RR

Rr

Round

Round

Wrinkled

GenotypePea seed shape Phenotype

This is This is complete dominancecomplete dominance because “wrinkled” is because “wrinkled” is hidden in a heterozygous genotype.hidden in a heterozygous genotype.

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GGrr

A cross of two pea plants, both with heterozygous A cross of two pea plants, both with heterozygous green seed pods and heterozygous round seeds green seed pods and heterozygous round seeds

would yield:would yield:

GR

GgRr

GgRrGr

gR

gr

GR Gr gR gr

GGRR

GGRr

GgRR

GgRr

GGRr

GgRr

Ggrr

GgRR

GgRr

ggRR

ggRr

GgRr

Ggrr

ggRr

ggrr

There is only one chance in 16 that both recessive There is only one chance in 16 that both recessive traits will be expressed.traits will be expressed.

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Complex Patterns of

Heredity

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Incomplete DominanceProduce a

heterozygous offspring

with a blended

phenotype

BUT THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED.

Two homozygous parents

with different

phenotypesred + white = pink

r r + w w = r w

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Incomplete Dominance

r r

w

w

rw rw

rw rw

Crossing homozygous parents to produce F1

generation

THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED.

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r w

r

w

rr rw

rw ww

Crossing heterozygous F1 generation to produce F2

generationTHE ALLELES REMAINED DISTINCT; THE ORIGINAL PHENOTYPES REAPPEAR IN THE F2 GENERATION

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CodominanceBoth alleles in the

heterozygote express themselves fully.

Example: Blood types

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Codominance

A person homozygous for Type A blood

And a person homozygous for Type B blood

Will produce a child that will

demonstrate both Type A and Type B

blood (Type AB)

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Traits are blended

(red + white = pink)

CodominanceBoth traits are

expressed (A + B = AB)

In summary:

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PleiotropyA single gene affects more than one trait.

For example, sickle-cell disease results from one gene, but it has numerous effects on the

body.

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Polygenic Traits

A trait controlled by more than one gene.

Eye color is an example of a polygenic trait.

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PleitrophySingle gene affects more

than one trait.

Polygenic TraitSingle trait

controlled by more than one

gene.

In summary:

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CarrierCarrieris heterozygous for the trait.is heterozygous for the trait.

does not express the trait.does not express the trait.

can pass the trait to offspring.can pass the trait to offspring.

An individual who carries a recessive An individual who carries a recessive trait…trait…

XXC C XXcc

For Example:For Example: This female is heterozygous for colorblindness.

She has normal vision.

She can pass the trait on to her son who will be colorblind.

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Without sexual reproduction,we would have very little genetic variation.

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With sexual reproduction, there is great variety in the appearance of offspring.

BASIC GENETICS

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