Genetics.2012

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Genetics

copyright cmassengale 1

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Gregor Mendel(1822-1884)

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Reproduction in Flowering Plants

Pollen contains sperm

Produced by the stamen

Ovary contains eggs

Found inside the flower


Pollen carries sperm to theeggs for fertilization

Self-fertilization canoccur in the same flower

Cross-fertilization can

occur between flowers

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Mendels

Experiments


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Antagonistic Pairs

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Monohybrid

Crosses


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P1 Monohybrid Cross Trait: Seed Shape

Alleles: R

Round r

Wrinkled

Cross: Roundseeds x Wrinkled seeds

RR x rr


R

R

rr

Rr

RrRr

Rr

Genotype:Rr

Phenotype:Round

GenotypicRatio: All alike

Phenotypic

Ratio: All alike

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P1 Monohybrid Cross Review

Homozygous dominant x Homozygousrecessive

OffspringallHeterozygous(hybrids)

Offspring calledF1 generation

Genotypic & Phenotypic ratio isALLALIKE


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F1 Monohybrid Cross Trait: Seed Shape

Alleles: R

Round r

Wrinkled

Cross: Roundseeds x Round seeds

Rr x Rr


R

r

rR

RR

rrRr

Rr

Genotype:RR, Rr, rr

Phenotype:Round &wrinkled

G.Ratio: 1:2:1

P.Ratio: 3:1

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Mendels Laws


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Law of Segregation

During the formation of gametes (eggs or

sperm), the two alleles responsible for a

trait separate from each other.

Alleles for a trait are then "recombined"

at fertilization, producing the genotype for

the traits of the offspring.


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Applying the Law of Segregation

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Law of Independent Assortment

Alleles for differenttraits are

distributed to sex cells (& offspring)

independently of one another.

This law can be illustrated using

dihybrid crosses.


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Dihybrid Cross

A breeding experiment that tracksthe inheritance of two traits.

Mendels Law of IndependentAssortment

a. Each pair of alleles segregates

independently during gamete formation b. Formula: 2n (n = # of heterozygotes)


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Dihybrid Cross

Traits: Seed shape & Seed color

Alleles: R roundr wrinkledY yellow

y green


RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations

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Dihybrid Cross


RY Ry rY ry

RY

Ry

rY

ry

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Dihybrid Cross


RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic

ratio

RY Ry rY ry

RY

Ry

rY

ry

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Dihybrid Cross


Round/Yellow: 9

Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1

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Summary of Mendels laws


LAWPARENT

CROSSOFFSPRING

DOMINANCE TT x tttall x short

100% Tttall

SEGREGATIONTt x Tttall x tall

75% tall25% short

INDEPENDENTASSORTMENT

RrGg x RrGground & green

xround & green

9/16 round seeds & green

pods3/16 round seeds & yellow

pods3/16 wrinkled seeds & green

pods1/16 wrinkled seeds & yellow

pods

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Incomplete DominanceandCodominance


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Incomplete Dominance

F1 hybrids have an appearance somewhat inbetween the phenotypes of the two parentalvarieties.

Example: snapdragons (flower)

red (RR) x white (rr)

RR = red flower

rr = white flower


R

R

r r

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Rr

Rr

Rr

Rr

R

R

r

All Rr = pink(heterozygous pink)

produces theF1 generation

r

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Codominance

Two alleles are expressed (multiplealleles) in heterozygous individuals.

Example: blood type

1. type A = IAIA or IAi 2. type B = IBIB or IBi 3. type AB = IAIB 4. type O = ii


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Codominance Problem

Example: homozygous male Type B (IBIB)

x heterozygousfemale Type A (IAi)


IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

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Another Codominance Problem


Example: male Type O (ii)x

female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

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Codominance

Question:If a boy has a blood type O and

his sister has blood type AB,what are the genotypes andphenotypes of their parents?

boy - type O (ii) X girl - typeAB (IAIB)


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Codominance

Answer:


IAIB

ii

Parents:

genotypes = IAi and IBiphenotypes = A and B

IB

IA i

i

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Sex-linked Traits

Traits (genes) located on the sexchromosomes

Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carriedon X chromosome


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Sex-linked Traits


Sex Chromosomes

XX chromosome - female Xy chromosome - male

fruit fly

eye color

Example: Eye color in fruit flies

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Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed

female)XRY x XrXr

Remember: the Y chromosome in malesdoes not carry traits. RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female


XR

Xr Xr

Y

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Sex-linked Trait Solution:


XR Xr

XrY

XR Xr

XrY

50% red eyedfemale

50% white eyedmale

XR

Xr Xr

Y

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Female Carriers


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Genetic PracticeProblems


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Breed the P1 generation

tall (TT) x dwarf (tt) pea plants


T

T

t t

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Solution:


T

T

t t

Tt

Tt

Tt

Tt All Tt = tall(heterozygous tall)

produces theF1 generation

tall (TT) vs. dwarf (tt) pea plants

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Breed the F1 generation

tall (Tt) vs. tall (Tt) pea plants


T

t

T t

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Solution:

i ht l 41

TT

Tt

Tt

tt

T

t

T tproduces the

F2 generation1/4 (25%) = TT1/2 (50%) = Tt

1/4 (25%) = tt1:2:1 genotype3:1 phenotype

tall (Tt) x tall (Tt) pea plants

Genetics.2012

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