Genetics 2

GeneticsGenetics

Heredity and GeneticsHeredity and Genetics• Heredity is the passing of

traits from parents to offspring

• Traits are controlled by genes, so GENETICS is the study of how traits are inherited through the action of alleles

• Austrian monk born in 1822 who is responsible

for the laws governing the inheritance of traits

• Between 1856 and 1863, Mendel cultivated and tested over 28,000 pea plants

• Mendel performed cross-pollination by transferring pollen from one plant to selected ova of other plants, thereby controlling which plants mixed

Gregor Mendel – “Father of Genetics”

Mendel’s ExperimentsMendel’s Experiments• Mendel produced pure strains by allowing

plants to self-pollinate, I.e., pollen (male gamete) from one plant fertilizes ova (female gamete) of same plant

Mendel’s Peas….Mendel’s Peas….

S

G

iI

yY

s

g

Mendel’s Peas….Mendel’s Peas….

T t

a

p

A

P

Mendel’s Results….Mendel’s Results….(F2 from crossing to heterozygous F1s)

Genetics TerminologyGenetics Terminology• Traits – any characteristic that can be passed from

parents to offspring• Heredity – the passing of traits from parents to

offspring• Alleles – one form (dominant or recessive) of a gene

• Sex cells have ONE form of a gene on their chromosomes

• Body cells have TWO forms or alleles for a single gene (you get one from mom and the other from dad)

• Dominant – is always expressed; masks a recessive trait

• Recessive – can only be expressed if there are no dominant alleles present

Genetics TerminologyGenetics Terminology• Dominant alleles are represented by a capital letter• Recessive alleles are represented by a lower case letter

Example: B = Brown eye color (dominant) b = Blue eye color (recessive)

• “Purebred” species have two of the same alleles; this is also called HOMOZYGOUS, e.g., BB (homozygous dominant) or bb (homozygous recessive)

• Species with two different alleles are called HETEROZYGOUS, e.g., Bb

• Genotype: the alleles present in the organism, i.e., BB, Bb, or bb

• Phenotype: the expression of the genes; what is observed

Genotype and Genotype and Phenotype in FlowerPhenotype in Flower

• All genes occur in pairs, so TWO alleles affect a trait.

• Possible combinations if:R = Red flowerr = Yellow flower

• Genotypes RR Rr rr• Phenotypes Red Red Yellow

Genetic CrossesGenetic Crosses• Monohybrid Cross: cross involving ONE

trait, e.g., eye color • Dihybrid Cross: cross involving TWO traits,

e.g., eye color and hair color• Offspring’s genotype and phenotype is

determined using a Punnett square

B

b

B

b

BB

bbBb

Bb

BrainPop - Heredity

Punnett Square, contPunnett Square, cont.

Generations in CrossesGenerations in Crosses• Parental Generation (P1) = the parental

generation in a breeding experiment• First Filial Generation (F1) = the first

generation of offspring in a breeding experiment

• Second Filial Generation (F2) = the second generation of offspring in a breeding experiment

TT Tttt TtTTTtTt tt

P1 Monohybrid CrossP1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds

YY X yy Crossing two true-breeding (pure)

plants

Yy Yy

Yy Yy

y

y

Y YGenotype: Yy

Phenotype: Yellow

Genotypic Ratio: 100% Yy

Phenotypic Ratio: 100% Yellow

F1 Monohybrid CrossF1 Monohybrid Cross• Trait: Seed Color• Alleles: Y – Yellow y – Green• Cross: Yellow seeds X Green seeds

Yy X YyCrossing to heterozygotes (hybrids)

YY Yy

Yy yy

Y

y

Y yGenotype: YY, Yy, yy

Phenotype: Yellow and Green

Genotypic Ratio: 25% YY, 50% Yy, 25% yy (1:2:1)

Phenotypic Ratio: 75% Yellow, 25% Green (3:1)

Dihybrid CrossesDihybrid Crosses• A breeding experiment that tracks the

inheritance of two traits• Mendel’s “Law of Independent Assortment”

• Each pair of alleles segregates independently during gamete formation

• Formula: 2n (n = # of heterozygotes)Example:

1. RrYy: 2n = 22 = 4 possible gametes RY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametes ABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD

Dihybrid CrossesDihybrid CrossesTraits: Seed shape & Seed colorTraits: Seed shape & Seed colorAlleles:Alleles: R round r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ryRY Ry rY ry RY Ry rY ryRY Ry rY ry

All possible gamete combinationsAll possible gamete combinations

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Dihybrid CrossDihybrid Cross

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry

copyright cmassengale

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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

RYRY RyRy rYrY ryry

RYRY

RyRy

rYrY

ryry


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Round/Yellow: 9Round/green: 3wrinkled/Yellow: 3wrinkled/green: 1

9:3:3:1


Test CrossTest Cross

Yy Yy

Yy Yy

y

y

Y YYy yy

Yy yy

•Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous yY

y

y

Offspring all yellow!

½ Offspring yellow; ½ Offspring green!

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Test CrossTest Cross• A mating between an individual of unknown

genotype and a homozygous recessive individual.• Example: bbC__ x bbcc

• BB = brown eyes• Bb = brown eyes• bb = blue eyes

• CC = curly hair• Cc = curly hair• cc = straight hair

bCbC b___b___

bcbc


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Test CrossTest Cross

Possible results:Possible results:bCbC b___b___

bcbc bbCc bbCc

C bCbC b___b___

bcbc bbCc bbccor

c


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Incomplete DominanceIncomplete Dominanceandand

CodominanceCodominance


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Incomplete DominanceIncomplete Dominance• F1 hybrids have an appearance somewhat in

between the phenotypes of the two parental varieties.

• Example: snapdragons (flower)red (RR)red (RR) x x white (rrwhite (rr)

RR = red flowerRR = red flowerrr = white flower

R

R

r r


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Incomplete DominanceIncomplete Dominance

RrRr

RrRr

RrRr

RrRr

RR

RR

rr

AllAll Rr Rr = = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

r


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Incomplete DominanceIncomplete Dominance


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CodominanceCodominance• Two alleles are expressed (multiple Two alleles are expressed (multiple

alleles) in heterozygous individuals.alleles) in heterozygous individuals.• Example: blood typeExample: blood type

• 1.1. type Atype A = I= IAAIIAA or I or IAAii• 2.2. type Btype B = I= IBBIIBB or I or IBBii• 3.3. type ABtype AB= I= IAAIIBB

• 4.4. type Otype O = ii= ii


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Codominance ProblemCodominance Problem• Example:homozygous male Type B (IBIB)

xheterozygous female Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB


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Another Codominance ProblemAnother Codominance Problem

• Example: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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•Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?

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


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• Answer:Answer:

IAIB

ii

Parents:Parents:genotypesgenotypes = IAi and IBiphenotypesphenotypes = A and B

IB

IA i

i


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Sex-linked TraitsSex-linked Traits• Traits (genes) located on the sex

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

on X chromosome


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Sex-linked TraitsSex-linked Traits

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye color

Example: Example: Eye color in fruit Eye color in fruit fliesflies


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Sex-linked Trait ProblemSex-linked Trait Problem

• Example: Eye color in fruit flies• (red-eyed male) x (white-eyed female)

XRY x XrXr

• Remember: the Y chromosome in males does 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:Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y


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Female CarriersFemale Carriers


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Genetics 2