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Mendelelian Genetics

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Gregor Mendel

(1822-1884)

Responsible for the Laws governing

Inheritance of Traits

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Gregor Johann MendelAustrian monkStudied the inheritance of traits in pea plantsHe found that the plants' offspring retained traits of the parentsCalled the “Father of Genetics"

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Mendel stated that physical traits are inherited as “particles”Mendel did not know that the “particles” were actually Chromosomes & DNA

Particulate Inheritance

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Genetic Terminology

Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity

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Types of Genetic CrossesMonohybrid cross - cross

involving a single traite.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height

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Punnett Square

Used to help solve genetics problems

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Designer “Genes”Alleles - two forms of a gene

(dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r)

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More Terminology

Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) (e.g. red, white)

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Genes and Environment Determine Characteristics

pH of soil can change the color of hydrangea plants

Temperature and season can change fur color of animals

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Generation “Gap”

Parental P1 Generation = the parental generation in a breeding experiment.

F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation)From breeding individuals from the P1 generation

F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation

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Following the Generations

Cross 2 Pure

PlantsTT x tt

Results in all

HybridsTt

Cross 2 Hybridsget

3 Tall & 1 ShortTT, Tt, tt

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Monohybrid Crosses

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Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Wrinkled seeds

RR x rr

P1 Monohybrid Cross

R

R

rr

Rr

RrRr

Rr

Genotype: Rr

Phenotype: Round

GenotypicRatio: All alike

PhenotypicRatio: All alike

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P1 Monohybrid Cross ReviewHomozygous dominant x

Homozygous recessiveOffspring all Heterozygous (hybrids)Offspring called F1 generation

Genotypic & Phenotypic ratio is ALL ALIKE

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Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Round seeds

Rr x Rr

F1 Monohybrid Cross

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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F1 Monohybrid Cross ReviewHeterozygous x heterozygousOffspring:

25% Homozygous dominant RR50% Heterozygous Rr25% Homozygous Recessive rrOffspring called F2 generationGenotypic ratio is 1:2:1Phenotypic Ratio is 3:1

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What Do the Peas Look Like?

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…And Now the Test Cross

Mendel then crossed a pure & a hybrid from his F2 generation

This is known as an F2 or test cross

There are two possible testcrosses:Homozygous dominant x HybridHomozygous recessive x Hybrid

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Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Round seeds x Round seeds

RR x Rr

F2 Monohybrid Cross (1st)

R

R

rR

RR

RrRR

Rr

Genotype: RR, Rr

Phenotype: Round

GenotypicRatio: 1:1

PhenotypicRatio: All alike

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Trait: Seed ShapeAlleles: R – Round r – WrinkledCross: Wrinkled seeds x Round seeds

rr x Rr

F2 Monohybrid Cross (2nd)

r

r

rR

Rr

rrRr

rr

Genotype: Rr, rr

Phenotype: Round & Wrinkled

G. Ratio: 1:1

P.Ratio: 1:1

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F2 Monohybrid Cross ReviewHomozygous x heterozygous

(hybrid)Offspring:50% Homozygous RR or rr50% Heterozygous RrPhenotypic Ratio is 1:1Called Test Cross because the offspring have SAME genotype as parents

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Results of Monohybrid Crosses

Inheritable factors or genes are responsible for all heritable characteristics

Phenotype is based on Genotype Each trait is based on two genes,

one from the mother and the other from the father

True-breeding individuals are homozygous ( both alleles) are the same

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Mendel’s Laws

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Law of Dominance

In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.

All the offspring will be heterozygous and express only the dominant trait.

RR x rr yields all Rr (round seeds)

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Law of Dominance

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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 different traits are distributed to sex cells (& offspring) independently of one another.

This law can be illustrated using dihybrid crosses.

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Test CrossA mating between an individual of unknown

genotype and a homozygous recessive individual.

Example: bbC__ x x bbcc

BB = brown eyesBb = brown eyesbb = blue eyes

CC = curly hairCc = curly haircc = straight hair

bC b___

bc

bc

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Test Cross

Possible results:

bCbC b___

bcbc bbCc bbCc

CC bC b___

bc bbCc bbccor

c

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Incomplete Dominanceand

Codominance

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Incomplete Dominance

F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.

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

RR = red flowerrr = white flower

R

R

r r

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Incomplete Dominance

RrRr

RrRr

RrRr

RrRr

R

R

r

All Rr = pink(heterozygous pink)

produces theF1 generation

r

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Incomplete Dominance

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CodominanceTwo alleles are expressed (multiple

alleles) in heterozygous individuals.Example: blood type

1. type A = IAIA or IAi2. type B = IBIB or IBi3. type AB= IAIB

4. type O = ii

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Codominance Problem

Example: homozygous male Type B (IBIB)

x heterozygous female 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: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 and phenotypes of their parents?

boy - type O (ii) X girl - type AB (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 sex chromosomes, X and Y

XX genotype for femalesXY genotype for malesMany sex-linked traits carried

on X chromosome

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Sex-linked Traits

Sex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye 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 males does not carry traits.

RR = red eyedRr = red eyedrr = white eyedXY = maleXX = female XR

Xr Xr

Y

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Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female50% white eyed

male

XR

Xr Xr

Y

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Female Carriers

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Dihybrid CrossA breeding experiment that tracks

the inheritance of two traits.

Mendel’s “Law of Independent Assortment”

a. Each pair of alleles segregates independently during gamete formation

b. Formula: 2n (n = # of heterozygotes)

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Question:How many gametes will be

produced for the following allele arrangements?

Remember: 2n (n = # of heterozygotes)

1. RrYy

2. AaBbCCDd

3. MmNnOoPPQQRrssTtQq

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Answer:1. RrYy: 2n = 22 = 4 gametes

RY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametesABCD ABCd AbCD AbCdaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

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Dihybrid CrossTraits: Seed shape & Seed colorAlleles: R round

r wrinkled Y 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

9:3:3:1 phenotypic ratio

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

9:3:3:1

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Next up, Pedigrees!