1 mendelelian genetics copyright cmassengale. 2 gregor mendel ( 1822-1884) responsible for the laws...

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Mendelelian Genetics

copyright cmassengale

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Gregor Mendel(1822-1884)

Responsible for the Laws governing

Inheritance of Traits


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Gregor Johann Mendel

Austrian monkStudied the inheritance of traits in pea plantsDeveloped the laws of inheritanceMendel's work was not recognized until the turn of the 20th century


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Gregor Johann Mendel

Between 1856 and 1863, Mendel cultivated and tested some 28,000 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 Crosses

Monohybrid 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


10copyright cmassengale

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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)


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Genotype & Phenotype in Flowers

Genotype of alleles:R = red flowerr = yellow flowerAll genes occur in pairs, so 2 alleles affect a characteristicPossible combinations are:

Genotypes RR  Rr rrPhenotypes RED RED  YELLOW


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Genotypes

Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid


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Eight Pea Plant Traits

Seed shape --- Round (R) or Wrinkled (r)Seed Color ---- Yellow (Y) or Green (y)

Pod Shape --- Smooth (S) or wrinkled (s)Pod Color --- Green (G) or Yellow (g)Seed Coat Color ---Gray (G) or White (g)Flower position---Axial (A) or Terminal (a)Plant Height --- Tall (T) or Short (t)

Flower color --- Purple (P) or white (p)


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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: RrPhenotype: RoundGenotypicRatio: All alikePhenotypicRatio: All alike


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P1 Monohybrid Cross Review

Homozygous dominant x Homozygous recessiveOffspring all Heterozygous (hybrids)Offspring called F1 generationGenotypic & 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, rrPhenotype: Round & wrinkledG.Ratio: 1:2:1P.Ratio: 3:1


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F1 Monohybrid Cross Review

Heterozygous 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 generationThis is known as an F2 or test crossThere 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, RrPhenotype: RoundGenotypicRatio: 1:1PhenotypicRatio: 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, rrPhenotype: Round & WrinkledG. Ratio: 1:1P.Ratio: 1:1


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F2 Monohybrid Cross Review

Homozygous 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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Practice Your Crosses

Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits


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Mendel’s Laws


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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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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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Dihybrid Cross

A breeding experiment that tracks the inheritance of two traits.Mendel’s “Law of Independent Assortment”a. Each pair of alleles segregates independently during gamete formationb. 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 gametes ABCD ABCd AbCD AbCd aBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes


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Dihybrid Cross

Traits: 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


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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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Test CrossA 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

bC b___

bc


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Test Cross

Possible results:

bC b___

bc bbCc bbCc

C bC b___

bc bbCc bbccor

c


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Summary of Mendel’s lawsLAW PARENT

CROSSOFFSPRING

DOMINANCE TT x tt tall x short

100% Tt tall

SEGREGATION Tt x Tt tall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGg round & green x round & green

9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods


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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 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 (multiple alleles) 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    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: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi

1/2 = 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 chromosomesSex chromosomes are X and YXX 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 XrXrRemember: 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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Genetic Practice Problems


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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:

TT

Tt

Tt

tt

T

t

T tproduces theF2 generation1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt1:2:1 genotype 3:1 phenotype

tall (Tt) x tall (Tt) pea plants


1 mendelelian genetics copyright cmassengale. 2 gregor mendel ( 1822-1884) responsible for the laws...

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