Mendelelian Genetics · 2012-02-07 · Gregor Johann Mendel Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until

Mendelian Genetics 1/6/2011

G. Podgorski, Biol 1010 1

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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 Mendel

�Austrian monk

�First to study characteristics of living things in successive generation


Gregor Johann Mendel�Studied the inheritance of traits in pea plants

�Developed the laws of inheritance

� Mendel'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 plants�He found that the plants' offspring retained traits of the parents�Called the “Father of Genetics"


Why Peas?

Quick reproductive cycle

Self-pollinating

He studied simple traits, traits that occurred in only one of two possible variations.



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Site of Gregor Mendel’s experimental garden in the Czech Republic


�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


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

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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 alowercase letter (r)


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 flower

All genes occur in pairs, so 2alleles affect a characteristic

Possible combinations are:

Genotypes RR Rr rr

Phenotypes RED RED YELLOW


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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Genes and Environment Determine Characteristics


Mendel’s Pea Plant Experiments




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Why peas, Pisum sativum?

�Can be grown in a small area�Produce lots of offspring �Produce pure plants when allowed to self-pollinateseveral generations �Can be artificially cross-pollinated


Reproduction in Flowering Plants

Pollen contains spermProduced by the stamen

Ovary contains eggsFound inside the flower

Pollen carries sperm to the eggs for fertilization

Self-fertilization can occur in the same flower

Cross-fertilization can occur between flowerscopyright cmassengale

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Mendel’s Experimental Methods

Mendel hand-pollinatedflowers using a paintbrush

He could snip the stamens to prevent self-pollination

Covered each flower with a cloth bag

He traced traits through the several generations


How Mendel BeganMendel produced purestrains by allowing the plants to self-pollinatefor several generations


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Eight Pea Plant Traits

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

Pod Shape --- Smooth (S) or wrinkled (ssss))))

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 (pppp)copyright cmassengale 24copyright cmassengale



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Mendel’s Experimental Results


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Did the observed ratio match the theoretical ratio?

The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled

Mendel’s observed ratio was 2.96:1

The discrepancy is due to statistical error

The larger the sample the more nearly the results approximate to the theoretical ratio


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 P1generation

F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation)From breeding individuals from the F1generation


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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


Monohybrid Crosses




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Trait: Seed Shape

Alleles: R – Round r – Wrinkled

Cross: 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


P1 Monohybrid Cross Review

� Homozygous dominant x Homozygous recessive

� Offspring all Heterozygous(hybrids)

� Offspring called F1 generation

� Genotypic & Phenotypic ratio is ALL ALIKE


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Trait: Seed Shape


Cross: 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


F1 Monohybrid Cross Review

� Heterozygous x heterozygous� Offspring:25% Homozygous dominant RR50% Heterozygous Rr25% Homozygous Recessive rr

� Offspring called F2 generation� Genotypic ratio is 1:2:1� Phenotypic Ratio is 3:1


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What Do the Peas Look Like?


…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 Shape


Cross: 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


Trait: Seed Shape


Cross: 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 Review

� Homozygous x heterozygous(hybrid)� Offspring:50% Homozygous RR or rr50% Heterozygous Rr

� Phenotypic Ratio is 1:1� Called Test Cross because the offspring have SAME genotype as parents


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


Results of Monohybrid Crosses

Inheritable factors or genes are responsible for all heritable characteristics

Phenotype is based on GenotypeEach 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)


Law of Dominance


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Law of Segregation

During the formation of gametes(eggs or sperm), the two allelesresponsible for a trait separatefrom each other.

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.


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.


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 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. MmNnOoPPQQRrssTtQqcopyright cmassengale 50

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 CrossTraits: Seed shape & Seed colorAlleles: R round

r wrinkledY yellowy green

RrYy x RrYy

RY Ry rY ry RY Ry rY ry

All possible gamete combinations


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 phenotypicratio

RY Ry rY ry

RY

Ry

rY

ry


Dihybrid Cross

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

9:3:3:1




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Test CrossA mating between an individual of unknown genotype and a homozygous recessiveindividual.

Example: bbC__ x bbcc

BB = brown eyesBb = brown eyesbb = blue eyes

CC = curly hairCc = curly haircc = straight hair

bC b___

bc


Test Cross

Possible results:

bC b___

bc bbCc bbCc

C bC b___

bc bbCc bbccor

c


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Summary of Mendel’s laws

LAWPARENT CROSS

OFFSPRING

DOMINANCE TT x tttall x short

100% Tt tall

SEGREGATIONTt x Tttall x tall

75% tall 25% short

INDEPENDENT ASSORTMENT

RrGg x RrGground & 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


Mendel’s Laws of Heredity

•Hereditary factors do not combine, but are passed intact (Law of Segregation)•Each member of the parental generation transmits only half of its hereditary factors to each offspring (with certain factors "dominant" over others) (Law of Dominance)•Different offspring of the same parents receive different sets of hereditary factors (Law of Independent Assortment)

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Incomplete Dominanceand

Codominance


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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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 IAi2. type B = IBIB or IBi3. type AB = IAIB

4. type O = ii


Other Factors: Multiple Alleles• Phenotypes are controlled by more than 1 allele. Eg. Blood types

are regulated by 3 separate genes.

• ABO Blood typing– Humans have multiple types of surface antigens on RBC's – The nature of these surface proteins determines a person's

Blood Type.– There are 3 alleles which determine blood type IA, IB, or IO. This

is referred to as having multiple alleles– Human blood types are designated as A, B or O.

• Type A denotes having the A surface antigen, and is denoted by IA

• Type B denotes having the B surface antigen, and is denoted by IB

• Type O denotes having neither A or B surface antigen, and is denoted by IO

– There are several blood type combinations possible• A• B• AB (Universal recipient)• O (Universal donor)

Blood & Immunity• A person can receive blood only when the donor's blood type does

not contain any surface antigen the recipient does not. This is because the recipient has antibodies which will attack any foreign surface protein.

• Thus, Type AB can accept any blood types because it will not attack A or B surface antigens. However, a type AB person could only donate blood to another AB person. They are known as Universal Recipients.

• Also, Type O persons are Universal donors because they have NO surface antigens that recipients' immune systems can attack. Type O persons can ONLY receive blood from other type O persons.

• There is another blood type factor known as Rh.• People are either Rh+ or Rh- based on a basic dominant/recessive

mechanism.• Not usually a problem except with pregnancy.• It is possible that an Rh- mother can carry an Rh+ fetus and develop

antibodies which will attack & destroy the fetal blood• This usually occurs with 2nd or 3rd pregnancies, and is detectable

and treatable.

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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 = IAi1/2 = IBi

i

IA IB

i


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


Other Factors: Continuous Variation

• Many traits may have a wide range of continuous values. Eg. Human height can vary considerably. There are not just "tall" or "short" humans

Other Factors

• Gene interaction: – Many biological pathways are governed by multiple

enzymes, involving multiple steps. If any one of these steps are altered. The end product of the pathway may be disrupted.

• Environmental effects:– Sometimes genes will not be fully expressed owing to

external factors. Example: Human height may not be fully expressed if individuals experience poor nutrition.

Chromosomes and Classical Genetics

• Walter Sutton in 1902 proposed that chromosomes were the physical carriers of Mendel's alleles

• Problems arose however regarding the following question:

• Why are the number of alleles which undergo independent assortment greater than the number of chromosomes of an organism?

• This was explained understanding of 2 additional factors; Sex Linkage and crossing over



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Sex-linked Traits

Traits (genes) located on the sex chromosomes

Sex chromosomes are X and YXX genotype for femalesXY genotype for malesMany sex-linked traits carried on Xchromosome

If a recessive allele exists on the X chromosome. It will not have a corresponding allele on the Y chromosome, and will therefore always be expressed


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 ProblemExample: 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


Sex-linked Trait Solution:

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyedfemale

50% white eyedmale

XR

Xr Xr

Y


Sex linkage example

Recessive gene for white eye color located on the Xw

chromosome of Drosophila.All Males which receive this gene during fertilization (50%) will express this.If a female receives the Xw

chromosome. It will usually not be expressed since she carries an X chromosome with the normal gene

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Female Carriers




Human Sex Linkage

• Hemophilia:– Disorder of the blood

where clotting does not occur properly due to a faulty protein.

– Occurs on the X chromosome, and is recessive.

• Thus a vast majority of those affected are males.

– First known person known to carry the disorder was Queen Victoria of England. Thus all those affected are related to European royalty.

Hemophilia and Royalty

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Genetic Practice Problems


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


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 generation

1/4 (25%) = TT1/2 (50%) = Tt1/4 (25%) = tt1:2:1 genotype3:1 phenotype

tall (Tt) x tall (Tt) pea plants


TERMS YOU MUST KNOW!!!

Trait

Gene

Allele

Dominant

Recessive

Heredity

Genetics

Hybrid

Phenotype

Genotype

Homozygous

Heterozygous

Vocabulary

• Genetics: The scientific study of heredity• Allele: Alternate forms of a gene/factor.• Genotype: combination of alleles an organism has.• Phenotype: How an organism appears.• Dominant: An allele which is expressed (masks the

other).• Recessive: An allele which is present but remains

unexpressed (masked)• Homozygous: Both alleles for a trait are the same.• Heterozygous: The organism's alleles for a trait are

different.

The Average

American Phenotype

Documents

Mendelelian Genetics · 2012-02-07 · Gregor Johann Mendel Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until