mendel and the principles of heredity

8/14/2019 Mendel and the Principles of Heredity

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Mendel and the Principles ofMendel and the Principles of

HeredityHeredityGregor Johan Mendel,Gregor Johan Mendel,an Austrian monk,an Austrian monk,

gave the firstgave the first

scientificscientificinterpretationinterpretation of theof theheredity mechanismheredity mechanismin 1865 after 8 yearsin 1865 after 8 yearsof experimental workof experimental workwith garden peaswith garden peas


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When Mendel began his studies of inheritanceWhen Mendel began his studies of inheritanceusing Pisum sativum, the garden pea, thereusing Pisum sativum, the garden pea, therewas no knowledge of chromosomes nor of thewas no knowledge of chromosomes nor of the

role and mechanisms of meiosis. Nevertheless,role and mechanisms of meiosis. Nevertheless,he was able to determine that distincthe was able to determine that distinct units ofunits ofinheritanceinheritance exist and to predict their behaviorexist and to predict their behavior

during the formation of gametes.during the formation of gametes.


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In 1866 Gregor Mendel published the resultsIn 1866 Gregor Mendel published the resultsof his experiments, but unfortunately , noof his experiments, but unfortunately , noattention was paid from scientists anywhere.attention was paid from scientists anywhere.


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Subsequent investigators, with access toSubsequent investigators, with access tocytological data, were able to relate theircytological data, were able to relate theirobservations of chromosome behavior duringobservations of chromosome behavior duringmeiosis to Mendels principles ofmeiosis to Mendels principles ofinheritance. Once this correlation was made,inheritance. Once this correlation was made,Mendels postulates were accepted as theMendels postulates were accepted as the

basis for the study ofbasis for the study oftransmission geneticstransmission genetics,,ororMendelian GeneticsMendelian Genetics


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Mendels achievements were rediscovered in 1900 by threeMendels achievements were rediscovered in 1900 by three

scientists who had obtained similar results and recognized hisscientists who had obtained similar results and recognized his

precedenceprecedence

Hugo de Vries (1848-1935)Hugo de Vries (1848-1935)

in Hollandin Holland

Carl Correns (1864-1933)Carl Correns (1864-1933)

in Germanyin Germany

Eric von Tschermack(1855-1927)Eric von Tschermack(1855-1927)

in Austriain Austria
http://images.google.ru/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/7/76/Hugo_de_Vries_2.jpg&imgrefurl=http://commons.wikimedia.org/wiki/File:Hugo_de_Vries_2.jpg&usg=__fp2YBs7sMggS04HIRlHmkmGvT2A=&h=584&w=434&sz=50&hl=ru&start=4&itbs=1&tbnid=oBxQz-coEHkGgM:&tbnh=135&tbnw=100&prev=/images%3Fq%3Dhugo%2Bde%2Bvries%26gbv%3D2%26hl%3Dru%26sa%3DG%26newwindow%3D1


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Blending Concept of Inheritance,Blending Concept of Inheritance,Before MendelBefore Mendel

Many scientists before Mendel had tried to discoverMany scientists before Mendel had tried to discoverhow biological characteristics are inherited. They hadhow biological characteristics are inherited. They hadcrossed plants or animals and had looked at thecrossed plants or animals and had looked at theoverall similarities between the offspring and theiroverall similarities between the offspring and their

parents.parents. The results were confusing: the offspring resembledThe results were confusing: the offspring resembled

one parent in some traits and the other parent in otherone parent in some traits and the other parent in othertraits, and apparently neither one in still other traits.traits, and apparently neither one in still other traits.

Blending Concept of Inheritance,Blending Concept of Inheritance, . This theory. This theorystated that offspring would possess traits intermediatestated that offspring would possess traits intermediatebetween those of different parents.between those of different parents.


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Mendel succeeded where earlierMendel succeeded where earlierinvestigators had failed owing toinvestigators had failed owing to

his brilliant insight andhis brilliant insight andmethodologymethodology


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MENDELS METHODMENDELS METHOD

1. Mendel did a statistical study (he had a mathematical1. Mendel did a statistical study (he had a mathematical

background).background). 2. He prepared his experiments carefully and conducted2. He prepared his experiments carefully and conducted

preliminary studies.preliminary studies. 3. Mendel paid attention to a single trait at a time3. Mendel paid attention to a single trait at a time

44. He chose the garden pea, Pisum sativum, because peas. He chose the garden pea, Pisum sativum, because peaswere easy to cultivate, had a short generation time, andwere easy to cultivate, had a short generation time, andcould be cross-pollinated.could be cross-pollinated.

55. From many varieties, Mendel chose 22 true-breading. From many varieties, Mendel chose 22 true-breadingvarieties for his experiments.varieties for his experiments.

66. True-breeding varieties had all offspring like the parents. True-breeding varieties had all offspring like the parentsand like each other.and like each other.


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True breeding pla


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genotype

True breeding

phenotype

True breedingplants


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Mendel studied Seven Traits orMendel studied Seven Traits orCharacteristics. EachCharacteristics. Each characteristiccharacteristicoccurred in Two Contrasting Traits:occurred in Two Contrasting Traits:


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F

1

Mendel studied the inheritance of seedMendel studied the inheritance of seed

shape first.shape first. Mendel crossed pure-breeding (alsoMendel crossed pure-breeding (also

referred to as true-breeding) smooth-referred to as true-breeding) smooth-

seeded plants with a variety that hadseeded plants with a variety that hadalways produced wrinkled seeds (60always produced wrinkled seeds (60fertilizations on 15 plants).fertilizations on 15 plants).

All resulting seeds were smooth.All resulting seeds were smooth.


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Mendel found that all seven characters he hadselected for study behaved in this way, in eachcase only one of two contrasting traits appearedin the F1 hybrids.


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genotype

phenotypeF1

dominant characteristic recessive characteristic

Homozygous organisms.


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YY yy

Y y

Yy

.

gametes

F1 generation


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Mendel called such traits( yellow

seeds, axial flowers, etc.)dominant


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And their alternatives ( wrinkled seeds, green peas,terminal flowers), he called recessive


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When contrasting traits are present in theparents they do not blend in the offspring,

but one is dominant and usually appears

fully developed, while the other is recessiveand temporarily drops out of sight..


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The following year, Mendel planted these seedsThe following year, Mendel planted these seeds

and allowed them to self-fertilize. He recoveredand allowed them to self-fertilize. He recovered7324 seeds: 5474 smooth and 1850 wrinkled.7324 seeds: 5474 smooth and 1850 wrinkled.

The ratio was very close to 3:ait1The ratio was very close to 3:ait1

A similar ratio appeared in all other crosses, inA similar ratio appeared in all other crosses, inevery case the dominant trait was about threeevery case the dominant trait was about three

times as common in the Ftimes as common in the F22 as the recessive traitas the recessive trait


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Yy Yy

Y y

Yy

.

gametes

F2generation

y Y

Yy yyYY


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1 1 1 1genotype

phenotype

F2


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From these types of experiments and observations,From these types of experiments and observations,Mendel concluded that the heritable factor for yellowMendel concluded that the heritable factor for yellow

ppeaseas was not lost in the F1 plants, but was masked bywas not lost in the F1 plants, but was masked by

the presence of the green-pthe presence of the green-peaseas factor.factor.We now call these heritable factorsWe now call these heritable factorsgenesgenes..


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Mendel formulated his hypothesis of inheritanceMendel formulated his hypothesis of inheritance

which can be divided into four parts:which can be divided into four parts:There are alternative forms forThere are alternative forms forgenesgenes, the units that, the units that

determine inheritable characteristics:determine inheritable characteristics:

The gene forThe gene forseedseed shapeshape existed in two alternativeexisted in two alternativeforms, one forforms, one forsmoothsmooth and one forand one forwrinkledwrinkled..Alternative forms of a gene are now calledAlternative forms of a gene are now called allelesalleles..


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For each inherited characteristic, an organism has twoFor each inherited characteristic, an organism has two

alleles, one inherited from each parent.alleles, one inherited from each parent.Mendel's experiment included onMendel's experiment included onee parental varietyparental varietywhich had a pair of alleles forwhich had a pair of alleles forsmoothsmooth seed shapeseed shapeand one which had a pair of alleles forand one which had a pair of alleles forwrinkledwrinkled

seedseed shapeshape . The F1 hybrids inherited from the parental plantsThe F1 hybrids inherited from the parental plants

one allele forone allele forsmoothsmooth seeseedd shapeshape and one alleleand one allele

forforwrinkled seewrinkled seedd shapeshape..


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A sperm or egg carries only one allele for each inheritedA sperm or egg carries only one allele for each inheritedcharacteristic, because allele pairs separate (segregate) fromcharacteristic, because allele pairs separate (segregate) from

each other during the production of gametes. At fertilization,each other during the production of gametes. At fertilization,the sperm and egg unite with both contributing their alleles.the sperm and egg unite with both contributing their alleles.

This restores the gene to the paired condition.This restores the gene to the paired condition.In Mendel's experiment, each gamete of a parental plantIn Mendel's experiment, each gamete of a parental plant

carried one allele forcarried one allele forseedseed shapeshape, specifying either, specifying eithersmoothsmooth ororwrinkledwrinkled..

Cross-pollination to produce the F1 resulted in theCross-pollination to produce the F1 resulted in thecombination found in this generation.combination found in this generation.


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Mendel's hypothesis explains the 3:1 ratio of progeny plantMendel's hypothesis explains the 3:1 ratio of progeny planttypes he observed in the F2 generation.types he observed in the F2 generation.

It predicts that the F1 hybrids (It predicts that the F1 hybrids (SsSs) will produce two classes of) will produce two classes ofgametes when the pairs separate during gamete formation.gametes when the pairs separate during gamete formation.Half will receive aHalf will receive a smooth seedsmooth seed ((SS) allele and the other half) allele and the other half

thethe wrinkled seedwrinkled seed allele (allele (ss).).


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During self-pollination, these two classes of alleles unite in aDuring self-pollination, these two classes of alleles unite in arandom manner.random manner.

Eggs containingEggs containing smooth seedsmooth seed alleles have equal chances ofalleles have equal chances ofbeing fertilized by sperm carryingbeing fertilized by sperm carrying wrinkledwrinkled --seeseed alleles ord alleles or

sperm carryingsperm carrying yellow- seedyellow- seed alleles.alleles.Since the same is true for eggs containingSince the same is true for eggs containing greengreen--seeseed alleles,d alleles,there are four equally likely combinations of sperm and eggs.there are four equally likely combinations of sperm and eggs.

The combinations resulting from a genetic cross may beThe combinations resulting from a genetic cross may bepredicted by using a device called apredicted by using a device called aPunnett SquarePunnett Square..


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F

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Mendel's Principle of Segregation statesthat an individual inherits a unit ofinformation (allele) about a trait from

each parent. During gamete formation,the alleles segregate from each other.


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Mendel realized the need to conduct his experimentson more complex situations. He performedexperiments tracking two seed traits: shape andcolor. A cross concerning two traits is known as adihybrid cross.


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F

1

Mendel started with true-breeding plants thatMendel started with true-breeding plants that

had smooth, yellow seeds and crossed themhad smooth, yellow seeds and crossed themwith true-breeding plants having green,with true-breeding plants having green,wrinkled seeds. Allwrinkled seeds. Allplantsplants in the Fin the F11 had smoothhad smoothyellow seeds.yellow seeds.


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F

1

. The F2 plants self-fertilized, and produced. The F2 plants self-fertilized, and produced

four phenotypes:four phenotypes: 315 smooth yellow315 smooth yellow 108 smooth green108 smooth green

101 wrinkled yellow101 wrinkled yellow 32 wrinkled green32 wrinkled green


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Mendel analyzed each trait for separate inheritanceMendel analyzed each trait for separate inheritance


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Mendel analyzed each trait for separate inheritanceMendel analyzed each trait for separate inheritanceas if the other trait were not present.The 3:1 ratioas if the other trait were not present.The 3:1 ratiowas seen separately and was in accordance with thewas seen separately and was in accordance with the

Principle of Segregation. The segregation of S and sPrinciple of Segregation. The segregation of S and salleles must have happened independently of thealleles must have happened independently of thesegregation of Y and y alleles.segregation of Y and y alleles.


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The chance of any gamete having a Y is 1/2; the chance ofThe chance of any gamete having a Y is 1/2; the chance ofany one gamete having a S is 1/2.The chance of a gameteany one gamete having a S is 1/2.The chance of a gametehaving both Y and S is the product of their individualhaving both Y and S is the product of their individualchances (or 1/2 X 1/2 = 1/4). The chance of two gameteschances (or 1/2 X 1/2 = 1/4). The chance of two gametes

forming any given genotype is 1/4 X 1/4 (remember, theforming any given genotype is 1/4 X 1/4 (remember, theproduct of their individual chances). Thus, the Punnettproduct of their individual chances). Thus, the PunnettSquare has 16 boxes. Since there are more possibleSquare has 16 boxes. Since there are more possiblecombinations to produce a smooth yellow phenotypecombinations to produce a smooth yellow phenotype

(SSYY, SsYy, SsYY, and SSYy), that phenotype is more(SSYY, SsYy, SsYY, and SSYy), that phenotype is morecommon in the F2.common in the F2.


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When two individuals differ from each other in

two or more pairs of factors, the inheritance of

one pair is quite independent of the inheritance

of others


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mendel and the principles of heredity

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