Introduction to GeneticsIntroduction to Genetics

For thousands of years farmers For thousands of years farmers and herders have been selectively and herders have been selectively breeding their plants and animals breeding their plants and animals to produce more useful hybrids. to produce more useful hybrids.

It was somewhat of a hit or miss It was somewhat of a hit or miss process since the actual process since the actual mechanisms governing mechanisms governing inheritance were unknown. inheritance were unknown.

Knowledge of these genetic Knowledge of these genetic mechanisms finally came as a mechanisms finally came as a result of careful laboratory result of careful laboratory breeding experiments carried out breeding experiments carried out over the last century and a half.over the last century and a half.

Intro. To GeneticsIntro. To Genetics By the 1890's, the invention By the 1890's, the invention

of better microscopes of better microscopes allowed biologists to discover allowed biologists to discover the basic facts of cell division the basic facts of cell division and sexual reproduction. and sexual reproduction.

The focus of The focus of geneticsgenetics research then shifted to research then shifted to understanding what really understanding what really happens in the transmission happens in the transmission of hereditary traits from of hereditary traits from parents to children.parents to children.

Gregor MendelGregor Mendel A number of hypotheses A number of hypotheses

were suggested to explain were suggested to explain heredity, but Gregor Mendel, heredity, but Gregor Mendel, a little known Central a little known Central European monk, was the European monk, was the only one who got it more or only one who got it more or less right. less right.

His ideas had been His ideas had been published in 1866 but largely published in 1866 but largely went unrecognized until went unrecognized until 1900, which was long after 1900, which was long after his deathhis death

Gregor MendelGregor MendelHis early adult life was His early adult life was

spent in relative obscurity spent in relative obscurity doing basic genetics doing basic genetics research and teaching high research and teaching high school mathematics, school mathematics, physics, and Greek in physics, and Greek in Brünn (now in the Czech Brünn (now in the Czech Republic).Republic).

In his later years, he In his later years, he became the abbot (friar) of became the abbot (friar) of his monastery and put his monastery and put aside his scientific work.aside his scientific work.

Gregor MendelGregor Mendel While Mendel's research While Mendel's research

was with plants, the was with plants, the basic underlying basic underlying principles of heredity principles of heredity that he discovered also that he discovered also apply to people and apply to people and other animals because other animals because the mechanisms of the mechanisms of heredity are essentially heredity are essentially the same for all complex the same for all complex life forms.life forms.

Mendel and PeasMendel and Peas Through the selective cross-Through the selective cross-

breeding of common pea breeding of common pea plants over many plants over many generations, Mendel generations, Mendel discovered that certain traits discovered that certain traits show up in offspring without show up in offspring without any blending of parent any blending of parent characteristics.characteristics.

For instance, the pea flowers For instance, the pea flowers are either purple or white--are either purple or white--intermediate colors do not intermediate colors do not appear in the offspring of appear in the offspring of cross-pollinatedcross-pollinated pea plants. pea plants.

Mendel and PeasMendel and Peas

Mendel observed seven Mendel observed seven traits traits (specific (specific characteristic) that are easily recognized and characteristic) that are easily recognized and apparently only occur in one of two forms:apparently only occur in one of two forms: flower colorflower color is purple or white is purple or white flower positionflower position is axial or terminal is axial or terminal stem lengthstem length is long or short is long or short seed shapeseed shape is round or wrinkled is round or wrinkled seed colorseed color is yellow or green is yellow or green pod shapepod shape is inflated or constricted is inflated or constricted pod colorpod color is yellow or green is yellow or green

Mendel’s PeasMendel’s Peas The pea plant was favorable organism for these studies The pea plant was favorable organism for these studies

because it was self-fertilizing because it was self-fertilizing When he made crosses, he followed only 1 or 2 (out of his 7) When he made crosses, he followed only 1 or 2 (out of his 7)

traits (characters) at a timetraits (characters) at a time He employed a very consistent method:He employed a very consistent method:

-- Opened flower & placed pollen from one type onto the stigma Opened flower & placed pollen from one type onto the stigma

Mendel’s methodsMendel’s methods

• Mendel covered each Mendel covered each flower with little bagflower with little bag

• When pods were ripe When pods were ripe harvested them and harvested them and planted seedsplanted seeds

He He countedcounted the the number of each type number of each type of offspring and of offspring and carefully recorded all carefully recorded all of his data.of his data.

Mendel’s First ExperimentMendel’s First Experiment Crossed (P1):Crossed (P1): Pure breeding Pure breeding TallTall x x

Pure breeding Pure breeding ShortShort (Dwarf) (Dwarf) (P1) = parental generation (pure (P1) = parental generation (pure

breed)breed) Pure (true) breedingPure (true) breeding means that if means that if

the plants were allowed to self-the plants were allowed to self-pollinate, they’re offspring would be pollinate, they’re offspring would be identical to themselves.identical to themselves.

PredictionsPredictions: The offspring would be:: The offspring would be:

All tallAll tall

All shortAll shortAll intermediateAll intermediateSome would be tall and some shortSome would be tall and some short

Mendel’s 1st and 2nd Mendel’s 1st and 2nd ExperimentExperiment

1st Exp1st Exp (P1) (P1): Crossed: CrossedPure Tall x Pure ShortPure Tall x Pure Short

All offspring (FAll offspring (F11): ): All TallAll Tall These offspring of the These offspring of the

parental generation are parental generation are calledcalled hybridshybrids,, which are which are offspring with different traits offspring with different traits than parents.than parents.

(F(F11)) = first filia (son or = first filia (son or daugther)daugther)

2nd Exp2nd Exp: Bred F: Bred F11 Results: Ratio of 787 tall to Results: Ratio of 787 tall to

277 short 277 short (3:1)(3:1)

Mendel’s Principle of Mendel’s Principle of SegregationSegregation

Mendel assumed that the two Mendel assumed that the two “Factors”“Factors” for each for each trait must exist in the parental germ cells trait must exist in the parental germ cells producing the gametes (pollen / egg)producing the gametes (pollen / egg)

These “factors” are called These “factors” are called allelesalleles.. Each allele came from the parents and were Each allele came from the parents and were

united in united in fertilizationfertilization In forming pollen and egg, the two alleles for any In forming pollen and egg, the two alleles for any

trait must separate and go into different gametestrait must separate and go into different gametes This became known as Mendel’s “This became known as Mendel’s “Principle of Principle of

SegregationSegregation””

Mendel’s Third ExperimentMendel’s Third Experiment

Crossed one of the Crossed one of the FF11 talltall

plants with its plants with its dwarf parentdwarf parent::

FF11 Tall x Dwarf (P Tall x Dwarf (P11))Possible Outcomes:Possible Outcomes:

All would be tallAll would be tall

Mixture of Tall & DwarfMixture of Tall & Dwarf

All would be intermediateAll would be intermediateExperimental results —>Experimental results —>

50% 50%

Mendel’s NotationMendel’s Notation After the third experiment, Mendel formulated his After the third experiment, Mendel formulated his

“Principles of Dominance”“Principles of Dominance” which states that some which states that some alleles are dominant and some are recessive.alleles are dominant and some are recessive.

Used capital letter to denote what he called the Used capital letter to denote what he called the dominant dominant form of the trait: form of the trait: T = tallT = tall

Used lower case letter to denote what he called the Used lower case letter to denote what he called the recessive recessive trait: trait: t = short (dwarf)t = short (dwarf)

Thus for the Tall and Dwarf crosses:Thus for the Tall and Dwarf crosses:TT TT = Original pure-breeding tall parent= Original pure-breeding tall parenttttt = Original pure-breeding short parent = Original pure-breeding short parentTtTt = Hybrid F = Hybrid F11 offspring offspring

Pure-breeding forms later called Pure-breeding forms later called homozygoushomozygous Hybrids later called Hybrids later called heterozygousheterozygous

Mendel’s Experiment 3Mendel’s Experiment 3 Mendel recognized that it Mendel recognized that it

is not always possible to is not always possible to tell what offspring will be tell what offspring will be like by inspecting the like by inspecting the parentparent

Mendel could test if tall Mendel could test if tall plants were pure-breeds plants were pure-breeds (homozygotes) or hybrid (homozygotes) or hybrid (heterozygotes) by the (heterozygotes) by the “back-cross” or “back-cross” or “test-“test-cross” cross”

Test Cross: Test Cross: Crossing a Crossing a organism with the organism with the dominant phenotype dominant phenotype with the same organism with the same organism with the recessive with the recessive phenotype to determine phenotype to determine the genotype of the the genotype of the dominant organism.dominant organism.

Tt

tt

TT

tt

Not in your notes

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Writing Mendel’s Crosses Writing Mendel’s Crosses Using His Notational SystemUsing His Notational System

It was now possible It was now possible to account for the to account for the 3:1 ratio in the F3:1 ratio in the F1 1 in in

Mendel’s second Mendel’s second experiment:experiment:

This method of This method of calculating traits of calculating traits of offspring in each offspring in each generation is called generation is called a a Punnett squarePunnett square..

Punnett SquaresPunnett Squares The gene combination The gene combination

that might result from a that might result from a genetic cross can be genetic cross can be determined by drawing determined by drawing a Punnett square.a Punnett square.

Punnett squares help to Punnett squares help to determine determine phenotypesphenotypes and and genotypesgenotypes..

PhenotypePhenotype – Physical – Physical characteristics (Purple)characteristics (Purple)

GenotypeGenotype – Genetic – Genetic make-up (BB or Bb)make-up (BB or Bb)

Independent AssortmentIndependent Assortment After showing alleles After showing alleles

segregated during the segregated during the formation of gametes, formation of gametes, Mendel wondered if they Mendel wondered if they did so independently.did so independently.

In other words, does the In other words, does the segregation of one pair of segregation of one pair of alleles affect the alleles affect the segregation of another set segregation of another set of alleles?of alleles?

ExampleExample: Does the gene : Does the gene for seed color have for seed color have anything to do with the anything to do with the gene for seed shape?gene for seed shape?

Independent AssortmentIndependent Assortment

To answer these To answer these questions, Mendel questions, Mendel performed an experiment performed an experiment to follow two different to follow two different genes as they passed from genes as they passed from one generation to the next.one generation to the next.

This experiment is known This experiment is known as the as the Two-Factor CrossTwo-Factor Cross or or Dihybrid crossDihybrid cross..

Mendel Extended His Mendel Extended His AnalysisAnalysis

Mendel chose two different Mendel chose two different characteristics of the pea:characteristics of the pea:

-- Seed CoatSeed Coat: : Smooth Smooth

Wrinkled Wrinkled

-- Seed colorSeed color:: Yellow Yellow

Green Green

Two-Factor (dihybrid) CrossTwo-Factor (dihybrid) Cross First, Mendel crossed First, Mendel crossed

pure-breeding plants pure-breeding plants that produced only that produced only round yellow peas round yellow peas (RRYY) with plants that (RRYY) with plants that produced wrinkled produced wrinkled green peas (rryy).green peas (rryy).

All of the FAll of the F11 offspring offspring produced round yellow produced round yellow peas (RrYy).peas (RrYy).

This proved round and This proved round and yellow must be yellow must be dominant alleles.dominant alleles.

Try doing the Punnett Try doing the Punnett square.square.

RYRY RYRY RYRY RYRY

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

ryry RrYyRrYy RrYyRrYy RrYyRrYy RrYyRrYy

RRYY

r

r

y

y

Independent assortmentIndependent assortmentThis cross does not This cross does not

indicate whether indicate whether genes assorted genes assorted independently.independently.

This cross only This cross only provided hybrids provided hybrids plants (Fplants (F11) needed to ) needed to

produce an Fproduce an F22

generation that would generation that would provide the answers provide the answers to Mendel's question.to Mendel's question.

ResultsResults Mendel knew the FMendel knew the F11

had a genotype of had a genotype of RrYy RrYy (heterozygous).(heterozygous).

How would the How would the alleles segregate alleles segregate into the Finto the F22

Try the Punnett Try the Punnett Square for the Square for the cross of Fcross of F11 hybrids hybrids

producing and Fproducing and F22

generation.generation.

ResultsResults

After Mendel After Mendel counted the counted the offspring of the offspring of the dihybrid cross, dihybrid cross, he could make he could make the following the following conclusion.conclusion.

Mendel Could Now Make A Mendel Could Now Make A Second Generalization:Second Generalization:

Genes for different Genes for different traits segregate traits segregate independently during independently during the formation of the formation of gametes.gametes.

This became known as This became known as Mendel’s third Mendel’s third principle: principle: Independent Independent AssortmentAssortment

A Summary of Mendel’s PrinciplesA Summary of Mendel’s Principles Mendel’s principles form the basis of the modern Mendel’s principles form the basis of the modern

science of genetics. These principles can be science of genetics. These principles can be summarized as follows:summarized as follows:

1.1. The inheritance of biological characteristics is determined The inheritance of biological characteristics is determined by individual units known as by individual units known as genesgenes. Genes are passed . Genes are passed from parents to their offspring.from parents to their offspring.

2.2. In cases in which two or more forms In cases in which two or more forms (alleles)(alleles) of the gene of the gene for a single trait exist, some form of the gene may be for a single trait exist, some form of the gene may be dominant and the other recessive dominant and the other recessive (Principles of (Principles of Dominance)Dominance)..

3.3. In most sexually reproductive organisms, each adult has In most sexually reproductive organisms, each adult has two copies of each gene; one from each parent. These two copies of each gene; one from each parent. These genes are segregated from each other when gametes are genes are segregated from each other when gametes are formed formed (Principles of Segregation).(Principles of Segregation).

4.4. The alleles for different genes usually segregate The alleles for different genes usually segregate independently of one another independently of one another (Principles of independent (Principles of independent assortment).assortment).

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