Gregor Mendel and Gregor Mendel and his Laws of his Laws of InheritanceInheritance

MENDEL’S LAWS OF HEREDITYMENDEL’S LAWS OF HEREDITY

I. WHY MENDEL SUCCEEDEDI. WHY MENDEL SUCCEEDED Gregor Mendel – father of geneticsGregor Mendel – father of genetics everything we know about genetics everything we know about genetics

began with Mendel began with Mendel He did the 1He did the 1stst studies of studies of heredityheredity – the – the

passing of characteristics to offspringpassing of characteristics to offspring GeneticsGenetics – study of heredity – study of heredity The characteristics passed on called The characteristics passed on called

traitstraits

•Traits Traits - Specific characteristics that vary - Specific characteristics that vary from one individual to anotherfrom one individual to another

•HybridsHybrids-The offspring of crosses -The offspring of crosses between parents with different between parents with different characteristicscharacteristics

•GenesGenes – portions of DNA that determine – portions of DNA that determine traitstraits

•AllelesAlleles - The different forms of a gene - The different forms of a gene

1. MENDEL CHOSE HIS 1. MENDEL CHOSE HIS SUBJECT CAREFULLYSUBJECT CAREFULLY

Used garden peas for his studiesUsed garden peas for his studies Pea plants have male & female Pea plants have male & female

gametesgametes (sex cells) (sex cells) Male & female on same flowerMale & female on same flower He could control the He could control the fertilizationfertilization

processprocess Not many traits to keep track ofNot many traits to keep track of

Mendel crossed the plantsMendel crossed the plants

FertilizationFertilization - the uniting of male and - the uniting of male and female gametes (sex cells)female gametes (sex cells)

CrossCross - combining gametes from - combining gametes from parents with different traitsparents with different traits

For example: he For example: he crossed a purple-crossed a purple-flowered plant with flowered plant with a white-flowered a white-flowered plant.plant.

2. MENDEL WAS A CAREFUL 2. MENDEL WAS A CAREFUL RESEARCHERRESEARCHER

USED CAREFULLY CONTROLLED USED CAREFULLY CONTROLLED EXPERIMENTSEXPERIMENTS

STUDIED ONE TRAIT AT A TIMESTUDIED ONE TRAIT AT A TIME KEPT DETAILED DATAKEPT DETAILED DATA

II. MENDEL’S MONOHYBRID II. MENDEL’S MONOHYBRID CROSSESCROSSES

MENDEL STUDIED 7 TRAITS CAREFULLYMENDEL STUDIED 7 TRAITS CAREFULLY Mendel crossed plants w/ different traits Mendel crossed plants w/ different traits

to see what traits the offspring would to see what traits the offspring would havehave

These offspring are called These offspring are called hybridshybrids – – (offspring of parents w/ different traits)(offspring of parents w/ different traits)

A A monohybridmonohybrid cross is one that looks at cross is one that looks at only only oneone trait (let’s look at plant height – trait (let’s look at plant height – tall or short)tall or short)

Mendel noted that the size Mendel noted that the size of pea plants varied. He of pea plants varied. He cross-bred these pea cross-bred these pea plants to find some plants to find some surprising surprising results.results.

A. THE 1A. THE 1STST GENERATION GENERATION

Mendel crossed two plants – 1 tall & Mendel crossed two plants – 1 tall & 1 short (they came from tall & short 1 short (they came from tall & short populations)populations)

These plants are called the parent These plants are called the parent generation (generation (P generationP generation))

The offspring were all called the 1The offspring were all called the 1stst filial generation (filial generation (FF11 generation generation))

All the offspring were tall (the short All the offspring were tall (the short plants were totally excluded)plants were totally excluded)

Mendel’s cross between tall pea plants yielded all tall Mendel’s cross between tall pea plants yielded all tall pea plants. His cross between small pea plants pea plants. His cross between small pea plants yielded all small pea plants. yielded all small pea plants.

Mendel’s cross between tall pea plants and small pea Mendel’s cross between tall pea plants and small pea plants yielded all tall pea plants. plants yielded all tall pea plants.

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FF11 GENERATION GENERATIONP GENERATIONP GENERATION

B. THE 2B. THE 2NDND GENERATION GENERATION

Next, Mendel crossed two plants Next, Mendel crossed two plants from the from the FF11 generation generation

The offspring from this cross are The offspring from this cross are called the 2called the 2ndnd filial generation ( filial generation (FF22 GENERATIONGENERATION))

Mendel found that ¾ of the offspring Mendel found that ¾ of the offspring were tall & ¼ were short (the short were tall & ¼ were short (the short plants reappeared!!!!!!)plants reappeared!!!!!!)

Mendel then crossed these second generation tall pea Mendel then crossed these second generation tall pea plants and ended up with 1 out 4 being small. Fplants and ended up with 1 out 4 being small. F22 GENERATIONGENERATION

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OR ANOTHER OR ANOTHER TRAIT: TRAIT:

FLOWER COLORFLOWER COLOR

TO GO ANY FURTHER, WE TO GO ANY FURTHER, WE MUST UNDERSTAND ALLELES, MUST UNDERSTAND ALLELES, DOMINANCE, & SEGREGATIONDOMINANCE, & SEGREGATION

GenesGenes – a section of DNA that codes for – a section of DNA that codes for one proteinone protein These genes are what control & produce These genes are what control & produce

traitstraits The genes Mendel studied came in two The genes Mendel studied came in two

forms (tall/short - round/wrinkled - forms (tall/short - round/wrinkled - yellow/green…….etc.)yellow/green…….etc.)

Alternate forms of a gene are called Alternate forms of a gene are called allelesalleles

Alleles are represented by a one or two Alleles are represented by a one or two letter symbol (e.g. T for tall, t for short)letter symbol (e.g. T for tall, t for short)

ALLELES CONT’DALLELES CONT’D

THESE 2 ALLELES ARE NOW KNOWN THESE 2 ALLELES ARE NOW KNOWN TO BE FOUND ON COPIES OF TO BE FOUND ON COPIES OF CHROMOSOMES – ONE FROM EACH CHROMOSOMES – ONE FROM EACH PARENTPARENT

THE RULE OF DOMINANCETHE RULE OF DOMINANCE A A dominantdominant trait is the trait that will trait is the trait that will

always be expressed if at least one always be expressed if at least one dominant allele is presentdominant allele is present

The dominant allele is The dominant allele is alwaysalways represented by a capital letterrepresented by a capital letter

A recessive trait will A recessive trait will onlyonly be expressed if be expressed if bothboth alleles are recessive alleles are recessive

Recessive traits are represented by a Recessive traits are represented by a lower case letterlower case letter

BY CONVENTION when choosing a letter to represent the BY CONVENTION when choosing a letter to represent the alleles:alleles:

The dominant trait is given a capitol letter, the lowercase of that The dominant trait is given a capitol letter, the lowercase of that same letter is the recessive trait.  DO NOT MIX LETTERS. same letter is the recessive trait.  DO NOT MIX LETTERS.  Pick one and stick to it. Pick one and stick to it.

Also, some letters are better than others.  Capital S looks a lot Also, some letters are better than others.  Capital S looks a lot like a lowercase (s).  Pick a different letter...like a lowercase (s).  Pick a different letter...

      Okay                                                   Better (use H for hair)Okay                                                   Better (use H for hair)

Short  hair  = SS                                    HHShort  hair  = SS                                    HHShort hair = Ss                                       HhShort hair = Ss                                       HhLong hair = ss                                        hh Long hair = ss                                        hh 

DOMINANCE CONT’DDOMINANCE CONT’D

LET’S USE TALL & SHORT PEA LET’S USE TALL & SHORT PEA PLANTS FOR AN EXAMPLEPLANTS FOR AN EXAMPLE

WHICH OF THESE WILL SHOW THE WHICH OF THESE WILL SHOW THE DOMINANT & RECESSIVE TRAIT?DOMINANT & RECESSIVE TRAIT?

TT Tt TT Tt tttt

DOMINANT TRAIT RECESSIVE TRAITDOMINANT TRAIT RECESSIVE TRAIT

Practice picking letters.... the following traits are Practice picking letters.... the following traits are

found in the common found in the common Shirtus americanusShirtus americanus..

A.A. Polka dots are dominant to stripes.Polka dots are dominant to stripes.

B.B. Long sleeves are dominant to short sleeves.Long sleeves are dominant to short sleeves.

C.C. Collared shirts are recessive.Collared shirts are recessive.

D.D. Buttons are dominant over snaps.Buttons are dominant over snaps.

E.E. Pockets are recessive.   Pockets are recessive.   

When an organism is studied for three different genes and has When an organism is studied for three different genes and has the alleles AABbCC, it is homozygous for A and C genes but the alleles AABbCC, it is homozygous for A and C genes but heterozygous for the B gene.heterozygous for the B gene.

A = antennaA = antennaa = no antennaa = no antenna

B = blackB = blackb = purpleb = purple

C = chompersC = chompersc = no chompersc = no chompers

So what would thisSo what would thisorganism look like?organism look like?

THE LAW OF SEGREGATIONTHE LAW OF SEGREGATION

MENDEL ASKED HIMSELF……..”HOW MENDEL ASKED HIMSELF……..”HOW DID THE RECESSIVE SHORT PLANTS DID THE RECESSIVE SHORT PLANTS REAPPEAR IN THE F2 GENERATION?”REAPPEAR IN THE F2 GENERATION?”

HE CONCLUDED THAT EACH TALL HE CONCLUDED THAT EACH TALL PLANT FROM THE F1 GENERATION PLANT FROM THE F1 GENERATION CARRIED TWO ALLELES, 1 DOMINANT CARRIED TWO ALLELES, 1 DOMINANT TALL ALLELE & ONE RECESSIVE TALL ALLELE & ONE RECESSIVE SHORT ALLELESHORT ALLELE

SO ALL WERE TtSO ALL WERE Tt

SEGREGATION CONT’DSEGREGATION CONT’D HE ALSO CONCLUDED THAT ONLY HE ALSO CONCLUDED THAT ONLY

ONE ALLELE FROM EACH PARENT ONE ALLELE FROM EACH PARENT WENT TO EACH OFFSPRINGWENT TO EACH OFFSPRING

HIS CORRECT HYPOTHESIS WAS HIS CORRECT HYPOTHESIS WAS THAT SOMEHOW DURING THAT SOMEHOW DURING FERTILIZATION, THE ALLELES FERTILIZATION, THE ALLELES SEPARATED (SEGREGATED) & SEPARATED (SEGREGATED) & COMBINED WITH ANOTHER ALLELE COMBINED WITH ANOTHER ALLELE FROM THE OTHER PARENTFROM THE OTHER PARENT

The law of segregation states that The law of segregation states that during gamete formation, the alleles during gamete formation, the alleles separate to different gametesseparate to different gametes

F1 GENERATIONF1 GENERATION

FATHERFATHER MOTHERMOTHER

T tT t T tT t

TT TT TT tt tt ttF2 GENERATIONF2 GENERATION

- the law of dominance explained the - the law of dominance explained the heredity of the offspring of the f1 heredity of the offspring of the f1 generationgeneration

- the law of segregation explained the - the law of segregation explained the heredity of the f2 generationheredity of the f2 generation

PHENOTYPES & GENOTYPESPHENOTYPES & GENOTYPES

PHENOTYPEPHENOTYPE – THE WAY AN ORGANISM – THE WAY AN ORGANISM LOOKS AND BEHAVES – ITS PHYSICAL LOOKS AND BEHAVES – ITS PHYSICAL CHARACTERISTICS (i.e. – TALL, GREEN, CHARACTERISTICS (i.e. – TALL, GREEN, BROWN HAIR, BLUE EYES, ETC.)BROWN HAIR, BLUE EYES, ETC.)

GENOTYPEGENOTYPE – THE GENE COMBONATION – THE GENE COMBONATION (ALLELIC COMBINATION) OF AN (ALLELIC COMBINATION) OF AN ORGANISM – (i.e. – TT, Tt, tt, ETC.)ORGANISM – (i.e. – TT, Tt, tt, ETC.) HOMOZYGOUSHOMOZYGOUS – 2 ALLELES ARE THE SAME – 2 ALLELES ARE THE SAME HETEROZYGOUSHETEROZYGOUS – 2 ALLELES DIFFERENT – 2 ALLELES DIFFERENT