GeneticsGenetics

““The real key to explaining The real key to explaining Inheritance”Inheritance”

ObjectivesObjectives

To understand the experiments and To understand the experiments and conclusions of conclusions of Gregor MendelGregor Mendel

To compare To compare MonohybridMonohybrid and and DihybridDihybrid crossescrosses

To compare To compare Complete, Incomplete Complete, Incomplete and Co-dominanceand Co-dominance

To understand how To understand how Multiple Alleles Multiple Alleles workwork

Gregor MendelGregor Mendel Austrian Monk in the Austrian Monk in the

1800’s1800’s Experimented with Experimented with

pea plants pea plants Kept detailed records Kept detailed records

of several of several characteristics for characteristics for generations generations – HeightHeight– Flower color Flower color – Pod colorPod color– Shape of peas and podsShape of peas and pods– Position of flowers on Position of flowers on

the plantthe plant

Mendel’s ObservationsMendel’s Observations

If he had tall plants in one section of the If he had tall plants in one section of the garden, they always produced tall garden, they always produced tall offspringoffspring

Similarly if he had short plants in another Similarly if he had short plants in another section the same would occur (short section the same would occur (short offspring)offspring)

He called these plants He called these plants Pure LinesPure Lines– Tall x Tall = TallTall x Tall = Tall– Short x Short = ShortShort x Short = Short– Mendel's ExperimentMendel's Experiment

Mendel’s ExperimentMendel’s Experiment He decided to cross (breed) these pure linesHe decided to cross (breed) these pure lines Pure Line Tall x Pure Line ShortPure Line Tall x Pure Line Short The result was The result was ALLALL tall plants tall plants He decided to cross these plants with each He decided to cross these plants with each

otherother To his surprise, this resulted in both To his surprise, this resulted in both tall tall and and

short short plantsplants The ratio was exactly 3 to 1 (tall to short)The ratio was exactly 3 to 1 (tall to short)

ParentsParents Tall (Pure Line) x Short (Pure Line)Tall (Pure Line) x Short (Pure Line)

First First Generation Generation (F1)(F1)

All Tall PlantsAll Tall Plants

Second Second Generation Generation (F2)(F2)

Tall & Short Plants Tall & Short Plants 3 to 1 ratio (75% Tall & 25% Short)3 to 1 ratio (75% Tall & 25% Short)

Mendel’s Conclusions(4)Mendel’s Conclusions(4)

All characteristics are controlled by All characteristics are controlled by factors inherited from your parentsfactors inherited from your parents

There are two factors for every There are two factors for every trait trait (characteristic)(characteristic)– These traits can be hair color, length of These traits can be hair color, length of

nose, skin tone, height, etc.nose, skin tone, height, etc. One factor can dominate anotherOne factor can dominate another

– ‘‘Mask’ the otherMask’ the other These factors separate from each other These factors separate from each other

during sex cell formation (meiosis)during sex cell formation (meiosis)

Modernizing Mendel’s Modernizing Mendel’s ConclusionsConclusions

We now call these We now call these ‘factors’ ‘factors’ AllelesAlleles

Alleles can be Alleles can be DominantDominant or or RecessiveRecessive

We represent Alleles with We represent Alleles with letters:letters:– Capitals = Dominant Capitals = Dominant

AlleleAllele– Lowercase = Recessive Lowercase = Recessive

AlleleAllele– *Dominant Alleles always *Dominant Alleles always

go firstgo first GenotypeGenotype is the combination of is the combination of

alleles you havealleles you have PhenotypePhenotype is the expressed is the expressed

traittrait Homozygous –Homozygous – 2 alleles that 2 alleles that

are the same.are the same. Heterozygous – Heterozygous – 2 alleles that 2 alleles that

are differentare different

GenotypeGenotype PhenotypePhenotype

TTTT Tall PlantTall Plant

TtTt Tall PlantTall Plant

tttt Short PlantShort Plant

BBBB Brown EyesBrown Eyes

BbBb Brown EyesBrown Eyes

bbbb Blue EyesBlue Eyes

Types of GenotypesTypes of Genotypes

Genotype in Genotype in LettersLetters

Genotype in Genotype in WordsWords

PhenotypePhenotype

TTTT Homozygous Homozygous DominantDominant

Tall PlantTall Plant

TtTt HeterozygousHeterozygous Tall PlantTall Plant

tttt Homozygous Homozygous RecessiveRecessive

Short PlantShort Plant

Genotypes can also be expressed in wordsGenotypes can also be expressed in words

‘‘Homo’ – Latin prefix for ‘the same’Homo’ – Latin prefix for ‘the same’

The Punnett SquareThe Punnett Square Mendel started his Mendel started his

experiments crossing experiments crossing a pure line of tall a pure line of tall plants (TT) with a pure plants (TT) with a pure line of short plants (tt)line of short plants (tt)

ParentsParents (Tt x Tt) (Tt x Tt) CrossCross

TT tt

TT

tt

ParentsParents (TT x tt) (TT x tt) CrossCross

TT TT

tt

tt

• He then took those hybrids (Tt) and crossed them with one another

Sample Problem # 1Sample Problem # 1

Widow’s peak is Widow’s peak is dominant to straight dominant to straight hairlinehairline

A homozygous A homozygous woman with widow’s woman with widow’s peak reproduces peak reproduces with a man with a with a man with a homozygous man homozygous man with straight hairlinewith straight hairline

What hairline will What hairline will their children (F1) their children (F1) have?have?

Hairline Hairline CrossCross

WW WW

ww

ww

• What genotype (in words) can we expect from a homozygous dominant, homozygous recessive cross?

• ALL offspring will be heterozygous

Sample # 1 ContinuedSample # 1 Continued Now if an offspring of that cross Now if an offspring of that cross

reproduces with another of the same reproduces with another of the same genotype, what hairlines can we expect genotype, what hairlines can we expect for their children?for their children?

What genotype(s) (in words) can we What genotype(s) (in words) can we expect from a cross such as this? expect from a cross such as this?

What ratio of phenotype?What ratio of phenotype?

Our FindingsOur Findings Homozygous dominant crossed with Homozygous dominant crossed with

homozygous recessive yields homozygous recessive yields heterozygous offspring (100%)heterozygous offspring (100%)

A heterozygous crossed with heterozygous A heterozygous crossed with heterozygous yields three options:yields three options:– Homozygous DominantHomozygous Dominant– HeterozygousHeterozygous– Homozygous RecessiveHomozygous Recessive

The ratio of those options are:The ratio of those options are:– A genotype Ratio of 1:2:1 (HD:H:HR)A genotype Ratio of 1:2:1 (HD:H:HR)– A phenotype Ratio of 3:1 (Dominant Trait : A phenotype Ratio of 3:1 (Dominant Trait :

Recessive)Recessive)

Sample Problem # 2Sample Problem # 2

Attached earlobes are Attached earlobes are recessive to un-attached recessive to un-attached (hanging) lobes(hanging) lobes

A homozygous dominant A homozygous dominant man (for the trait) mates man (for the trait) mates with a heterozygous with a heterozygous woman (for same trait)woman (for same trait)

What are the chances of What are the chances of their offspring having their offspring having attached earlobes?attached earlobes?

The chances of their The chances of their child having a child child having a child with attached earlobes with attached earlobes is 0%, 0 in 4, 0/4is 0%, 0 in 4, 0/4

Let’s follow Mendel’s Let’s follow Mendel’s ExperimentExperiment

P1 – P1 –

F1 –F1 –

F2 –F2 –

Let’s try some problemsLet’s try some problems

1. Genetics Practice Problems1. Genetics Practice Problems

2. Bikini Bottom Genetics 2. Bikini Bottom Genetics

Variations in DominanceVariations in Dominance All of the eight traits Mendel All of the eight traits Mendel

looked at had two alleles looked at had two alleles (dominant & recessive) (dominant & recessive)

This ‘all-or-nothing’ expression This ‘all-or-nothing’ expression is known as is known as complete complete dominancedominance

Another form is Another form is incomplete incomplete dominancedominance

This is where neither of the This is where neither of the alleles can be considered alleles can be considered dominant or recessivedominant or recessive

The phenotype is a The phenotype is a mixing mixing of of the two different allelesthe two different alleles

The classic case is with The classic case is with flowersflowers

Red flowers crossed with Red flowers crossed with white flowers gives us pink white flowers gives us pink flowersflowers

Incomplete DominanceIncomplete Dominance In order to represent these In order to represent these

alleles we need to make a alleles we need to make a changechange

Since we’re talking about color Since we’re talking about color we’ll use the letter Cwe’ll use the letter C

For each color we add the For each color we add the capital superscript to the allele capital superscript to the allele

Capital designation always Capital designation always indicates indicates Incomplete Incomplete DominanceDominance

Therefore:Therefore:– CCRR = Red Allele = Red Allele– CCWW = White Allele = White Allele

What the genotypes mean:What the genotypes mean:– CCRR C CRR = Red = Red – CCWW C CWW = White = White– CCRR C CWW = Pink = Pink– CCWW C CRR = Pink = Pink

Codominance with FlowersCodominance with Flowers With codominance neither With codominance neither

allele is dominant, they both allele is dominant, they both show throughshow through

Lets look at flower colorLets look at flower color In codominance a plant with In codominance a plant with

red flowers mating with one red flowers mating with one having white flowers produces having white flowers produces a plant with what?a plant with what?

Both red & white flowersBoth red & white flowers So again we write the alleles So again we write the alleles

differently differently Lower case designation Lower case designation

indicates indicates CodominanceCodominance For flower color:For flower color:

– CCrr C Crr Red Flowers Red Flowers– CCww C Cww = White Flowers = White Flowers– CCrr C Cww = Red & White Flowers = Red & White Flowers

Lets cross a homozygous Lets cross a homozygous red plant with a red plant with a heterozygous oneheterozygous one

What are the chances of our What are the chances of our baby plant having red & baby plant having red & white flowers?white flowers?

Multiple AllelesMultiple Alleles

Some traits are Some traits are controlled by multiple controlled by multiple allelesalleles

It still only takes two It still only takes two alleles to make a genealleles to make a gene

However there are more However there are more than two options for than two options for allelesalleles

For example, the eye For example, the eye color of fruit flies have color of fruit flies have four options for allelesfour options for alleles– EE11 = Red = Red– EE22 = Apricot = Apricot– EE33 = Honey = Honey– EE44 = White = White

Each allele is dominant to Each allele is dominant to the remainingthe remaining– EE11 is dominant to E is dominant to E22, E, E33, & E, & E44

– EE22 is dominant to E is dominant to E33 & E & E44

– EE33 is dominant to E is dominant to E44

– EE44 is the recessive allele is the recessive allele Options for genes would be:Options for genes would be:

– EE11 E E44 = Red Eyes = Red Eyes– EE22 E E33 = Apricot = Apricot– EE44 E E44 = White = White

Examples of Multiple Examples of Multiple Alleles:Alleles:– Blood TypesBlood Types– Types of CornTypes of Corn– Feather Color (in some Feather Color (in some

cases)cases)

Let’s try a Multiple Allele Let’s try a Multiple Allele CrossCross

Monohybrids & DihybridsMonohybrids & Dihybrids Everything we talked about Everything we talked about

so far have been examples of so far have been examples of monhybrid crossesmonhybrid crosses

Monohybrids – crosses that Monohybrids – crosses that are looking at are looking at one set one set of of allelesalleles

We can also do dihybrid We can also do dihybrid crossescrosses

Dihybrids – crosses that are Dihybrids – crosses that are looking at looking at two sets two sets of allelesof alleles

The following are two The following are two examples examples – A monohybrid crossA monohybrid cross– A dihybrid crossA dihybrid cross

The color is just coding for The color is just coding for different phenotypesdifferent phenotypes

Independent AssortmentIndependent Assortment

Dihybrid ExampleDihybrid Example In peas plants tall is dominant to short and purple flowers are In peas plants tall is dominant to short and purple flowers are

dominant to whitedominant to white So if a pea plant is heterozygous for height and heterozygous for So if a pea plant is heterozygous for height and heterozygous for

flower color what is their genotype?flower color what is their genotype?– TtPpTtPp

During the creation of sex cells (meiosis) there are four During the creation of sex cells (meiosis) there are four combinations of alleles the gamete (sex cell) could hold:combinations of alleles the gamete (sex cell) could hold:– TP (Dominant & Dominant alleles)TP (Dominant & Dominant alleles)– Tp (Dominant & Recessive alleles)Tp (Dominant & Recessive alleles)– tP (Recessive & Dominant alleles)tP (Recessive & Dominant alleles)– tp (Two recessive alleles)tp (Two recessive alleles)

If a plant that is heterozygous for both traits reproduced with a If a plant that is heterozygous for both traits reproduced with a plant having the same genotype, what might the phenotype of plant having the same genotype, what might the phenotype of their offspring be?their offspring be?

Dihybrid Cross SummaryDihybrid Cross Summary

If we look at the combinations of If we look at the combinations of phenotypes possible from a dihybrid phenotypes possible from a dihybrid cross between two heterozygous cross between two heterozygous organisms for two traits we find a ratio organisms for two traits we find a ratio of = 9:3:3:1of = 9:3:3:1

9 = Tall plants with purple flowers9 = Tall plants with purple flowers 3 = Tall Plants with white flowers3 = Tall Plants with white flowers 3 = Short Plants with purple flowers 3 = Short Plants with purple flowers 1 = Short Plant with white flowers1 = Short Plant with white flowers

Test CrossTest Cross

Dihybrid Cross PracticeDihybrid Cross Practice

Dihybrid Cross ProblemsDihybrid Cross Problems

Guinea Pig ActivityGuinea Pig Activity

Mendel's Peas and the Pedigree Mendel's Peas and the Pedigree ChartChart