GeneticsGenetics

Mendelian GeneticsMendelian Genetics

GeneticsGenetics – The branch of Biology that – The branch of Biology that studies heredity.studies heredity.

HeredityHeredity – The passing on of – The passing on of characteristics from parent to offspring.characteristics from parent to offspring.

Since early days of farming humans have Since early days of farming humans have been involved with genetics and been involved with genetics and heredity.heredity.

The first to actively study inheritance The first to actively study inheritance was Gregor Mendelwas Gregor Mendel

MendelMendel Mendel was an Austrian monk who studied Mendel was an Austrian monk who studied

pea plants.pea plants. Why would you choose to study peas Why would you choose to study peas

rather than an elephant?rather than an elephant? Traits Mendel studied:Traits Mendel studied:

Seed shapeSeed shape Seed colorSeed color Flower positionFlower position Pod colorPod color Pod shapePod shape Plant height Plant height

Pure breeding and inbreedingPure breeding and inbreeding Pure breedingPure breeding – Organisms that always – Organisms that always

produce the same type of offspring.produce the same type of offspring. Tall plants always produce tall plantsTall plants always produce tall plants Red flowers always produced red flowersRed flowers always produced red flowers

Mendel did all of his genetic work before Mendel did all of his genetic work before the discovery of DNA, chromosomes, the discovery of DNA, chromosomes, genes, evolution and natural selection.genes, evolution and natural selection.

Mendel used mathematics to analyze his Mendel used mathematics to analyze his data.data.

Monohybrid crossesMonohybrid crosses

Monohybrid crossMonohybrid cross – Is a cross between two – Is a cross between two parent organisms that differ in only one trait. parent organisms that differ in only one trait.

Mendel first crossed a pure breeding tall plant Mendel first crossed a pure breeding tall plant to a pure breeding short plant.to a pure breeding short plant.

Parental (PParental (P11)) tall x short tall x short

First generation (FFirst generation (F11)) tall tall (crossed with another F(crossed with another F11))

Second generation (FSecond generation (F22)) 3 Tall : 1 short3 Tall : 1 short

Mendel concludedMendel concluded

There are two There are two ““factorsfactors”” for each trait for each trait These factors or genes have different These factors or genes have different

allelesalleles AlleleAllele – A gene form for each variation of – A gene form for each variation of

an organism.an organism. So a plant could have 2 alleles for tall or 2 So a plant could have 2 alleles for tall or 2

alleles for short, or one from each.alleles for short, or one from each. Mendel further concluded that Mendel further concluded that ““factorsfactors””

were either were either dominantdominant or or recessiverecessive..

Dominant and recessiveDominant and recessive

Dominant allelesDominant alleles – Are traits that are visible. – Are traits that are visible. Observable traits of organisms that masks a Observable traits of organisms that masks a recessive form (T)recessive form (T)

Recessive allelesRecessive alleles – Hidden trait of an – Hidden trait of an organism that is masked by a dominant organism that is masked by a dominant trait. (t)trait. (t)

Possible combinations of traits in Mendelian Possible combinations of traits in Mendelian Genetics:Genetics: TT, tt, or Tt TT, tt, or Tt The dominant letter is always written firstThe dominant letter is always written first Once you select a letter to represent a trait you Once you select a letter to represent a trait you

must not change it.must not change it.

MendelMendel’’s Law of Segregations Law of Segregation

A parent passes on at random only one A parent passes on at random only one allele for each trait to each offspringallele for each trait to each offspring

TTTT or or Tt Tt or or tt tt

TT TT T t T t t tt t

Alleles give an organism its individual Alleles give an organism its individual appearance.appearance.

Phenotype and GenotypePhenotype and Genotype PhenotypePhenotype – the way an organism looks – the way an organism looks

and feels. Its PHYSICAL appearance.and feels. Its PHYSICAL appearance. GenotypeGenotype – The allele (gene) combination – The allele (gene) combination

an organism contains.an organism contains. Phenotype Phenotype tall, short, round tall, short, round Genotype Genotype TT, tt, Rr TT, tt, Rr HomozygousHomozygous – Organism has two alleles – Organism has two alleles

for the trait that are the same (TT or tt)for the trait that are the same (TT or tt) HeterozygousHeterozygous – Two alleles for a trait are – Two alleles for a trait are

different (Tt) different (Tt) Always show the dominant trait Always show the dominant trait

Mendelian genetics and Punnet Mendelian genetics and Punnet squaressquares

P = TT x tt P = TT x tt Tt Tt F1 cross = Tt x TtF1 cross = Tt x Tt F2 generation = 3 tall:1 shortF2 generation = 3 tall:1 short

T t

T

t

TT Tt

Tt tt

tall

tall short

tall

Genotypic ratio 1TT:2Tt:1tt

Phenotypic ratio 3 tall:1 short

Practice problemPractice problem Cross a homozygous blue flower with a heterozygous Cross a homozygous blue flower with a heterozygous

purple flower.purple flower. What color is dominant? What color is dominant?

PurplePurple What color is recessive? What color is recessive?

BlueBlue How do you know? How do you know?

The heterozygote always shows the dominant traitThe heterozygote always shows the dominant trait What is the genotype of the Purple flower?What is the genotype of the Purple flower?

PpPp What is the genotype of the blue flower? What is the genotype of the blue flower?

pppp What are the genotypic and phenotypic ratios of the What are the genotypic and phenotypic ratios of the

cross?cross?

Seat workSeat work Cross the following: TT x Tt, tt x tt, Tt X ttCross the following: TT x Tt, tt x tt, Tt X tt

T = tall, t = short, what are the genotypic T = tall, t = short, what are the genotypic and phenotypic ratios?and phenotypic ratios?

Cross a homozygous red flower with a Cross a homozygous red flower with a heterozygous blue flowerheterozygous blue flower

Cross a homozygous recessive to a Cross a homozygous recessive to a homozygous dominant pink flowerhomozygous dominant pink flower

Cross a heterozygous white flower to Cross a heterozygous white flower to another heterozygous white floweranother heterozygous white flower

Dihybrid CrossDihybrid Cross

A dihybrid cross involves two different traits.A dihybrid cross involves two different traits. Mendel crossed true breeding round and Mendel crossed true breeding round and

yellow seeds with green and wrinkled seeds.yellow seeds with green and wrinkled seeds. The Genotypes are:The Genotypes are:

RR = roundRR = round rr = wrinkled rr = wrinkled YY = yellowYY = yellow yy = greenyy = green

The genotypes for this dihybrid cross are: RRYY x rryy

When writing dihybrid crosses, we always keep the alleles together RrYy, rrYY, Rryy

Dihybrid crossDihybrid cross True breeding PTrue breeding P11 = RRYY x rryy = RRYY x rryy Resulting FResulting F11 = RrYy = RrYy All are round and yellowAll are round and yellow Mendel then crossed the FMendel then crossed the F11 together and together and

observed the following:observed the following: 9 round and yellow:3 round and green:3 wrinkled 9 round and yellow:3 round and green:3 wrinkled

and yellow: 1 wrinkled and green.and yellow: 1 wrinkled and green. Law of independent assortmentLaw of independent assortment – Genes for – Genes for

different traits are inherited independently of different traits are inherited independently of each other.each other. That means in the above example, the seed color does That means in the above example, the seed color does

not affect the seed shape, and the seed shape does not affect the seed shape, and the seed shape does not affect the seed color.not affect the seed color.

Dihybrid crossDihybrid cross

See side board for how to use a See side board for how to use a punnet square for a dihybrid cross.punnet square for a dihybrid cross.

RRYY x rryy, RrYy x RrYy, Rryy x RrYyRRYY x rryy, RrYy x RrYy, Rryy x RrYy R=round, r=wrinkledR=round, r=wrinkled Y=yellow, y=greenY=yellow, y=green

Non-Mendelian Genetics Non-Mendelian Genetics

The exceptions to the ruleThe exceptions to the rule

Non-Mendelian GeneticsNon-Mendelian Genetics Even though a trait is not simply Even though a trait is not simply

dominant or recessive, geneticists can dominant or recessive, geneticists can still predict and explain their still predict and explain their appearance.appearance.

We will discuss 3 examples of non-We will discuss 3 examples of non-mendelian genetics:mendelian genetics: Incomplete dominanceIncomplete dominance Co-dominanceCo-dominance Sex-linked traitsSex-linked traits

Incomplete DominanceIncomplete Dominance

When the phenotype of the When the phenotype of the heterozygote is an intermediate of the heterozygote is an intermediate of the two homozygotes.two homozygotes.

Sometimes called blendingSometimes called blending Red flower x white flower Red flower x white flower pink flower pink flower RR – redRR – red xx WW – white WW – white R R

W

W

RWpin

k

RWpink

RW

RWpink pink

Notice that I used a capital letter for both and distinguished them by using different letters

The only difference is in how we express the genotype and the appearance of the organism

Incomplete dominanceIncomplete dominance

Cross two pink flowers, what do you Cross two pink flowers, what do you think the phenotypes will be?think the phenotypes will be?

RW x RWRW x RW R W

R

W

RR RW

RW

WWwhitepink

pinkred

Co-dominanceCo-dominance

Both alleles are expressed equally.Both alleles are expressed equally. black rabbit x white rabbit black rabbit x white rabbit spotted spotted

rabbitrabbit With co-dominance since both alleles With co-dominance since both alleles

are seen we use capital letters and the are seen we use capital letters and the prime symbol (prime symbol (‘‘))

Co-dominanceCo-dominance

BB BB (black)(black) x B x B’’BB ’’ (white)(white) BB BB’’ (black and (black and white)white)

BBBB’’ x BB x BB’’ B B ’

B

B ’

BB

Black

BB’

Black & white spots

BB’

Black & white spots

B ’B ’

White

Sex linked traitsSex linked traits Also called x linked.Also called x linked. Sex linked traits are controlled by genes Sex linked traits are controlled by genes

located on sex chromosomes.located on sex chromosomes. Sex chromosomesSex chromosomes determine the gender of an determine the gender of an

individual. In humans, the sex chromosomes individual. In humans, the sex chromosomes are the 23are the 23rdrd pair. pair. These are the only chromosomes that are not These are the only chromosomes that are not

homologous!homologous! AutosomesAutosomes – pairs of matching homologous – pairs of matching homologous

chromosomes. (all the chromosomes except chromosomes. (all the chromosomes except the sex chromosomesthe sex chromosomes

Sex linked traitsSex linked traits

In humans as well as fruit flies, males In humans as well as fruit flies, males are XY, females are XX.are XY, females are XX.

Males produce gametes that will have Males produce gametes that will have either the X or Y sex chromosome.either the X or Y sex chromosome.

Females produce gametes that will Females produce gametes that will just carry the X sex chromosome.just carry the X sex chromosome.

This means that the gender of a child This means that the gender of a child is determined by the father. is determined by the father.

Sex linked traits Sex linked traits

Color blindness in humans and eye Color blindness in humans and eye color in fruit flies are two examples color in fruit flies are two examples of sex linked traits.of sex linked traits.

See the side board for how to work See the side board for how to work on a sex linked trait on a sex linked trait

KaryotypesKaryotypes

A karyotype is a picture of an individualA karyotype is a picture of an individual’’s s chromosomes.chromosomes.

Karyotypes can be used to determine if Karyotypes can be used to determine if there have been mutations to a there have been mutations to a chromosome.chromosome. Mutations like duplication or deletionsMutations like duplication or deletions

Human males and females have different Human males and females have different Karyotypes because of differences in the Karyotypes because of differences in the sex chromosomes (X and Y chromosomes). sex chromosomes (X and Y chromosomes).

PedigreePedigree Pedigrees are graphical representations Pedigrees are graphical representations

used to predict the likelihood of the used to predict the likelihood of the appearance of a trait.appearance of a trait.

Symbols of a pedigree:Symbols of a pedigree:Affected male

Unaffected male

Affected female Unaffected female

Known heterozygote, male

Known heterozygote, female

A mating

offspring

I

II

1 2 3 4

PedigreesPedigrees Used to determine the possibility of a Used to determine the possibility of a

genetic defect or probability of a genetic defect or probability of a specific trait.specific trait.

See side board for a pedigreeSee side board for a pedigree

Odds and EndsOdds and Ends Test crossTest cross is cross of an individual of is cross of an individual of

unkown genotype with an individual of unkown genotype with an individual of known genotype in order to determine known genotype in order to determine the unknown genotypethe unknown genotype

A carrier is another term for a A carrier is another term for a heterozygous individual.heterozygous individual.

A A hybridhybrid is an offspring produced when is an offspring produced when two varieties of plants or animals are two varieties of plants or animals are mated. Hybrids often exhibit greater mated. Hybrids often exhibit greater vigor and size than their parents. A vigor and size than their parents. A phenomena termed hybrid vigor.phenomena termed hybrid vigor.

Chromosome structure and Chromosome structure and allele locusallele locus

Allele for purple flowers

Locus for flower-color gene

Allele for white flowers

Homologouspair ofchromosomes

Linked genesLinked genes

Linked genes Linked genes are genes for two are genes for two different traits that are located close different traits that are located close together on the same chromosome.together on the same chromosome. Linked genes tend to travel together Linked genes tend to travel together

during crossing overduring crossing over

Closely linked NotLinked

PleiotropyPleiotropy

Most genes have multiple Most genes have multiple phenotypic effects, a property phenotypic effects, a property called called pleiotropy pleiotropy

For example, pleiotropic alleles are For example, pleiotropic alleles are responsible for the multiple responsible for the multiple symptoms of certain hereditary symptoms of certain hereditary diseases, such as cystic fibrosis diseases, such as cystic fibrosis and sickle-cell diseaseand sickle-cell disease

EpistasisEpistasis In In epistasisepistasis, a gene at one locus alters , a gene at one locus alters

the phenotypic expression of a gene at a the phenotypic expression of a gene at a second locussecond locus

For example, in mice and many other For example, in mice and many other mammals, coat color depends on two mammals, coat color depends on two genesgenes

One gene determines the pigment color One gene determines the pigment color (with alleles (with alleles BB for black and for black and bb for brown) for brown)

The other gene (with alleles The other gene (with alleles CC for color for color and and cc for no color) determines whether for no color) determines whether the pigment will be deposited in the hairthe pigment will be deposited in the hair

Polygenic InheritancePolygenic Inheritance• Quantitative charactersQuantitative characters are are

those that vary in the population those that vary in the population along a continuumalong a continuum

• Quantitative variation usually Quantitative variation usually indicates indicates polygenic inheritancepolygenic inheritance, , an additive effect of two or more an additive effect of two or more genes on a single phenotypegenes on a single phenotype

• Skin color in humans is an example Skin color in humans is an example of polygenic inheritanceof polygenic inheritance

Multiple Allele TraitsMultiple Allele Traits

Most genes exist in populations in more than Most genes exist in populations in more than two allelic formstwo allelic forms

For example, the four phenotypes of the For example, the four phenotypes of the ABO blood group in humans are determined ABO blood group in humans are determined by three alleles for the enzyme (I) that by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood attaches A or B carbohydrates to red blood cells: cells: IIAA, , IIBB, and , and ii..

The enzyme encoded by theThe enzyme encoded by the I IAA allele adds allele adds the A carbohydrate, whereas the enzyme the A carbohydrate, whereas the enzyme encoded by the encoded by the IIBB allele adds the B allele adds the B carbohydrate; the enzyme encoded by the carbohydrate; the enzyme encoded by the ii allele adds neitherallele adds neither

35

Any of a set of three or more alleles, Any of a set of three or more alleles, or alternative states of a gene, only or alternative states of a gene, only two of which can be present at any two of which can be present at any given time in a diploid organism.given time in a diploid organism. Example: Example: PPhysical characteristics such as eye hysical characteristics such as eye

color, skin color, height and coat color in animals color, skin color, height and coat color in animals …these are also …these are also POLYGENIC traitsPOLYGENIC traits controlled controlled by many genes interacting together!by many genes interacting together!

copyright cmassengale

Multiple Alleles and Multiple Alleles and Codiminance - BLOOD TYPINGCodiminance - BLOOD TYPING

Blood is a complex, living tissue that contains many Blood is a complex, living tissue that contains many cell types and proteins. Distinct molecules called cell types and proteins. Distinct molecules called agglutinogens (a type of antigen) are attached to agglutinogens (a type of antigen) are attached to the surface of red blood cells. the surface of red blood cells. There are two different types of antigen markers, type There are two different types of antigen markers, type ““AA ””

and type and type ““BB””. Each type has different properties.. Each type has different properties. The ABO blood type classification system uses the presence The ABO blood type classification system uses the presence

or absence of these molecules to categorize blood into four or absence of these molecules to categorize blood into four types.types.

type Atype A = AA or AO= AA or AO type Btype B = BB or BO= BB or BO type ABtype AB = AB= AB type Otype O = OO (sometimes ii)= OO (sometimes ii)•A, B, AB, and O are the POSSIBLE

PHENOTYPES•A and B are DOMINANT alleles•i or type O is the RECESSIVE allele

Chart of Blood TypesChart of Blood Types

WHEN BLOOD TYPES MIX

Blood plasma is packed with proteins called antibodies. The body produces a wide variety of antibodies that will recognize and attack foreign molecules that may enter from the outside world. A person’s plasma does not contain any antibodies that will bind to molecules that are part of his or her own body.

When conducting a blood transfusion, it is important to carefully match the donor and recipient blood types. If the donor blood cells have surface molecules that are different from those of the recipient, antibodies in the recipient’s blood recognize the donor blood as foreign. This triggers an immune response resulting in blood clotting. If the donor blood cells have surface molecules that are the same as those of the recipient, the recipient’s body will not see them as foreign and will not mount an immune response.

There are two special blood types when it comes to blood transfusions. People with type O blood are universal donors because there are no molecules on the surface of the red blood cells that can trigger an immune response. People with type AB blood are universal recipients because they do not have any antibodies that will recognize type A or B surface molecules.

Note: Blood cells are covered with a variety of surface molecules. For simplicity, only type A and B surface molecules are shown here.

Nature and Nurture: The Nature and Nurture: The Environmental Impact on PhenotypeEnvironmental Impact on Phenotype

Another departure from Mendelian genetics Another departure from Mendelian genetics arises when the phenotype for a character arises when the phenotype for a character depends on environment as well as genotypedepends on environment as well as genotype

The The norm of reactionnorm of reaction is the phenotypic is the phenotypic range of a genotype influenced by the range of a genotype influenced by the environmentenvironment

For example, hydrangea flowers of the same For example, hydrangea flowers of the same genotype range from blue-violet to pink, genotype range from blue-violet to pink, depending on soil aciditydepending on soil acidity

Fig. 14-14