CHAPTER 10CHAPTER 10

Mendelian GeneticsMendelian Genetics

MENDELIAN GENETICSMENDELIAN GENETICS

GeneticsGenetics - branch of biology that - branch of biology that studies how genetic characteristics studies how genetic characteristics are inheritedare inherited

First investigated by Gregor Mendel, First investigated by Gregor Mendel, an Augustinian monk (1822-1884)an Augustinian monk (1822-1884)

His work was not accepted until 1900 His work was not accepted until 1900 (30 years later)(30 years later)

The study of inheritance is often The study of inheritance is often called Mendelian genetics.called Mendelian genetics.

GENE INHERITANCE GENE INHERITANCE Diploid organisms have two forms of a Diploid organisms have two forms of a

gene – one from each parent gene – one from each parent Several different forms (alleles) of each Several different forms (alleles) of each

gene may exist within a populationgene may exist within a population AllelesAlleles are found at the same location are found at the same location

on a chromosome (called the on a chromosome (called the locuslocus)) Ex: In humans there are two alleles for Ex: In humans there are two alleles for

ear lobe shape (attached and ear lobe shape (attached and unattached)unattached)

Ear Lobe ShapeEar Lobe Shape

GENE INHERITANCEGENE INHERITANCE An organism’s An organism’s genomegenome is the set of all is the set of all

its genes.its genes. GenotypeGenotype – genetic makeup of the – genetic makeup of the

organismorganism PhenotypePhenotype – how alleles are expressed – how alleles are expressed

(physical characteristics)(physical characteristics) Alleles can be described as: Alleles can be described as:

DominantDominant – trait that is expressed – trait that is expressed Designated by a capital letter (EE)Designated by a capital letter (EE)

RecessiveRecessive – trait is masked by dominant – trait is masked by dominant and not expressedand not expressed

Designated by a lowercase letter (ee)Designated by a lowercase letter (ee)

GENE INHERITANCEGENE INHERITANCE HomozygousHomozygous - alleles are of the - alleles are of the

same typesame type both are dominant or both are both are dominant or both are

recessiverecessive AA, aaAA, aa

HeterozygousHeterozygous – alleles are different – alleles are different one is dominant and the other is one is dominant and the other is

recessiverecessive AaAa

GENE INHERITANCEGENE INHERITANCE

In homozygous individuals, the trait In homozygous individuals, the trait expressed depends on what alleles are expressed depends on what alleles are present (AA, aa)present (AA, aa)

Heterozygous individuals are known as Heterozygous individuals are known as carrierscarriers::

Only dominant allele is expressed orOnly dominant allele is expressed or Trait will be expressed only under certain Trait will be expressed only under certain

conditionsconditions Ex: traits which are sex-linkedEx: traits which are sex-linked

The environment also can determine The environment also can determine whether or not a trait is expressedwhether or not a trait is expressed

““nature vs. nurture”nature vs. nurture”

Dark color in cats is expressed on the parts of the body that stay cool.

The gene for freckles expresses itself more fully when a person is exposed to the sun.

GENE INHERITANCE:GENE INHERITANCE:CODOMINANCECODOMINANCE

Both alleles are Both alleles are expressed (dominant)expressed (dominant)

Ex: petal color in Ex: petal color in snapdragonssnapdragons

Homozygous plants Homozygous plants (RR, WW) produce (RR, WW) produce either red or white either red or white flowersflowers

Heterozygous plants Heterozygous plants (RW) produce pink (RW) produce pink colored flowers colored flowers

GENE INHERITANCE:GENE INHERITANCE:X-LINKED GENESX-LINKED GENES

Refers to genes found on X-Refers to genes found on X-chromosomes chromosomes

Also called sex-linked genesAlso called sex-linked genes

These genes are inherited together These genes are inherited together because they are found on the same because they are found on the same chromosome – called a linkage groupchromosome – called a linkage group

Females get two copies of X-linked Females get two copies of X-linked genes (XX), males get one copy (XY)genes (XX), males get one copy (XY)

GENE INHERITANCE:GENE INHERITANCE:X-LINKED GENESX-LINKED GENES

The Y is much smaller than the X The Y is much smaller than the X chromosome, thus it has less genes.chromosome, thus it has less genes.

In men, because there is only one X, In men, because there is only one X, genes on the X-chromosomes will be genes on the X-chromosomes will be expressed.expressed.

X-linked genes with abnormal traits are:X-linked genes with abnormal traits are: color blindnesscolor blindness HemophiliaHemophilia brown teethbrown teeth muscular dystrophy (Becker’s and muscular dystrophy (Becker’s and

Duchenne’s)Duchenne’s) These are much more common in menThese are much more common in men

SEX CHROMOSOMESSEX CHROMOSOMES

MENDEL’S LAW OF MENDEL’S LAW OF HEREDITYHEREDITY

Gregor Mendel developed a method Gregor Mendel developed a method to predict inheritanceto predict inheritance

His work focused on:His work focused on: Garden peas - determined dominant and Garden peas - determined dominant and

recessive traitsrecessive traits Obvious characteristics (Height, flower Obvious characteristics (Height, flower

color, seed shape, etc.)color, seed shape, etc.) One trait at a timeOne trait at a time

Lead to the development of three Lead to the development of three lawslaws

MENDEL’S LAWSMENDEL’S LAWS Law of dominance Law of dominance — — an allele that is an allele that is

expressed over another allele is said to be expressed over another allele is said to be dominant.dominant.

Law of segregationLaw of segregation— — during meiosis, alleles during meiosis, alleles separate from one another into different separate from one another into different gametes, retaining their individualitygametes, retaining their individuality

Law of independent assortmentLaw of independent assortment——members members of one gene pair separate independently of of one gene pair separate independently of other gene pairsother gene pairs

Do not apply to linked genesDo not apply to linked genes

PROBABILITYPROBABILITY

Genetic problems are based on Genetic problems are based on probabilityprobability

The chance that an event will happenThe chance that an event will happen In In monohybrid crossesmonohybrid crosses, a single trait , a single trait

is studied. is studied. Crossing tall plants with short plants, Crossing tall plants with short plants,

green peas with yellow peas, etc.green peas with yellow peas, etc. Punnett squaresPunnett squares are tools that are are tools that are

used to determine probabilityused to determine probability

MONOHYBRID CROSSESMONOHYBRID CROSSES

A tall pea plant crossed with a tall A tall pea plant crossed with a tall pea plant will produce offspring that pea plant will produce offspring that are all tallare all tall

TT TT

TT TT

• T T• T T

• T T•T T

MONOHYBRID CROSSESMONOHYBRID CROSSES

A short pea plant crossed with a A short pea plant crossed with a short pea plant will produce offspring short pea plant will produce offspring that are all shortthat are all short

tt tt

tt tt • t t• t t

• t t•t t

MONOHYBRID CROSSESMONOHYBRID CROSSES A tall pea plant crossed with a short A tall pea plant crossed with a short

pea plant will produce all tall pea plant will produce all tall offspring:offspring:

tt tt TT TT

Tall is dominant - The phenotype is Tall is dominant - The phenotype is 100% tall. 100% tall.

The genotype is 100% heterozygous The genotype is 100% heterozygous tall (Tt)tall (Tt)

• T t• T t

• T t•T t

MONOHYBRID CROSSESMONOHYBRID CROSSES A heterozygous tall pea plant crossed with A heterozygous tall pea plant crossed with

a heterozygous tall pea plant will produce a heterozygous tall pea plant will produce varying offspringvarying offspring

TT t t TT

tt

One will be homozygous tall, two will be One will be homozygous tall, two will be heterozygous tall and one will be heterozygous tall and one will be homozygous short. homozygous short.

Genotypic ratio - 1:2:1Genotypic ratio - 1:2:1 Phenotypic ratio - 3:1 (3 tall, 1Phenotypic ratio - 3:1 (3 tall, 1 short) short)

• t t• T t

• T t•T T

MONOHYBRID CROSSES:MONOHYBRID CROSSES:CODOMINANCECODOMINANCE

A red snapdragon (RR) crossed with a white A red snapdragon (RR) crossed with a white snapdragon (WW) will produce offspring that are all snapdragon (WW) will produce offspring that are all pinkpink

WW W W

RR

RR

All offspring will be heterozygous, and because red All offspring will be heterozygous, and because red and white are codominant, all the flowers will be and white are codominant, all the flowers will be pink. pink.

Genotypic ratio: all are heterozygous (RWGenotypic ratio: all are heterozygous (RW)) Phenotypic ratio: all are pink Phenotypic ratio: all are pink

• RW•RW

• RW•RW

MONOHYBRID CROSSES: MONOHYBRID CROSSES: CODOMINANCECODOMINANCE

A heterozygous pink crossed with a heterozygous pink, will A heterozygous pink crossed with a heterozygous pink, will produce varying offspring:produce varying offspring:

WW RR

WW

RR

One will be homozygous white, two will be One will be homozygous white, two will be heterozygous pink, and one will be homozygous red. heterozygous pink, and one will be homozygous red.

The genotypic ratio will be 1:2:1 and the phenotypic The genotypic ratio will be 1:2:1 and the phenotypic ratio will also be 1:2:1.ratio will also be 1:2:1.

• RR•RW

• RW•WW

DIHYBRID CROSSESDIHYBRID CROSSES

Two pairs of alleles are followed from Two pairs of alleles are followed from parents to offspring. parents to offspring.

Crossing two individuals heterozygous Crossing two individuals heterozygous for earlobes and color of hair, the for earlobes and color of hair, the gametes can combine in 16 different gametes can combine in 16 different ways. ways.

EeDd x EeDdEeDd x EeDd E – free earlobes; e – attached earlobes; D E – free earlobes; e – attached earlobes; D

– dark hair; d – light hair– dark hair; d – light hair

DIHYBRID CROSSESDIHYBRID CROSSES EDED Ed Ed eD eD ed ed

EDED

EdEd

eDeD

eded

The probability for a given phenotype will be 9:3:3:1.The probability for a given phenotype will be 9:3:3:1. 9 free earlobes, dark hair9 free earlobes, dark hair 3 free earlobes, light hair3 free earlobes, light hair 3 attached earlobes, dark hair3 attached earlobes, dark hair 1 attached earlobes, light hair 1 attached earlobes, light hair

•eedd•eeDd•Eedd•EeDd

•eeDd•eeDD•EeDd•EeDD

•Eedd•EeDd•EEdd•EEDd

•EeDd•EeDD•EEDd•EEDD

ALTERNATIVE ALTERNATIVE INHERITANCE:INHERITANCE:

MULTIPLE ALLELESMULTIPLE ALLELES Some genes are expressed by the presence Some genes are expressed by the presence of more than 2 alleles.of more than 2 alleles.

Each person only gets two alleles, but in the Each person only gets two alleles, but in the population, there are more possibilities.population, there are more possibilities.

ABO blood type in humansABO blood type in humans Determined by 3 alleles (IA, IB, i)Determined by 3 alleles (IA, IB, i) 2 alleles are codominant, one is recessive2 alleles are codominant, one is recessive GenotypeGenotype PhenotypePhenotype IAIAIAIA type Atype A IAiIAi type Atype A IBIBIBIB type Btype B IBiIBi type Btype B IAIBIAIB type ABtype AB iiii type Otype O

ALTERNATIVE ALTERNATIVE INHERITANCE:INHERITANCE:

POLYGENIC INHERITANCEPOLYGENIC INHERITANCE Traits are determined by several Traits are determined by several

genes at different locigenes at different loci Genes may be found on the same Genes may be found on the same

chromosome or on different chromosomeschromosome or on different chromosomes Skin color in humans is an exampleSkin color in humans is an example

Dark skin is dominant over light skinDark skin is dominant over light skin 3 different loci for skin color3 different loci for skin color Expressed as many possible variationsExpressed as many possible variations

POLYGENIC INHERITANCEPOLYGENIC INHERITANCE

ALTERNATIVE ALTERNATIVE INHERITANCE:INHERITANCE:PLEIOTROPYPLEIOTROPY

Describes the multiple effects that a gene Describes the multiple effects that a gene may have on a phenotypemay have on a phenotype

Examples include PKU and Marfan Examples include PKU and Marfan SyndromeSyndrome

PKU – a single gene affects many chemical PKU – a single gene affects many chemical reactions (autosomal recessive disorder)reactions (autosomal recessive disorder)

Determines how a cell metabolizes Determines how a cell metabolizes phenylalanine (amino acid)phenylalanine (amino acid)

Causes retardation if not treated with a proper Causes retardation if not treated with a proper dietdiet

Very light skin colorVery light skin color

ALTERNATIVE ALTERNATIVE INHERITANCE:INHERITANCE:PLEIOTROPYPLEIOTROPY

Marfan syndrome affects the connective Marfan syndrome affects the connective tissue but can also affect other tissues tissue but can also affect other tissues (autosomal dominant disorder)(autosomal dominant disorder)

Symptoms include:Symptoms include: Disproportionately long arms, legs, and fingersDisproportionately long arms, legs, and fingers Skinniness and scoliosis of the spineSkinniness and scoliosis of the spine Myopia (near-sightedness) and dislocated Myopia (near-sightedness) and dislocated

lenseslenses Weak or defective heart valves, aneurysms, Weak or defective heart valves, aneurysms,

and aortic dissectionsand aortic dissections Collapsed lungs and sleep apneaCollapsed lungs and sleep apnea

MARFAN SYNDROMEMARFAN SYNDROME

ENVIRONMENTAL ENVIRONMENTAL INFLUENCESINFLUENCES

Gene expression may vary on an Gene expression may vary on an individual basisindividual basis

Polydactylism (6 fingers)Polydactylism (6 fingers) Hormones on voice changes at pubertyHormones on voice changes at puberty Male-pattern baldnessMale-pattern baldness PKU expression (proper diet prevents PKU expression (proper diet prevents

symptoms)symptoms) Familial diabetes can be delayed by Familial diabetes can be delayed by

reducing sugarreducing sugar TanningTanning

CHAPTER 10CHAPTER 10

Mendelian GeneticsMendelian Genetics

Dark color in cats is expressed on the parts of the body that stay cool.

The gene for freckles expresses itself more fully when a person is exposed to the sun.

• T T• T T

• T T•T T

• t t• t t

• t t•t t

• T t• T t

• T t•T t

• t t• T t

• T t•T T

• RW•RW

• RW•RW

• RR•RW

• RW•WW

•eedd•eeDd•Eedd•EeDd

•eeDd•eeDD•EeDd•EeDD

•Eedd•EeDd•EEdd•EEDd

•EeDd•EeDD•EEDd•EEDD