chapter 11 mendel & mutations father of genetics monk and teacher. experimented with purebred...

CHAPTER 11CHAPTER 11

MENDEL & MUTATIONSMENDEL & MUTATIONS

Father of GeneticsFather of Genetics Monk and teacher.Monk and teacher. Experimented with purebred tall and Experimented with purebred tall and

short peas. short peas.

Discovered some of the basic laws of Discovered some of the basic laws of heredity.heredity.

Studied seven purebred traits in peas.Studied seven purebred traits in peas. Called the stronger hereditary factor Called the stronger hereditary factor

dominant.dominant. Called the weaker hereditary factor Called the weaker hereditary factor

recessive.recessive. Presentation to the Science Society Presentation to the Science Society

in1866 went unnoticed. in1866 went unnoticed. He died in 1884 with his work still He died in 1884 with his work still

unnoticed.unnoticed. His work rediscovered in 1900.His work rediscovered in 1900. Known as the “Father of Known as the “Father of

Genetics”. Genetics”.

Mendel’s ObservationsMendel’s Observations

He noticed that peas are easy to breed for He noticed that peas are easy to breed for pure traits and he called the pure strains pure traits and he called the pure strains purebreds.purebreds.

He developed pure strains of peas for seven He developed pure strains of peas for seven different traits (i.e. tall or short, round or different traits (i.e. tall or short, round or wrinkled, yellow or green, etc.)wrinkled, yellow or green, etc.)

He crossed these pure strains to produce He crossed these pure strains to produce hybrids.hybrids.

He crossed thousands of plants and kept He crossed thousands of plants and kept careful records for eight years.careful records for eight years.

Mendel’s PeasMendel’s Peas In peas many traits appear in two forms (i.e. In peas many traits appear in two forms (i.e.

tall or short, round or wrinkled, yellow or tall or short, round or wrinkled, yellow or green.)green.)

The flower is the reproductive organ and the The flower is the reproductive organ and the male and female are both in the same flower.male and female are both in the same flower.

He crossed pure strains by putting the pollen He crossed pure strains by putting the pollen (male gamete) from one purebred pea plant on (male gamete) from one purebred pea plant on the pistil (female sex organ) of another the pistil (female sex organ) of another purebred pea plant to form a hybrid or purebred pea plant to form a hybrid or crossbred. crossbred.

Analyzing Mendel’s ResultsAnalyzing Mendel’s Results

Analyses using Punnett squares Analyses using Punnett squares demonstrate that Mendel’s results demonstrate that Mendel’s results reflect independent segregation of reflect independent segregation of gametes.gametes.

The Testcross:The Testcross: Can be used to determine the genotype Can be used to determine the genotype

of an individual when two genes are of an individual when two genes are involved.involved.

MENDEL’S LAWS OF HEREDITYMENDEL’S LAWS OF HEREDITY

WHY MENDEL SUCCEEDEDWHY MENDEL SUCCEEDED Gregor Mendol – father of geneticsGregor Mendol – father of genetics 11stst studies of studies of heredityheredity – the passing of – the passing of

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

traitstraits

From Genotype to PhenotypeFrom Genotype to Phenotype

Multiple Alleles:Multiple Alleles: Sometimes more than two alleles (multiple Sometimes more than two alleles (multiple

alleles) exist for a given trait in a population.alleles) exist for a given trait in a population. EX. ABO blood designation.EX. ABO blood designation. A and B are codominant.A and B are codominant. Rh Blood group:Rh Blood group:

Rh is a cell surface marker on red blood cellsRh is a cell surface marker on red blood cells About 85% of the population is Rh+ (have the About 85% of the population is Rh+ (have the

marker)marker) Problems: Mother is Rh negative has an Rh+ fetus.Problems: Mother is Rh negative has an Rh+ fetus.

MENDEL CHOSE HIS SUBJECT MENDEL CHOSE HIS SUBJECT CAREFULLYCAREFULLY

Used garden peas to studyUsed garden peas to study Have male & female Have male & female gametesgametes (sex cells) (sex cells) Male & female same flowerMale & female same flower Know what Know what pollinationpollination & & fertilizationfertilization

meanmean He could control the fertilization processHe could control the fertilization process Not many traits to keep track ofNot many traits to keep track of

MENDEL WAS A CAREFUL 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

MENDEL’S MONOHYBRID MENDEL’S MONOHYBRID CROSSESCROSSES

MENDEL STUDIED 7 TRAITS CAREFULLYMENDEL STUDIED 7 TRAITS CAREFULLY 11.111.1

Mendel crossed plants w/ diff. traits to Mendel crossed plants w/ diff. traits to see what traits the offspring would havesee what traits the offspring would have

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

THE 1THE 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 parental These plants are called the parental 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)

THE 2THE 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!!!!!!)

11.3 Mendel Proposes a Theory11.3 Mendel Proposes a Theory

By convention, genetic traits are assigned By convention, genetic traits are assigned a letter symbol referring to their more a letter symbol referring to their more common formcommon form dominant traits are represented by uppercase dominant traits are represented by uppercase

letters, and lower-case letters are used for letters, and lower-case letters are used for recessive traitsrecessive traits

for example, flower color in peas is for example, flower color in peas is represented as followsrepresented as follows

PP signifies purple signifies purple pp signifies white signifies white

Mendel Proposes a TheoryMendel Proposes a Theory The results from a cross between a true-breeding, The results from a cross between a true-breeding,

white-flowered plant (white-flowered plant (pppp) and a true breeding, ) and a true breeding, purple-flowered plant (purple-flowered plant (PPPP) can be visualized with ) can be visualized with a a Punnett squarePunnett square

A Punnett square lists the possible gametes from A Punnett square lists the possible gametes from one individual on one side of the square and the one individual on one side of the square and the possible gametes from the other individual on the possible gametes from the other individual on the opposite sideopposite side

The genotypes of potential offspring are The genotypes of potential offspring are represented within the squarerepresented within the square

Figure 11.7 A Punnett square Figure 11.7 A Punnett square analysisanalysis

Figure 11.8 How Mendel analyzed Figure 11.8 How Mendel analyzed 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 ALLELS ARE NOW KNOWN THESE 2 ALLELS 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

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

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 GENERATIONFATHERFATHER 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

ANSWER ON YOUR SHEETANSWER ON YOUR SHEET

TRAITS = BLUE SKIN & YELLOW SKINTRAITS = BLUE SKIN & YELLOW SKIN

BBBB – IS THIS HOMOZYGOUS OR – IS THIS HOMOZYGOUS OR HETEROZYGOUS?HETEROZYGOUS?

IS BLUE SKIN OR YELLOW SKIN IS BLUE SKIN OR YELLOW SKIN DOMINANT? DOMINANT?

HOMOZYGOUSHOMOZYGOUS

BLUEBLUE

MENDEL’S DIHYBRID CROSSESMENDEL’S DIHYBRID CROSSES

MONOHYBRID – MENDEL LOOKED AT MONOHYBRID – MENDEL LOOKED AT ONE TRAITONE TRAIT

IN HIS DIHYBRID CROSSES – HE IN HIS DIHYBRID CROSSES – HE LOOKED AT 2 TRAITS LOOKED AT 2 TRAITS

WANTED TO SEE IF TRAITS ARE WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INHERITED TOGETHER OR INDEPENDENTLYINDEPENDENTLY

DIHYBRID CROSSDIHYBRID CROSS

TOOK TWO TRUE BREEDING PLANTS TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS ------- (ROUND/WRINKLED SEEDS ------- YELLOW/GREEN SEEDS)YELLOW/GREEN SEEDS)

11STST GENERATION GENERATION WHAT WOULD HAPPEN IF HE CROSSED JUST WHAT WOULD HAPPEN IF HE CROSSED JUST

TRUE BREEDING ROUND W/ TRUE TRUE BREEDING ROUND W/ TRUE BREEDING WRINKLED (ROUND IS BREEDING WRINKLED (ROUND IS DOMINANT)DOMINANT)ALL THE OFFSPRING ARE ALL THE OFFSPRING ARE

ROUNDROUND

DIHYBRID CROSS – 1DIHYBRID CROSS – 1STST GENERATION CONT’DGENERATION CONT’D

SO WHAT DO YOU THINK HAPPENED SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW SEEDS WITH TRUE ROUND/YELLOW SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDSBREEDING WRINKLED/GREEN SEEDS

ALL THE F1 WERE ROUND ALL THE F1 WERE ROUND AND YELLOWAND YELLOW

DIHYBRID CROSS – 2DIHYBRID CROSS – 2NDND GENERATIONGENERATION

TOOK THE F1 PLANTS AND BRED TOOK THE F1 PLANTS AND BRED THEM TOGETHER (PHENOTYPE WAS THEM TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW)ROUND/YELLOW X ROUND/YELLOW)

22NDND GENERATION GENERATION FOUND ROUND/YELLOW - 9FOUND ROUND/YELLOW - 9 FOUND ROUND/GREEN - 3FOUND ROUND/GREEN - 3 FOUND WRINKLED/YELLOW - 3FOUND WRINKLED/YELLOW - 3 FOUND WRINKLED/GREEN - 1FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO) ( 9 : 3 : 3 : 1 RATIO)

EXPLANATION OF 2EXPLANATION OF 2NDND GENERATIONGENERATION

MENDEL CAME UP W/ 2MENDEL CAME UP W/ 2NDND LAW – THE LAW – THE LAW OF INDEPENDENT ASSORTMENTLAW OF INDEPENDENT ASSORTMENT GENES FOR DIFFERENT TRAITS ARE GENES FOR DIFFERENT TRAITS ARE

INHERITED INDEPENDENTLY FROM EACH INHERITED INDEPENDENTLY FROM EACH OTHEROTHER

THIS IS WHY MENDEL FOUND ALL THE THIS IS WHY MENDEL FOUND ALL THE DIFFERNENT COMBONATIONS OF TRAITSDIFFERNENT COMBONATIONS OF TRAITS

PUNNETT SQUARESPUNNETT SQUARES

A QUICK WAY TO FIND THE A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENOTYPES IN UPCOMING GENERATIONSGENERATIONS

11STST DRAW A BIG SQUARE AND DIVIDE DRAW A BIG SQUARE AND DIVIDE IT IN 4’SIT IN 4’S

PUNNETT SQUAREPUNNETT SQUARE

CROSS T T X TtCROSS T T X Tt

CONT’DCONT’D

T T X T tT T X T tTT TT

TT

tt

TT TT TT TT

TT tt TT tt

DIHYBRID CROSSESDIHYBRID CROSSES

A LITTLE DIFFERENTA LITTLE DIFFERENT H h G g X H h G gH h G g X H h G g MUST FIND OUT ALL THE POSSIBLE MUST FIND OUT ALL THE POSSIBLE

ALLELIC COMBONATIONSALLELIC COMBONATIONS USE THE FOIL METHOD LIKE IN MATHUSE THE FOIL METHOD LIKE IN MATH

H h G g X H h G gH h G g X H h G g

1. HG1. HG

2. Hg2. Hg

3. hG3. hG

4. hg4. hg

FOIL – FIRST, OUTSIDE, INSIDE, LASTFOIL – FIRST, OUTSIDE, INSIDE, LAST

BOTH PARENTS BOTH PARENTS ARE THE SAME ARE THE SAME

NOW LET’S DO A DIHYBRID NOW LET’S DO A DIHYBRID CROSSCROSS

H h G g X H h G gH h G g X H h G gHGHG HgHg hGhG hghg

HGHG

HgHg

hGhG

hghg

HHGGHHGG HHGgHHGg HhGGHhGG HhGgHhGg

HHGgHHGg HHggHHgg HhGgHhGg HhggHhgg

HhGGHhGG HhGgHhGg hhGGhhGG hhGghhGg

HhGgHhGg HhggHhgg hhGghhGg hhgghhgg

WHAT ARE THE PHENOTYPIC WHAT ARE THE PHENOTYPIC RATIO’S?RATIO’S?

H h G g X H h G gH h G g X H h G gHGHG HgHg hGhG hghg

HGHG

HgHg

hGhG

hghg

HHGGHHGG HHGgHHGg HhGGHhGG HhGgHhGg

HHGgHHGg HHggHHgg HhGgHhGg HhggHhgg

HhGGHhGG HhGgHhGg hhGGhhGG hhGghhGg

HhGgHhGg HhggHhgg hhGghhGg hhgghhgg

Figure 11.10 Analysis of a dihybrid crossFigure 11.10 Analysis of a dihybrid cross

PROBABILITYPROBABILITY

WILL REAL LIFE FOLLOW THE RESULTS WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE?FROM A PUNNETT SQUARE?

NO!!!!!! – A PUNNETT SQUARE ONLY NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCURSHOWS WHAT WILL PROBABLY OCCUR

IT’S A LOT LIKE FLIPPING A COIN – YOU IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITYALWAYS FOLLOW PROBABILITY

MEIOSISMEIOSIS

GENES, CHROMOSOMES, AND GENES, CHROMOSOMES, AND NUMBERSNUMBERS CHROMOSOMES HAVE 100’S OR 1000’S CHROMOSOMES HAVE 100’S OR 1000’S

OF GENESOF GENES GENES FOUND ON CHROMOSOMESGENES FOUND ON CHROMOSOMES

DIPLOID & HAPLOID CELLSDIPLOID & HAPLOID CELLS

ALL BODY CELLS ALL BODY CELLS (SOMATIC CELLS) (SOMATIC CELLS) HAVE HAVE CHROMOSOMES CHROMOSOMES IN PAIRS IN PAIRS

BODY CELLS ARE BODY CELLS ARE CALLED CALLED DIPLOIDDIPLOID CELLS ( CELLS (2n2n))

HUMANS HAVE HUMANS HAVE THE 2n # OF THE 2n # OF CHROMOSOMESCHROMOSOMES

DIPLOID AND HAPLOID CELLS DIPLOID AND HAPLOID CELLS CONT’DCONT’D

HAPLOIDHAPLOID CELLS CELLS ONLY HAVE 1 OF EACH TYPE OF ONLY HAVE 1 OF EACH TYPE OF

CHROMOSOME (DIPLOID CELLS HAVE 2 CHROMOSOME (DIPLOID CELLS HAVE 2 OF EACH TYPE)OF EACH TYPE)

SYMBOL IS (SYMBOL IS (nn)) SEX CELLS HAVE THE n # OF SEX CELLS HAVE THE n # OF

CHROMOSOMESCHROMOSOMES

HOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMES

HOMOLOGOUS CHROMOSOMESHOMOLOGOUS CHROMOSOMES ARE THE ARE THE PAIRED CHROMOSOMES THAT CONTAIN PAIRED CHROMOSOMES THAT CONTAIN THE THE SAMESAME TYPE OF GENTIC INFORMATION, TYPE OF GENTIC INFORMATION, SAME BANDING PATTERNS, SAME SAME BANDING PATTERNS, SAME CENTROMERE LOCATION, ETC.CENTROMERE LOCATION, ETC.

THEY MAY HAVE DIFFERENT ALLELES, SO THEY MAY HAVE DIFFERENT ALLELES, SO NOT PERFECTLY IDENTICAL NOT PERFECTLY IDENTICAL

WHY DO THEY HAVE DIFFERENT ALLELES?WHY DO THEY HAVE DIFFERENT ALLELES?

CAME FROM DIFFERENTCAME FROM DIFFERENT PARENTSPARENTS

IMPORTANT THINGS TO KNOW

CROSSING OVER – OCCURS DURING – OCCURS DURING PROPHASE IPROPHASE I CREATES CREATES GENETIC VARIABILITY

(RECOMBINATION OF GENES)(RECOMBINATION OF GENES) IN MEIOSIS I, IN MEIOSIS I, HOMOLOGOUS HOMOLOGOUS

CHROMOSOMES SEPARATECHROMOSOMES SEPARATE (ANAPHASE I) (ANAPHASE I) IN MEIOSIS II, IN MEIOSIS II, SISTER CHROMATIDS SISTER CHROMATIDS

SEPARATESEPARATE TETRADTETRAD – WHAT THE HOMOLOGOUS – WHAT THE HOMOLOGOUS

CHROMOSOMES ARE CALLED WHEN THEY CHROMOSOMES ARE CALLED WHEN THEY PAIR UP DURING PROPHASE IPAIR UP DURING PROPHASE I

Figure 11.11 The journey from DNA to Figure 11.11 The journey from DNA to phenotypephenotype

11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance

Often the expression of phenotype is Often the expression of phenotype is not straightforward not straightforward

Continuous variationContinuous variation characters can show a range of small characters can show a range of small

differences when multiple genes act differences when multiple genes act jointly to influence a characterjointly to influence a character

this type of inheritance is called this type of inheritance is called polygenicpolygenic

Figure 11.12 Height is a Figure 11.12 Height is a continuously varying charactercontinuously varying character


Pleiotropic effectsPleiotropic effects an allele that has more than one effect an allele that has more than one effect

on the phenotype is considered on the phenotype is considered pleiotropic: pleiotropic: one gene affects many one gene affects many characterscharacters

these effects are characteristic of many these effects are characteristic of many inherited disorders, such as cystic inherited disorders, such as cystic fibrosis and sickle-cell anemiafibrosis and sickle-cell anemia

Figure 11.13 Pleiotropic effects of Figure 11.13 Pleiotropic effects of the cystic fibrosis gene, the cystic fibrosis gene, cfcf


Incomplete dominanceIncomplete dominance not all alternative alleles are either fully not all alternative alleles are either fully

dominant or fully recessive in dominant or fully recessive in heterozygotesheterozygotes

in such cases, the alleles exhibit in such cases, the alleles exhibit incomplete dominanceincomplete dominance and produce a and produce a heterozygous phenotype that is heterozygous phenotype that is intermediate between those of the parentsintermediate between those of the parents

Figure 11.14 Incomplete Figure 11.14 Incomplete dominancedominance


Environmental effectsEnvironmental effects the degree to which many alleles are the degree to which many alleles are

expressed depends on the environmentexpressed depends on the environment for example, some alleles are heat-for example, some alleles are heat-

sensitivesensitive arctic foxes only produce fur pigment when arctic foxes only produce fur pigment when

temperatures are warmtemperatures are warm the the ch ch allele in Himalayan rabbits and Siamese allele in Himalayan rabbits and Siamese

cats encodes a heat-sensitive enzyme, called cats encodes a heat-sensitive enzyme, called tyrosinase, that controls pigment productiontyrosinase, that controls pigment production

tyrosinase is inactive at high temperaturestyrosinase is inactive at high temperatures

Figure 11.15 Environmental effects Figure 11.15 Environmental effects on an alleleon an allele


EpistasisEpistasis in some situations, two or more genes interact in some situations, two or more genes interact

with each other, such that one gene contributes with each other, such that one gene contributes to or masks the expression of the other geneto or masks the expression of the other gene

in in epistasisepistasis, one gene modifies the phenotypic , one gene modifies the phenotypic expression produced by the otherexpression produced by the other

for example, in corn, to produce and deposit for example, in corn, to produce and deposit pigment, a plant must possess at least one pigment, a plant must possess at least one functional copy of each of two genesfunctional copy of each of two genes

one gene controls pigment depositionone gene controls pigment deposition the other gene controls pigment productionthe other gene controls pigment production

Figure 11.16 How epistasis affects Figure 11.16 How epistasis affects kernel colorkernel color

Why is coat color in Labrador Why is coat color in Labrador retrievers an example of epistasis?retrievers an example of epistasis?

EE gene determines if dark pigment will be gene determines if dark pigment will be deposited in fur or notdeposited in fur or not

genotype genotype eeee, no pigment will be deposited in the , no pigment will be deposited in the fur, and it will be yellowfur, and it will be yellow

genotype genotype E_E_, pigment will be deposited in the fur, pigment will be deposited in the fur A second gene, the A second gene, the BB gene, determines how dark gene, determines how dark

the pigment will bethe pigment will be Yellow dogs with the genotype Yellow dogs with the genotype eebbeebb will have will have

brown pigment on their nose, lips, and eye rims, brown pigment on their nose, lips, and eye rims, while yellow dogs with the genotype while yellow dogs with the genotype eeB_eeB_ will will have black pigment in these areas.have black pigment in these areas.

Figure 11.17 The effect of epistatic Figure 11.17 The effect of epistatic interactions on coat color in dogsinteractions on coat color in dogs


CodominanceCodominance a gene may have more than two alleles a gene may have more than two alleles

in a populationin a population often, in heterozygotes, there is not a often, in heterozygotes, there is not a

dominant allele but, instead, both alleles are dominant allele but, instead, both alleles are expressedexpressed

these alleles are said to be these alleles are said to be codominantcodominant


The gene that determines ABO blood type in The gene that determines ABO blood type in humans exhibits more than one dominant humans exhibits more than one dominant alleleallele the gene encodes an enzyme that adds sugars to the gene encodes an enzyme that adds sugars to

lipids on the membranes of red blood cells lipids on the membranes of red blood cells these sugars act as recognition markers for cells in these sugars act as recognition markers for cells in

the immune systemthe immune system the gene that encodes the enzyme, designated the gene that encodes the enzyme, designated I, I,

has three alleles: has three alleles: IIAA,I,IBB, , andand ii different combinations of the three alleles produce four different combinations of the three alleles produce four

different phenotypes, or bloodtypes (A, B, AB, and O)different phenotypes, or bloodtypes (A, B, AB, and O) both both IIAA and and IIBB are dominant over are dominant over ii and also codominant and also codominant

Figure 11.19 Multiple alleles Figure 11.19 Multiple alleles controlling the ABO blood groupscontrolling the ABO blood groups

67

Inheritance of Blood TypeInheritance of Blood Type

11.8 Human Chromosomes11.8 Human Chromosomes

Nondisjunction may also affect the sex Nondisjunction may also affect the sex chromosomeschromosomes nondisjunction of the X chromosome nondisjunction of the X chromosome

creates three possible viable conditionscreates three possible viable conditions XXX femaleXXX female

usually taller than average but other symptoms varyusually taller than average but other symptoms vary XXY male (Klinefelter syndrome)XXY male (Klinefelter syndrome)

sterile male with many female characteristics and sterile male with many female characteristics and diminished mental capacitydiminished mental capacity

XO female (Turner syndrome)XO female (Turner syndrome) sterile female with webbed neck and diminished sterile female with webbed neck and diminished

staturestature

Figure 11.26 Nondisjunction of the Figure 11.26 Nondisjunction of the X chromosomeX chromosome

11.9 The Role of Mutations in 11.9 The Role of Mutations in Human HeredityHuman Heredity

Accidental changes in genes are Accidental changes in genes are called called mutationsmutations mutations occur only rarely and almost mutations occur only rarely and almost

always result in recessive allelesalways result in recessive alleles not eliminated from the population because not eliminated from the population because

they are not usually expressed in most they are not usually expressed in most individuals (heterozygotes)individuals (heterozygotes)

in some cases, particular mutant alleles have in some cases, particular mutant alleles have become more common in human populations become more common in human populations and produce harmful effects called and produce harmful effects called genetic genetic disordersdisorders

Table 11.3 Some Important Genetic Table 11.3 Some Important Genetic DisordersDisorders


To study human heredity, scientists To study human heredity, scientists examine crosses that have already examine crosses that have already been madebeen made they identify which relatives exhibit a they identify which relatives exhibit a

trait by looking at family trees or trait by looking at family trees or pedigreespedigrees

often one can determine whether a trait often one can determine whether a trait is sex-linked or autosomal and whether is sex-linked or autosomal and whether the trait’s phenotype is dominant or the trait’s phenotype is dominant or recessive recessive

Figure 11.27 A general pedigreeFigure 11.27 A general pedigree


Sickle-cell anemia Sickle-cell anemia is a recessive is a recessive hereditary disorderhereditary disorder affected individuals are homozygous affected individuals are homozygous

recessive and carry a mutated gene that recessive and carry a mutated gene that produces a defective version of hemoglobinproduces a defective version of hemoglobin

the hemoglobin sticks together inappropriately the hemoglobin sticks together inappropriately and produces a stiff red blood cell with a sickle-and produces a stiff red blood cell with a sickle-shapeshape

the cells cannot move through the blood vessels the cells cannot move through the blood vessels easily and tend to form clotseasily and tend to form clots

this causes sufferers to have intermittent illness and this causes sufferers to have intermittent illness and shortened life spansshortened life spans

Figure 11.29 Inheritance of sickle-cell anemiaFigure 11.29 Inheritance of sickle-cell anemia


The sickle-cell mutation to hemoglobin The sickle-cell mutation to hemoglobin affects the stickiness of the hemoglobin affects the stickiness of the hemoglobin protein surface but not its oxygen-binding protein surface but not its oxygen-binding abilityability

In heterozygous individuals, only some of In heterozygous individuals, only some of their red blood cells become sickled when their red blood cells become sickled when oxygen levels become lowoxygen levels become low this may explain why the sickle-cell allele is so this may explain why the sickle-cell allele is so

frequent among people of African descentfrequent among people of African descent the presence of the allele increases resistance to the presence of the allele increases resistance to

malaria infectionmalaria infection

Figure 11.30 Figure 11.30 The sickle-cell allele The sickle-cell allele confers resistance to confers resistance to malariamalaria

chapter 11 mendel & mutations father of genetics monk and teacher. experimented with purebred...

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