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Transcript of CHAPTER 11 MENDEL & MUTATIONS Father of Genetics Monk and teacher. Experimented with purebred tall...
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CHAPTER 11CHAPTER 11
MENDEL & MUTATIONSMENDEL & MUTATIONS
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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”.
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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.
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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.
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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.
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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.
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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
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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.
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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
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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
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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)
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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)
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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!!!!!!)
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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
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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
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Figure 11.7 A Punnett square Figure 11.7 A Punnett square analysisanalysis
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Figure 11.8 How Mendel analyzed Figure 11.8 How Mendel analyzed flower colorflower color
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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)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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)
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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
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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
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PUNNETT SQUAREPUNNETT SQUARE
CROSS T T X TtCROSS T T X Tt
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CONT’DCONT’D
T T X T tT T X T tTT TT
TT
tt
TT TT TT TT
TT tt TT tt
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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
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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
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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
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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
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Figure 11.10 Analysis of a dihybrid crossFigure 11.10 Analysis of a dihybrid cross
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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
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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
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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
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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
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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
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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
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Figure 11.11 The journey from DNA to Figure 11.11 The journey from DNA to phenotypephenotype
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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
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Figure 11.12 Height is a Figure 11.12 Height is a continuously varying charactercontinuously varying character
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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Figure 11.13 Pleiotropic effects of Figure 11.13 Pleiotropic effects of the cystic fibrosis gene, the cystic fibrosis gene, cfcf
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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Figure 11.14 Incomplete Figure 11.14 Incomplete dominancedominance
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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Figure 11.15 Environmental effects Figure 11.15 Environmental effects on an alleleon an allele
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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Figure 11.16 How epistasis affects Figure 11.16 How epistasis affects kernel colorkernel color
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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.
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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
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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11.6 Why Some Traits Don’t Show 11.6 Why Some Traits Don’t Show Mendelian InheritanceMendelian Inheritance
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
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Figure 11.19 Multiple alleles Figure 11.19 Multiple alleles controlling the ABO blood groupscontrolling the ABO blood groups
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67
Inheritance of Blood TypeInheritance of Blood Type
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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
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Figure 11.26 Nondisjunction of the Figure 11.26 Nondisjunction of the X chromosomeX chromosome
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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
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Table 11.3 Some Important Genetic Table 11.3 Some Important Genetic DisordersDisorders
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11.9 The Role of Mutations in 11.9 The Role of Mutations in Human HeredityHuman Heredity
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
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Figure 11.27 A general pedigreeFigure 11.27 A general pedigree
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11.9 The Role of Mutations in 11.9 The Role of Mutations in Human HeredityHuman Heredity
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
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Figure 11.29 Inheritance of sickle-cell anemiaFigure 11.29 Inheritance of sickle-cell anemia
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11.9 The Role of Mutations in 11.9 The Role of Mutations in Human HeredityHuman Heredity
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
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Figure 11.30 Figure 11.30 The sickle-cell allele The sickle-cell allele confers resistance to confers resistance to malariamalaria