genetics the scientific study of inheritance. terms allele barr body codominance dihybrid cross...
TRANSCRIPT
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GeneticsGenetics
The Scientific Study of Inheritance
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Terms Terms • Allele • Barr body• Codominance • Dihybrid cross• Dominant• Epistasis• Genotype• Heterozygous • Homozygous
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• Inbreeding
• Incomplete dominance
• Linkage
• Locus
• Multi-allelic
• Phenotype
• Pleiotropy
• Polygenic
• Recessive
• Sex-linked
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Gregor MendelGregor Mendel
• Monk
• Austria; Czech republic
• 1st to analyze inheritance in a scientific manner
• Scientific method
• Careful record-keeping
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Gregor MendelGregor Mendel
• Studied garden peas
– Easy to grow
– Produce lots of offspring
– Easily distinguished characteristics
• Fruit flies - today
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Gregor MendelGregor Mendel
• “Parents pass ‘factors’ to their offspring that are responsible for traits”
• ‘Factors’ = genes• Garden peas self-pollinate• True-breedingTrue-breeding = parents
produce offspring identical to themselves
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Gregor MendelGregor Mendel
• Control cross-pollination• Cross-pollination produced
hybrids – Called a ‘cross’
• HybridsHybrids = offspring with mixed traits
• TraitsTraits = inherited characteristic
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Crossed pure-breeding and got one trait. What happened to the white trait?
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Gregor MendelGregor Mendel
• Allowed F1’s to self-pollinate
• Produced F2 generation
• F2’s; 705 purple; 224 white
• 3:1 ratio• The heritable ‘factor’ for
white was ‘masked’ but was not destroyed
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Gregor Mendel - 4 Hypotheses:Gregor Mendel - 4 Hypotheses:
1. There are alternate forms for ‘factors’ that control heredity
2. For each characteristic, there are 2 factors inherited; one from each parent
3. A gamete carries only one form for each factor’; during fertilization, the 2 ‘factors’ unite
4. One form of the factor is fully expressed (visible) and the other has no effect
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Modern GeneticsModern Genetics• ‘FactorsFactors’ = genesgenes• Alternate ‘forms’Alternate ‘forms’ == allelesalleles • GenesGenes = sections of DNA; code for making
proteins• Expression of proteins determines trait• Dominant Allele= allele that IS expressed;
protein is expressed (made)• Recessive Allele = allele that is NOT expressed
(made); or masked; protein is not made
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Structure Of A ChromosomeStructure Of A Chromosome
• Chromosomes are homologous pairs
– Same size, banding, centromere location and genes
• Made of DNA
• Sections of chromosomes are genes
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Chromosome 1 Homologue
Gene Allele Allele
Two alleles to a gene; alleles
may be dominant or
recessive
From momFrom dad
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• GenotypeGenotype = an organism’s genetic makeup– PP
• PhenotypePhenotype = an organism’s expressed or physical traits– Purple
Modern GeneticsModern Genetics
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Mendel’s Principle of Mendel’s Principle of Segregation:Segregation:
Law of SegregationLaw of Segregation
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Principle of SegregationPrinciple of Segregation
• All organisms have 2 sets of homologous chromosomes; one from each parent;
– Diploid
• One allele located on each chromosome; one from mom, one from dad
– 2 alleles = 1 gene
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Principle of SegregationPrinciple of Segregation
• Pairs of alleles separate (segregate) during gamete formation
• 1 form of a ‘factor’ goes into 1 gamete while the other form separates and goes into another gamete (handout)
• LocusLocus = location of a gene on a chromosome; loci (pl.)
• Alleles are at the same locus on each homologous chromosome
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• HomozygousHomozygous = both alleles for the trait are the same (homo)
– PP, pp = homozygous
• HeterozygousHeterozygous = the two alleles are different
– Pp = heterozygous
Principle of SegregationPrinciple of Segregation
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Fertilization Fertilization • During fertilization, the sperm unites with the egg• 1 haploid sperm + 1 haploid egg = 1 diploid diploid
zygotezygote• Which sperm unites with which egg is by random
chance– Flipping a coin
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This is too hard to do!!!!
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Use The Laws of ProbabilityUse The Laws of Probability
• Probability = chance that something will occur
• How can we predict what will happen easier?
• Punnett SquarePunnett Square
• How does it work, you say?
• I’m so glad you asked ……
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Punnett Square Punnett Square
1. Use letters to represent each allele
a. Use the CAPITAL for dominant and small case for recessive
b. Ex. P = purple; p = white
c. T = tall; t = short
d. Y – yellow; y - green
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2. Draw a square PURPLE PURPLE x white
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3. Determine what letters to use to represent the alleles
Example:
a. Cross a PURPLEPURPLE with a white flower
b. . PURPLEPURPLE is dominant over white in pea plants so use P = PURPLEP = PURPLE and p = white
c. Every gene has 2 alleles so use 2 letters
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PPPP pp
Crossing a homozygous purple flower with a homozygous recessive white flower
PPPP pp
X
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4. Separate letters (alleles) around the square – this represents segregation
PP PP
p
p
BE CAREFUL HOW YOU
MAKE YOUR LETTERS!!
PPPP
pp
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5. Combine the letters (alleles) into each box of the square
PP PP
PURPLE PPPURPLE PP x white pp
p
p
PP PP
PP PP
pp pp
pp pp
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6. Determine the results
PP PPPPPP x pp = 4 Pp; and 4 PURPLEPURPLE
p
p
PPpp PPpp
PPpp PPpp
Genotype = 4 Pp
Phenotype = 4 PURPLEPURPLE
1 2
3 4
Purple Purple
Purple Purple
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Results:Results:
• Genotype – combination of letters (alleles);
– Pp
• Phenotype – appearance (what do they LOOK like?
– PurplePurple
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What If You Crossed heterozygous
purple with heterozygous purple?
PPpp x PPpp
PPppPPpp
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Separate letters (alleles) around the
square
PP
PPp
p
p
PP
PPp
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Combine the letters (alleles) in the
squares
PP
p
p
PP PPPP PPp
pPP pp
PurplePurple PurplePurple
PurplePurple white
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PP
PPp x PPp
p
PPPP PP
PPpp
pp
pppp
pp
PP
Genotypes –
Phenotypes –
1 - PP
2 - Pp
1- pp
3 - PURPLE3 - PURPLE
1 - white
PurplePurple PurplePurple
PurplePurple white
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Practice ProblemsPractice Problems
• Tall is dominant to short
• What genotypic and phenotypic results would be expected if you crossed a HOMOZYGOUS tall with a HOMOZYGOUS short?
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Practice ProblemsPractice Problems
• What genotypic and phenotypic results would
be expected if you crossed a HOMOZYGOUS tall
with a HOMOZYGOUS short?
T T
t t
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T T
Tt Tt
Tt Ttt
t
Genotypes -
4 - Tt
Phenotypes -
4 - tall
100% tall
Tall
Tall
Tall
Tall
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• In pea plants, yellow is dominant to green. What results would be expected if you crossed a homozygous yellow with a homozygous green?
• Homozygous = same• Yellow – Y; green – y• Homozygous yellow = YY• Homozygous green = yy
Practice:
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Y Y
y
y
Yy Yy
Yy Yy
Yellow Yellow
Yellow Yellow
Genotype – 4 Yy
Phenotype – 4 yellow; 100% yellow
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• Black fur is dominant to brown fur in mice. What results should you expect if you crossed a homozygous black with a homozygous brown?– Black is dominant so use B; brown - b– Homozygous black = BB– Homozygous brown = bb
Practice
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B B
b
b
BbBb BbBb
BbBb BbBb
Black Black
Black Black
Genotype – 100% Bb
Phenotype – 100% black
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Law of Independent Law of Independent AssortmentAssortment
Are Traits Inherited Together (dependently) or Separately (independently)?
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Law of Independent Assortment Law of Independent Assortment
• Round (R) is dominant to wrinkled (r)
• Yellow (Y) is dominant to green (y)
• Result from crossing two traits?
– If you inherit a dominant trait does the other trait also have to be dominant?
• Dihybrid cross – result of crossing two traits together
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Dihybrid CrossDihybrid Cross
• Homozygous (pure-breeding) round (RR), and yellow (YY) with:
• Homozygous recessive; wrinkled (rr), green (yy)
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Dihybrid CrossDihybrid Cross
• Are the two traits inherited together (in a ‘package’) or can they be inherited separately?
• Mendel crossed the P’s (yellow, round x green, wrinkled)
– F1’s were all dominant (yellow, round)
• Allowed the F1’s to self-pollinate
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Dihybrid CrossDihybrid Cross
• 9:3:3:1 ratio
• 9/16 = yellow, round
• 3/16 = yellow, wrinkled
• 3/16 = green, round
• 1/16 = green, wrinkled
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Independent Assortment:Independent Assortment:
YyRr
YR Yr yR yr
Parent: 1 & 2
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YR Yr yR yr
YR
Yr
yR
yr
Law of Independent Assortment
Yy Rr
Yy Rr
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YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
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Law of Independent Assortment:Law of Independent Assortment:
• Each pair of alleles segregates independently of other pairs of alleles
• Can recombine independently of each other
• Genetic Genetic vvaarriiaattiioonn – Biggest cause of genetic variation in sexually Biggest cause of genetic variation in sexually
reproducing organismsreproducing organisms
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Independent AssortmentIndependent Assortment
• Budgies inherit two colors INDEPENDENTLY
• Color (Yellow) or no color on the outer surface of the feather
• MelaninMelanin or no melanin in the inner core of the feather
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Variation and Patterns of VariationVariation and Patterns of Variation
• Wild type - most common traits in the wild
– Budgies = green feathers
• Knowing patterns and rules of inheritance allows breeders to produce blues, yellows, and whites
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Budgie ColorBudgie Color
Two genes inherited separately1. Outside color of feather2. Inside color of feather
Independent assortment; the two characteristics are inherited independently of each other
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GreenGreen = Y_B_
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Blue = yyB_
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Yellow; Y_bb
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White; yybb
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Test CrossTest Cross
How can We Use Genetics to Determine if Our Organism is Pure-
breeding?
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Test crossTest cross
• Mate an individual whose genotype is not known (dominant phenotype) with a homozygous recessive for that trait
• Ex. Is your favorite Labrador a ‘pure’ black or does he carry a recessive allele?
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Test crossTest cross
• Cross the unknown with a homozygous recessive
• Eight puppies born, 3 are brown (recessive)
• ?
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B B
b
b
Bb Bb
Bb Bb
If the unknown is homozygous (pure) then all the offspring are dominant
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B b
b
b
Bb bb
Bb bb
If the unknown is heterozygous (carrier) then some offspring are recessive
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Variations of MendelVariations of Mendel
1.1. Complete dominance Complete dominance
2.2. Incomplete dominanceIncomplete dominance
3.3. CodominanceCodominance
4.4. Multiple allelesMultiple alleles
5.5. PleiotropyPleiotropy
6.6. Polygenic inheritancePolygenic inheritance
7.7. LinkageLinkage
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Incomplete DominanceIncomplete Dominance
• Dominant allele does not totally mask recessive allele
• Some recessive trait is expressed: blended
RedRed x white = pink pink
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Curly hair + straight hair = wavy
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Incomplete DominanceIncomplete Dominance
• Heterozygotes express a trait between the dominant and recessive
• Familial hypercholesterolemia
– hh = very high cholesterol
– Hh = mild cholesterol
– HH = low cholesterol; ‘normal’
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CodominanceCodominance
• Both traits are EQUALLY dominant;
• Both traits are expressed (not blended)
– Roan color
– Sickle cell
– Blood types
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CodomCodominanceinance • Two different traits and both show
equally– Roan color– Blood types
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Blood TypesBlood Types
• Antigens = proteins on the surface of red blood cells (RBC’s)
• Antibodies = proteins floating in the plasma of blood that bind with ‘foreign’ proteins (antigens)
• Antibodies stick to ‘foreign’ antigens forming a clot
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Blood TypesBlood Types
• ‘B’ into ‘A’ causes a clot
• ‘A’ into ‘B’ causes a clot
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Blood TypesBlood Types
• Antibodies will be the opposite of the antigens
– “A” blood will have “B” antibodies
– ‘B’ blood will have ‘A’ antibodies
• Antibodies are like guard dogs; they attack foreign cells with the wrong antigens
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Blood TypesBlood Types
• CodominanceCodominance
• Multiple allelesMultiple alleles = 1 gene but three possible allele combinations– A, B, O
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Blood Types: PhenotypesBlood Types: Phenotypes
• AntigensAntigens = proteins on the surface of cells (RBC’s)
• Cell-to-cell recognition
• AntibodiesAntibodies = proteins floating in the plasma of blood that bind with ‘foreign’ proteins (antigens)
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Blood Types: PhenotypesBlood Types: Phenotypes
• Antibodies agglutinate to antigens that are ‘foreign’
• Agglutinate = clot, clump
• “B” into “A” causes agglutination
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Blood Types: PhenotypesBlood Types: Phenotypes
• Blood type = type of antigens on the surface
• Antibodies will be the opposite of the antigens
• “A” blood will have “B” antibodies
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‘A’ antigens
A
‘B’ Antibodies
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‘B’ antigens
B
‘A’ Antibodies
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‘B’ antigens
B
‘B’ AntibodiesB antibodies attach
to B antigens; causes blood to agglutinate
Person with ‘A’ blood:
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‘A’ antigens
A
‘A’ Antibodies
Person with ‘B’ blood:
‘A’ antigens are attacked by ‘A’
antibodies
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A, B antigens
AB
No antibodies
Person with AB blood:
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No antigens
O
A and B antibodies
Person with O blood:
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Blood Types: GenotypesBlood Types: Genotypes
• Dominant allele = I
• Recessive allele = i (inability)
• II, Ii, ii
• Dominant allele can carry A or B
• Ia or IB
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Blood TypesBlood Types
• 2 alleles for each gene:
• ‘A’ = IAIA or IA i
• ‘B’ = IBIB or IB i
• ‘AB’ = IAIB
• ‘O’ (zero) = ii
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phenotype genotype antigens antibodies Receive
From:
A IAIA
IAi
A Anti-B A or O
B IBIB
IBi
B Anti-A B or O
AB IAIB A,B None A, B, O
O ii None Anti-A
Anti-B
O
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How to do Punnett Squares With How to do Punnett Squares With BloodBlood Types: Types:
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IA i
IA
IA
IAIA
IAIA
IA i
IA i
Heterozygous
Homozygous
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Can 2 people With A and B Can 2 people With A and B Blood Have a Child With O Blood Have a Child With O
Blood?Blood?
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IA i
IB
i
IAIB
IA i
IB i
i i
Heterozygous A
Heterozygous B
AB B
A O
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Pleiotropy Pleiotropy
• One gene has multiple effects
• Sickle-cell anemia; p. 160
– Abnormal blood cells
– Difficulty breathing
– Brain, heart, kidney damage
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PleiotropyPleiotropy:: Heterozygote AdvantageHeterozygote Advantage
• High incidence of sickle-cell in areas where there is a lot of malaria
• Malaria does not effect sickle-cell
• People w/ sickle-cell don’t suffer malaria
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Polygenic InheritancePolygenic Inheritance
• Multiple genes produces a continuous effect; very dark-very light
– Skin, hair, eye color
– 6 – 10 alleles
– AABBCC - aabbcc
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LinkageLinkage
• Early 1900’s; TH Morgan
• Fruit flies
• Inheritance patterns did not follow Mendelian Laws of Probability (?)
• Genes are linked
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LinkageLinkage
• Genes on the same chromosome are inherited together
• Sex – linked genes
• Gene located on the sex chromosome (usually X)
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Sex linkage and Punnett Sex linkage and Punnett SquaresSquares
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LinkageLinkage
• The sex-linked trait is usually on the X chromosome
• X X = female• X Y = male• XH = ‘normal’• Xh = hemophilia
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HemophiliaHemophilia
• Sex-linked, recessive disorder
• ‘Bleeders disease’; lack protein for blood clotting
• Czar Nicholas’ son “Nicki”; p. 168
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XH
XH
XH
Y
XH XH XH XH
XH Y XH Y
Normal phenotypes:
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XH
XH
Xh
Y
XH XH XH Xh
XH Y Xh Y
Hemophilia phenotype:
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Sex-linked Traits:Sex-linked Traits:
• Hemophilia
• Duchenne’s Muscular dystrophy
• Color-blindness
• Mostly males
• Smartness
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Sex-linked Traits: Y ChromosomeSex-linked Traits: Y Chromosome
• “Maleness”
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Censored
Censored
Censored
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Pedigrees Pedigrees
• Tracing traits back over generations
• Dominance does NOT mean that a phenotype is ‘normal’ or more common
• Wild type
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PedigreesPedigrees
• Dominance = heterozygote displays the trait
• Recessive expression occurs only if the genotype is homozygous
• bb, tt, ff
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PedigreesPedigrees
• Used to predict probability of genetic disorders
• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype
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Human DisordersHuman Disorders
• Single gene:
• 2 types;
– Dominant expression
– Recessive expression
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Human Disorders: Dominant Human Disorders: Dominant
• Only requires one allele for trait to be expressed
– Polydactyly; multiple fingers
– Achondroplasia; dwarfism, heterozygotes
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Human Disorders: DominantHuman Disorders: Dominant
• Most dominant disorders are not lethalMost dominant disorders are not lethal – Huntington’s diseaseHuntington’s disease; midlife
expression, degeneration of the nervous system
– HypercholesterolemiaHypercholesterolemia – high cholesterol; heart disease
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Human Disorders: RecessiveHuman Disorders: Recessive
• Homozygous for the disorder to be expressed
– Cystic fibrosis; Cystic fibrosis;
– Sickle cell anemiaSickle cell anemia
– Tay-Sachs diseaseTay-Sachs disease
– PKUPKU
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Fetal TestingFetal Testing
• AmniocentesisAmniocentesis = removal of amniotic fluid (surrounds the developing baby); 20 ml– Biochemical tests (spina bifida, infections)– Cells karyotyping (Down’s, Tay-Sachs)
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Fetal TestingFetal Testing
• Chorionic villus samplingChorionic villus sampling (CVS) fetal cells removed from placenta
– Karyotyped quickly
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Fetal TestingFetal Testing
• UltrasoundUltrasound = view of baby
• FetoscopyFetoscopy = direct view of baby
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Recessive DisordersRecessive Disorders
• Cystic fibrosis – whites; build up of mucus in lungs, pancreas
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Recessive DisordersRecessive Disorders
• Sickle cell anemiaSickle cell anemia – Black and SE Asia; 1/500 (lethal), 1/10 carrier;
– Codominant – one allele is normal, other forms hemoglobin that crystallizes in low oxygen
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Recessive DisordersRecessive Disorders
• Tay-SachsTay-Sachs – Jewish; lack gene that produces enzyme that breaks down lipids in the brain; causes brain degeneration, lethal by age 3-4
• PKU – phenylketonuria; lack the gene needed to make the enzyme that breaks down phenylalanine. Phenylalanine accumulates causing nervous disorders. Treated with diet
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Pedigrees Pedigrees
• Tracing traits back over generations
• Dominance does NOT mean that a phenotype is ‘normal’ or more common
• Wild type
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PedigreesPedigrees
• Used to predict probability of genetic disorders
• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype
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Pedigree Pedigree
• Family tree• Shows how a trait is passed down from one
generation to the next
= male
= female
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Pedigree number 1
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Pedigree number 2
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Pedigree number 3
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