Post on 27-Mar-2015
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GeneticsGenetics
The Scientific Study of Inheritance
Terms Terms • Allele • Barr body• Codominance • Dihybrid cross• Dominant• Epistasis• Genotype• Heterozygous • Homozygous
• Inbreeding
• Incomplete dominance
• Linkage
• Locus
• Multi-allelic
• Phenotype
• Pleiotropy
• Polygenic
• Recessive
• Sex-linked
Gregor MendelGregor Mendel
• Monk
• Austria; Czech republic
• 1st to analyze inheritance in a scientific manner
• Scientific method
• Careful record-keeping
Gregor MendelGregor Mendel
• Studied garden peas
– Easy to grow
– Produce lots of offspring
– Easily distinguished characteristics
• Fruit flies - today
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
Gregor MendelGregor Mendel
• Control cross-pollination• Cross-pollination produced
hybrids – Called a ‘cross’
• HybridsHybrids = offspring with mixed traits
• TraitsTraits = inherited characteristic
Crossed pure-breeding and got one trait. What happened to the white trait?
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
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
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
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
Chromosome 1 Homologue
Gene Allele Allele
Two alleles to a gene; alleles
may be dominant or
recessive
From momFrom dad
• GenotypeGenotype = an organism’s genetic makeup– PP
• PhenotypePhenotype = an organism’s expressed or physical traits– Purple
Modern GeneticsModern Genetics
Mendel’s Principle of Mendel’s Principle of Segregation:Segregation:
Law of SegregationLaw of Segregation
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
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
• 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
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
This is too hard to do!!!!
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 ……
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
2. Draw a square PURPLE PURPLE x white
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
PPPP pp
Crossing a homozygous purple flower with a homozygous recessive white flower
PPPP pp
X
4. Separate letters (alleles) around the square – this represents segregation
PP PP
p
p
BE CAREFUL HOW YOU
MAKE YOUR LETTERS!!
PPPP
pp
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
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
Results:Results:
• Genotype – combination of letters (alleles);
– Pp
• Phenotype – appearance (what do they LOOK like?
– PurplePurple
What If You Crossed heterozygous
purple with heterozygous purple?
PPpp x PPpp
PPppPPpp
Separate letters (alleles) around the
square
PP
PPp
p
p
PP
PPp
Combine the letters (alleles) in the
squares
PP
p
p
PP PPPP PPp
pPP pp
PurplePurple PurplePurple
PurplePurple white
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
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?
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
T T
Tt Tt
Tt Ttt
t
Genotypes -
4 - Tt
Phenotypes -
4 - tall
100% tall
Tall
Tall
Tall
Tall
• 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:
Y Y
y
y
Yy Yy
Yy Yy
Yellow Yellow
Yellow Yellow
Genotype – 4 Yy
Phenotype – 4 yellow; 100% yellow
• 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
B B
b
b
BbBb BbBb
BbBb BbBb
Black Black
Black Black
Genotype – 100% Bb
Phenotype – 100% black
Law of Independent Law of Independent AssortmentAssortment
Are Traits Inherited Together (dependently) or Separately (independently)?
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
Dihybrid CrossDihybrid Cross
• Homozygous (pure-breeding) round (RR), and yellow (YY) with:
• Homozygous recessive; wrinkled (rr), green (yy)
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
Dihybrid CrossDihybrid Cross
• 9:3:3:1 ratio
• 9/16 = yellow, round
• 3/16 = yellow, wrinkled
• 3/16 = green, round
• 1/16 = green, wrinkled
Independent Assortment:Independent Assortment:
YyRr
YR Yr yR yr
Parent: 1 & 2
YR Yr yR yr
YR
Yr
yR
yr
Law of Independent Assortment
Yy Rr
Yy Rr
YR Yr yR yr
YR YYRR YYRr YyRR YyRr
Yr YYRr YYrr YyRr Yyrr
yR YyRR YyRr yyRR yyRr
yr YyRr Yyrr yyRr yyrr
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
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
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
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
GreenGreen = Y_B_
Blue = yyB_
Yellow; Y_bb
White; yybb
Test CrossTest Cross
How can We Use Genetics to Determine if Our Organism is Pure-
breeding?
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?
Test crossTest cross
• Cross the unknown with a homozygous recessive
• Eight puppies born, 3 are brown (recessive)
• ?
B B
b
b
Bb Bb
Bb Bb
If the unknown is homozygous (pure) then all the offspring are dominant
B b
b
b
Bb bb
Bb bb
If the unknown is heterozygous (carrier) then some offspring are recessive
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
Incomplete DominanceIncomplete Dominance
• Dominant allele does not totally mask recessive allele
• Some recessive trait is expressed: blended
RedRed x white = pink pink
Curly hair + straight hair = wavy
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’
CodominanceCodominance
• Both traits are EQUALLY dominant;
• Both traits are expressed (not blended)
– Roan color
– Sickle cell
– Blood types
CodomCodominanceinance • Two different traits and both show
equally– Roan color– Blood types
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
Blood TypesBlood Types
• ‘B’ into ‘A’ causes a clot
• ‘A’ into ‘B’ causes a clot
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
Blood TypesBlood Types
• CodominanceCodominance
• Multiple allelesMultiple alleles = 1 gene but three possible allele combinations– A, B, O
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)
Blood Types: PhenotypesBlood Types: Phenotypes
• Antibodies agglutinate to antigens that are ‘foreign’
• Agglutinate = clot, clump
• “B” into “A” causes agglutination
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
‘A’ antigens
A
‘B’ Antibodies
‘B’ antigens
B
‘A’ Antibodies
‘B’ antigens
B
‘B’ AntibodiesB antibodies attach
to B antigens; causes blood to agglutinate
Person with ‘A’ blood:
‘A’ antigens
A
‘A’ Antibodies
Person with ‘B’ blood:
‘A’ antigens are attacked by ‘A’
antibodies
A, B antigens
AB
No antibodies
Person with AB blood:
No antigens
O
A and B antibodies
Person with O blood:
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
Blood TypesBlood Types
• 2 alleles for each gene:
• ‘A’ = IAIA or IA i
• ‘B’ = IBIB or IB i
• ‘AB’ = IAIB
• ‘O’ (zero) = ii
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
How to do Punnett Squares With How to do Punnett Squares With BloodBlood Types: Types:
IA i
IA
IA
IAIA
IAIA
IA i
IA i
Heterozygous
Homozygous
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?
IA i
IB
i
IAIB
IA i
IB i
i i
Heterozygous A
Heterozygous B
AB B
A O
Pleiotropy Pleiotropy
• One gene has multiple effects
• Sickle-cell anemia; p. 160
– Abnormal blood cells
– Difficulty breathing
– Brain, heart, kidney damage
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
Polygenic InheritancePolygenic Inheritance
• Multiple genes produces a continuous effect; very dark-very light
– Skin, hair, eye color
– 6 – 10 alleles
– AABBCC - aabbcc
LinkageLinkage
• Early 1900’s; TH Morgan
• Fruit flies
• Inheritance patterns did not follow Mendelian Laws of Probability (?)
• Genes are linked
LinkageLinkage
• Genes on the same chromosome are inherited together
• Sex – linked genes
• Gene located on the sex chromosome (usually X)
Sex linkage and Punnett Sex linkage and Punnett SquaresSquares
LinkageLinkage
• The sex-linked trait is usually on the X chromosome
• X X = female• X Y = male• XH = ‘normal’• Xh = hemophilia
HemophiliaHemophilia
• Sex-linked, recessive disorder
• ‘Bleeders disease’; lack protein for blood clotting
• Czar Nicholas’ son “Nicki”; p. 168
XH
XH
XH
Y
XH XH XH XH
XH Y XH Y
Normal phenotypes:
XH
XH
Xh
Y
XH XH XH Xh
XH Y Xh Y
Hemophilia phenotype:
Sex-linked Traits:Sex-linked Traits:
• Hemophilia
• Duchenne’s Muscular dystrophy
• Color-blindness
• Mostly males
• Smartness
Sex-linked Traits: Y ChromosomeSex-linked Traits: Y Chromosome
• “Maleness”
Censored
Censored
Censored
Pedigrees Pedigrees
• Tracing traits back over generations
• Dominance does NOT mean that a phenotype is ‘normal’ or more common
• Wild type
PedigreesPedigrees
• Dominance = heterozygote displays the trait
• Recessive expression occurs only if the genotype is homozygous
• bb, tt, ff
PedigreesPedigrees
• Used to predict probability of genetic disorders
• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype
Human DisordersHuman Disorders
• Single gene:
• 2 types;
– Dominant expression
– Recessive expression
Human Disorders: Dominant Human Disorders: Dominant
• Only requires one allele for trait to be expressed
– Polydactyly; multiple fingers
– Achondroplasia; dwarfism, heterozygotes
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
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
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)
Fetal TestingFetal Testing
• Chorionic villus samplingChorionic villus sampling (CVS) fetal cells removed from placenta
– Karyotyped quickly
Fetal TestingFetal Testing
• UltrasoundUltrasound = view of baby
• FetoscopyFetoscopy = direct view of baby
Recessive DisordersRecessive Disorders
• Cystic fibrosis – whites; build up of mucus in lungs, pancreas
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
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
Pedigrees Pedigrees
• Tracing traits back over generations
• Dominance does NOT mean that a phenotype is ‘normal’ or more common
• Wild type
PedigreesPedigrees
• Used to predict probability of genetic disorders
• CarriersCarriers = individuals who do not express the trait but have the recessive allele in their genotype
Pedigree Pedigree
• Family tree• Shows how a trait is passed down from one
generation to the next
= male
= female
Pedigree number 1
Pedigree number 2
Pedigree number 3