chapter 14 draw 13 boxes on your paper probability & genetics calculating probability of making...
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Chapter 14
Draw 13 boxes on your paper
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Probability & genetics
• Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin– probability of tossing heads? – probability making a B gamete?
50%
100%BB
B
B
Bb
B
b
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Probability & genetics
• Outcome of 1 toss has no impact on the outcome of the next toss– probability of tossing heads each time?– probability making a B gamete each
time?50%
50%
Bb
B
b
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Calculating probabilityPp x Pp
sperm egg
1/2 1/2
offspring
=x 1/4P P PP
P p Pp
1/2 1/2 =x 1/4p p pp
p P
P pmale / sperm
P
pfem
ale
/ eg
gs
PP Pp
Pp pp
1/2 1/2 =x 1/4
1/2 1/2 =x 1/41/2
+
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Rule of multiplication• Chance that 2 or more independent
events will occur together– probability that 2 coins tossed at the same
time will land heads up
– probability of Pp x Pp pp
1/2 x 1/2 = 1/4
1/2 x 1/2 = 1/4Pp
P
p
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Calculating probability in crosses
Use rule of multiplication to predict crosses
YyRr YyRrx
yyrr
?%
Yy Yyx Rr Rrx
1/4
1/4
1/16
yy rr
x
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Apply the Rule of Multiplication
Got it?Try this!
AABbccDdEEFf AaBbccDdeeFfx
AabbccDdEeFF
Bb x Bb bbcc x cc ccDd x Dd DdEE x ee EeFf x Ff FF
AA x Aa Aa 1/21/4
11/2
11/4 1/64
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Rule of addition
• Chance that an event can occur 2 or more different ways – sum of the separate probabilities– probability of Bb x Bb Bb
sperm egg offspring
1/2 1/2 =x 1/4B b Bb
1/2 1/2 =x 1/4b B Bb
1/4
1/4+
1/2
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Box #1
• How are probability and genetics related?
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Pedigree analysis
• Pedigree analysis reveals Mendelian patterns in human inheritance– data mapped on a family tree
= male = female = male w/ trait = female w/ trait
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Genetic counseling• Pedigree can help us understand the past &
predict the future• Thousands of genetic disorders are inherited
as simple recessive traits– from benign conditions to deadly diseases– albinism– cystic fibrosis– Tay sachs– sickle cell anemia– PKU
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Genetic testing
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Recessive diseases• The diseases are recessive because the allele
codes for either a malfunctioning protein or no protein at all– Heterozygotes (Aa)
• carriers
• have a normal phenotype because one “normal” allele produces enough of the required protein
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Box#2
solve the following problem in box #2:In purple people eaters, one-horn is dominant
and no horns is recessive. Show the cross of a purple people eater that is heterozygous for horns with a purple people eater that does not have horns. Summarize the genotypes and phenotypes of the possible offspring.
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Box #3
answer the following questions in box #3In humans, curly hair is dominant over straight hair. A
woman heterozygous for hair curl marries a man with straight hair and they have children.
1. What is the genotype of the mother?2. What gametes can she produce?3. What gametes can the father produce?4. What is the probability that the 1st child will have
curly hair?5. What is the probability the 2nd child will have curly
hair?
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Box#4• What is the probability of each of the following sets
of parents producing the given genotypes in their offspring?
Parents Genotype
Offspring Genotype
Probability
Aa x Aa Aa
Aa x aa Aa
AaBb x AaBB AABB
AaBb x AABb Aabb
AaBb x AaBb AaBb
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Heterozygote crosses• Heterozygotes as carriers of recessive alleles
Aa x Aa
A amale / sperm
A
a
fem
ale
/ eg
gs
AA
Aa aa
Aa
Aa
A
a
Aa
A
a
AA
Aa aa
Aa
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Cystic fibrosis
• Primarily people of European descent– strikes 1 in 2500 births
• 1 in 25 people of European descent is a carrier (Aa)
– normal allele codes for a membrane protein that transports Cl- across cell membrane• defective or absent channels limit transport of Cl- (& H2O) across cell
membrane• thicker & stickier mucus coats around cells • mucus build-up in the pancreas, lungs, digestive tract & causes
bacterial infections– without treatment children die before 5;
with treatment can live past their late 20s
normal lung tissue
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Normal Lungs Chloride channelTransports chloride through protein channel out of cellOsmotic effects: H2O follows Cl-
airway
cells lining lungs
Cl-
Na+
mucus secreting glands
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Cystic fibrosis
airway
cells lining lungs
Cl-
Na+
bacteria & mucus build up
thickened mucus hard to secrete
damaged lung tissue
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Tay-Sachs• Primarily Jews of eastern European (Ashkenazi)
descent & Cajuns– strikes 1 in 3600 births
• 100 times greater than incidence among non-Jews or Mediterranean (Sephardic) Jews
– non-functional enzyme fails to breakdown lipids in brain cells
• symptoms begin few months after birth
• seizures, blindness & degeneration of motor & mental performance
• child usually dies before 5yo
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Sickle cell anemia• Primarily Africans
– strikes 1 out of 400 African Americans– caused by substitution of a single amino acid in
hemoglobin– when oxygen levels are low, sickle-cell
hemoglobin crystallizes into long rods• deforms red blood cells into
sickle shape• sickling creates pleiotropic
effects = cascade of other symptoms
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Sickle cell anemia
• Substitution of one amino acid in polypeptide chain
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Sickle cell phenotype
• 2 alleles are codominant – both normal & mutant hemoglobins are
synthesized in heterozygote (Aa)– carriers usually healthy, although some suffer
some symptoms of sickle-cell disease under blood oxygen stress
• exercise
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Heterozygote advantage
• Sickle cell frequency– high frequency of heterozygotes
is unusual for allele with severe detrimental effects in homozygotes
• 1 out of 400 African Americans
• Suggests some selective advantage of being heterozygous– sickle cell: resistance to malaria?– cystic fibrosis: resistance to cholera?
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Heterozygote advantage• Malaria
– single-celled eukaryote parasite spends part of its life cycle in red blood cells
• In tropical Africa, where malaria is common:– homozygous dominant individuals die of malaria– homozygous recessive individuals die of sickle cell anemia– heterozygote carriers are relatively free of both
• High frequency of sickle cell allele in African Americans is vestige of African roots ������
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Malaria
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Prevalence of Malaria
Prevalence of SickleCell Anemia
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Huntington’s chorea• Dominant inheritance
– on end of chromosome 4• one of 1st genes to be identified• mutation = CAG repeats = glutamine amino acid
– build up of protein “huntingtin” in neurons (brain) causing cell death
• memory loss• muscle tremors, jerky movements = “chorea”• early death (10-20 years after onset)
– onset age 30-50
1872
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Genetics & culture• Why do all cultures have a taboo against incest?
– laws or cultural taboos forbidding marriages between close relatives are fairly universal
• Fairly unlikely that 2 unrelated carriers of same rare harmful recessive allele will meet & mate– but matings between close relatives increase risk
• “consanguineous” (same blood) matings– individuals who share a
recent common ancestor are more likely to carry same recessive alleles
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AA
Aa aa
Aa
A hidden disease reveals itself
AA x Aa
A Amale / sperm
A
a
fem
ale
/ eg
gs
Aa
AA
Aa
AA
Aa Aax
A amale / sperm
A
afe
mal
e / e
gg
s AA
Aa aa
Aa
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Box #5
• What is the heterozygote advantage?
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Beyond Mendel’s Lawsof Inheritance
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Extending Mendelian genetics• Mendel worked with a simple system
– peas are genetically simple– most traits are controlled by a single gene– each gene has only 2 alleles, 1 of which
is completely dominant to the other• The relationship between
genotype & phenotype is rarely that simple
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Incomplete dominance• Heterozygote shows an intermediate, blended
phenotype– example:
• RR = red flowers• rr = white flowers• Rr = pink flowers
– make 50% less color
RR Rr rr
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Incomplete dominancetrue-breeding
red flowerstrue-breeding white flowers
XP
100%
100% pink flowersF1
generation(hybrids)
self-pollinate
25%whit
eF2generation
25%
red1:2:
1
50%pink
It’s likeflipping 2 pennies!
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Incomplete dominance
CRCW x CRCW
CR CW
male / sperm
CR
CW
fem
ale
/ eg
gs _____
_____
_____
____
1:2:1
____
____
____
1:2:1
%genotype
%phenotype
CRCR
CRCW
CRCW
CWCW
____
____
_____
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Box #6
Certain breeds of cattle show incomplete dominance in coat color. When pure breeding red cows are bred with pure breeding white cows, the offspring are roan (a pinkish coat color). Summarize the genotypes and phenotypes of the possible offspring when a roan cow is mated with a roan bull.
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Box #7
Cows also have a second gene for horn vs hornless cattle. The allele for horns dominates the allele for hornless. If a bull and cow are heterozygous for both genes, what are the probabilities for each possible phenotype?
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Co-dominance• 2 alleles affect the phenotype equally &
separately– not blended phenotype– example: ABO blood groups– 3 alleles
• IA, IB, i• IA & IB alleles are co-dominant to each other
– both antigens are produced• both IA & IB are dominant to i allele
– produces glycoprotein antigen markers on the surface of red blood cells
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Box #8
• What is incomplete dominance?• What is co-dominance?
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Box #9
A man with type AB blood marries a woman with type B blood. Her mother has type O blood. List the expected phenotype and genotype frequencies of their children.
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Pleiotropy • Most genes are pleiotropic
– one gene affects more than one phenotypic character
• wide-ranging effects due to a single gene• dwarfism (achondroplasia) • gigantism (acromegaly)
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Acromegaly: André the Giant
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Epistasis • one gene will mask the expression of another
gene – coat color in mice = 2 separate genes
• C,c: pigment (C) or no pigment (c)
• B,b: more pigment (black=B) or less (brown=b)
• cc = albino, no matter B allele
• 9:3:3:1 becomes 9:3:4
How would you know thatdifference wasn’t random chance?
Chi-square test!
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Epistasis in Labrador retrievers• 2 genes: (E,e) & (B,b)
– pigment (E) or no pigment (e)– pigment concentration: black (B) to brown (b)
E–B–E–bbeeB–eebb
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Box #10
• make up an example of epistasis
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Polygenic inheritance• Some phenotypes determined by additive
effects of 2 or more genes on a single character– phenotypes on a continuum– human traits
• skin color• height• weight• eye color• intelligence• behaviors
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Box #11
describe in your own words, polygenic inheritance.
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enzyme
Skin color: Albinism• However albinism can be
inherited as a single gene trait
Johnny & Edgar Winter
albinoAfricans
melanin = universal brown color
tyrosine melaninalbinism
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OCA1 albino Bianca Knowlton
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Sex linked traits• Genes are on sex chromosomes
– as opposed to autosomal chromosomes– first discovered by T.H. Morgan at Columbia U.– Drosophila breeding
• good genetic subject– prolific– 2 week generations– 4 pairs of chromosomes– XX=female, XY=male
1910 | 1933
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autosomal
chromosome
s
sexchromosome
s
Classes of chromosomes
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Huh!Sex matters?!
F2generation
100%red-eye female
50% red-eye male50% white eye male
Discovery of sex linkage
P X
F1generation(hybrids)
100%red eye
offspring
true-breeding white-eye male
true-breedingred-eye female
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Genetics of Sex• In humans & other mammals, there are 2 sex
chromosomes: X & Y– 2 X chromosomes
• develop as a female: XX• gene redundancy,
like autosomal chromosomes– an X & Y chromosome
• develop as a male: XY• no redundancy
50% female : 50% male
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box# #12
The genes for hemophilia are located on the X chromosome. It is a recessive disorder. List the possible genotypes and phenotypes of the children from a man normal for blood clotting and a woman who is a carrier. (Hint: you have to keep track of what sex the children are).
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Genes on sex chromosomes• Y chromosome
– few genes other than SRY• sex-determining region• master regulator for maleness• turns on genes for production of male hormones
– many effects = pleiotropy!
• X chromosome– other traits beyond sex determination
• mutations:– hemophilia– Duchenne muscular dystrophy– color-blindness
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Human X chromosome• Sex-linked
– usually means“X-linked”
– more than 60 diseases traced to genes on X chromosome
Duchenne muscular dystrophyBecker muscular dystrophy
Ichthyosis, X-linkedPlacental steroid sulfatase deficiencyKallmann syndromeChondrodysplasia punctata, X-linked recessive
HypophosphatemiaAicardi syndromeHypomagnesemia, X-linkedOcular albinismRetinoschisis
Adrenal hypoplasiaGlycerol kinase deficiency
Incontinentia pigmentiWiskott-Aldrich syndromeMenkes syndrome
Charcot-Marie-Tooth neuropathyChoroideremiaCleft palate, X-linkedSpastic paraplegia, X-linked, uncomplicatedDeafness with stapes fixation
PRPS-related gout
Lowe syndrome
Lesch-Nyhan syndromeHPRT-related gout
Hunter syndromeHemophilia BHemophilia AG6PD deficiency: favismDrug-sensitive anemiaChronic hemolytic anemiaManic-depressive illness, X-linkedColorblindness, (several forms)Dyskeratosis congenitaTKCR syndromeAdrenoleukodystrophyAdrenomyeloneuropathyEmery-Dreifuss muscular dystrophyDiabetes insipidus, renalMyotubular myopathy, X-linked
Androgen insensitivity
Chronic granulomatous diseaseRetinitis pigmentosa-3
Norrie diseaseRetinitis pigmentosa-2
Sideroblastic anemiaAarskog-Scott syndrome
PGK deficiency hemolytic anemia
Anhidrotic ectodermal dysplasia
AgammaglobulinemiaKennedy disease
Pelizaeus-Merzbacher diseaseAlport syndrome
Fabry disease
Albinism-deafness syndrome
Fragile-X syndrome
Immunodeficiency, X-linked,with hyper IgM
Lymphoproliferative syndrome
Ornithine transcarbamylase deficiency
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Map of Human Y chromosome?< 30 genes on Y chromosome Sex-determining Region Y (SRY)
Channeling Flipping (FLP)Catching & Throwing (BLZ-1)
Self confidence (BLZ-2)note: not linked to ability gene
Devotion to sports (BUD-E)
Addiction to death &destruction movies (SAW-2)
Scratching (ITCH-E)Spitting (P2E) linked
Inability to express affection over phone (ME-2) Selective hearing loss (HUH)
Total lack of recall for dates (OOPS)
Air guitar (RIF)
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Sex-linked traits summary• X-linked
– follow the X chromosomes– males get their X from their mother– trait is never passed from father to son
• Y-linked– very few genes / traits– trait is only passed from father to son– females cannot inherit trait
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X-inactivation• Female mammals inherit 2 X chromosomes
– one X becomes inactivated during embryonic development• condenses into compact object = Barr body• which X becomes Barr body is random
– patchwork trait = “mosaic”
XHXh
XH
Xh
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X-inactivation & tortoise shell cat
• 2 different cell lines in cat
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Box #13
Why does X inactivation exists in females and not males?
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Male pattern baldness
• Sex influenced trait– autosomal trait influenced by sex hormones
• age effect as well = onset after 30 years old– dominant in males & recessive in females
• B_ = bald in males; bb = bald in females
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Nature vs. Nurture
• Phenotype is controlled by both environment & genes
Color of Hydrangea flowers is influenced by soil pH
Human skin color is influenced by both genetics & environmental conditions
Coat color in arctic fox influenced by heat sensitive alleles