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CHAPTER 14 MENDEL & THE GENE IDEA
14.1 Mendel used the scientific approach to identify two laws of inheritance
14.2 The laws of probability govern Mendelian inheritanceI. Intro
A. Scale 0-11. 0= will not occur2. 1= certain to occur
B. Important lesson of probability 1. Each event is independent of the next
a. Alleles of 1 gene segregate into gametes independently of another gene’s alleles
C. Probability can help us predict the outcome of the fusion of gametes
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II. The multiplication and addition rules applied to monohybrid crossesA. Multiplication rule
1. Prediction of 2 independent events occurring simultaneouslya. Multiply all independent event probabilities
e.x Tossing pennies Event 1 Probability of tails = 1/2 Event 2 Probability of tails = 1/2 What is the probability of 2 coins flipped simultaneously of landing on tails
1/2 (event 1) x 1/2 (event 2)= 1/4
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e.x F1 cross Rr X Rr R=round r= wrinkleEvent 1 Probability of egg receiving R = 1/2
Probability of egg receiving r = 1/2 Event 2 Probability of sperm receiving R = 1/2
Probability of sperm receiving r = 1/2 What is probability of egg + sperm = RR
1/2 (egg with R) X 1/2 (Sperm with R) = 1/4
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B. Addition rule1. Probability of 1 of 2 or more mutually exclusive events will
occur is calculated by adding their individual probabilitiesa. Probability of 1 of 2 or more related but independent events
will occur is calculated by adding their individual probabilities
e.x Rr X RrWhat is probability of heterozygous round (Rr)
Rr = 1/4 rR = 1/4
1/2
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e.x Cross = Rr X rrWhat is probability of RrWhat is probability of rr
1/2 Rr
1/2 rr
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III. Solving complex genetics problems with the rules of probabilityA. Dihybrids
1. Cross = YyRr X YyRra. What fraction of offspring would be predicted to have
YyRR1. Step 1- due to independent assortment you can deal
with the 2 genes separately a. Set up a monohybrid cross for each
2. Step 2- Now use the laws of probability 1/2 Yy x 1/4 RR = 1/8 YyRR or 2/16
1/4 YY
1/2 Yy
1/4 yy
1/4 RR
1/2 Rr
1/4 rr
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2. Practice= TTQq X TtQqa. What is the frequency of the genotype TTQq in the F2
generationB. Trihybrids
1. Cross = QqTtRr X Qqttrra. What fraction of offspring would be predicted to exhibit
the recessive phenotype for at least 2 of the three characteristics 1. Step 1- List all possible genotypes of offspring fulfilling
conditionqqttRRqqttRrqqTTrrqqTtrrQQttrrQqttrrqqttrr
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2. Step 2- List all possible genotypes based on crossqqttRRqqttRrqqTTrrqqTtrrQQttrrQqttrrqqttrr
3. Monohybrid punnett square
1/4 QQ
1/2 Qq
1/4 qq
1/2 Tt
1/2 tt
1/2 Rr
1/2 rr
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4. Implement multiplication and addition rulesqqttRr 1/4 (probability of qq) X 1/2 (tt) X 1/2 (Rr) = 1/16qqTtrr 1/4 X 1/2 X 1/2 = 1/16Qqttrr 1/2 X 1/2 X 1/2 = 1/8 or 2/16QQttrr 1/4 X 1/2 X 1/2 = 1/16qqttrr 1/4 X 1/2 X 1/2 = 1/16
Chance of at least 2 recessive traits = 6/16 or 3/8 14.3 Inheritance patterns are often more complex than predicted by
simple Mendelian geneticsI. Extending Mendelian genetics for a single gene
A. Degrees of dominance 1. Complete dominance
a. Mendel’s workb. One allele overshadows/masks the otherc. Homozygous dominant & heterozygous phenotypically the
same
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2. Incomplete dominancea. Offspring are phenotypically intermediate between 2
parents1. Heterozygous flowers produce less red pigment than red
homozygote
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3. Codominancea. Both alleles of a gene are expressed phenotypicallyb. ABO blood grouping
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B. Relationship between dominance & phenotype1. How is dominance achieved
a. Alleles=nucleotide sequence proteins function1. Individual alleles do not interact2. Dominance or recessive is achieved through allele
expressionb. Ex Mendel’s peas
1. Round (dominant) & wrinkled (recessive)a. Round allele codes for enzymeb. Wrinkled allele codes for defective enzyme
c. Tay-sachs disease1. Disease manifests when enzymes cannot breakdown
certain lipids in the braina. Seizures, blindness, degeneration of motor & mental
performance, & death2. Homozygous dominant & heterozygous = no
manifestation3. Homozygous recessive = manifestation
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2. Dominance/recessive a matter of viewpointa. Tay-sachs disease
1. Organismal level dominant/recessive2. Biochemical level incomplete dominance
a. Homozygous dominant = complete functional enzyme production
b. Heterozygous = functional enzyme & nonfunctional enzyme production but enough function to prevent manifestation
c. Homozygous = complete nonfunctional enzymeC. Frequency of dominance
1. Dominant allele not always the higher frequencya. Polydactyly
D. Multiple alleles1. Blood groups (ABO)
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E. Pleiotropy1. 1 gene affecting multiple phenotypes2. Garden pea gene for flower color also influences seed color
II. Extending Mendelian genetics for two or more genesA. Epistasis
1. Gene at 1 locus alters the phenotypic expression of a gene at a second locus
2. Ex Mice – Black (B) dominant to brown (b) coat colora. However, a different gene for
color (C) controls the release of the pigments needed for hair color1. bb = brown but if ccbb will
be albino
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B. Polygenic inheritance1. Multiple genes controlling a particular phenotype
a. Phenotype exists as a continuum1. Quantitative characters2. Height & skin color
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III. Nature and nurture: The environmental impact on phenotype