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 inheritance I. Intro A. Scale = will not occur 2. 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

II. The multiplication and addition rules applied to monohybrid crosses A. Multiplication rule 1. Prediction of 2 independent events occurring simultaneously a. 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

e.x  F 1 cross Rr X Rr R=round r= wrinkle Event 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

B. Addition rule 1. Probability of 1 of 2 or more mutually exclusive events will occur is calculated by adding their individual probabilities a. Probability of 1 of 2 or more related but independent events will occur is calculated by adding their individual probabilities e.x  Rr X Rr What is probability of heterozygous round (Rr) Rr = 1/4 rR = 1/4 1/2

e.x  Cross = Rr X rr What is probability of Rr What is probability of rr 1/2 Rr 1/2 rr

III. Solving complex genetics problems with the rules of probability A. Dihybrids 1. Cross = YyRr X YyRr a. What fraction of offspring would be predicted to have YyRR 1. 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

2. Practice= TTQq X TtQq a. What is the frequency of the genotype TTQq in the F2 generation B. Trihybrids 1. Cross = QqTtRr X Qqttrr a. 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 condition qqttRR qqttRr qqTTrr qqTtrr QQttrr Qqttrr qqttrr

2. Step 2- List all possible genotypes based on cross qqttRR qqttRr qqTTrr qqTtrr QQttrr Qqttrr qqttrr 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

4. Implement multiplication and addition rules qqttRr 1/4 (probability of qq) X 1/2 (tt) X 1/2 (Rr) = 1/16 qqTtrr 1/4 X 1/2 X 1/2= 1/16 Qqttrr 1/2 X 1/2 X 1/2= 1/8 or 2/16 QQttrr 1/4 X 1/2 X 1/2= 1/16 qqttrr 1/4 X 1/2 X 1/2 = 1/16 Chance of at least 2 recessive traits = 6/16 or 3/  Inheritance patterns are often more complex than predicted by simple Mendelian genetics I. Extending Mendelian genetics for a single gene A. Degrees of dominance 1. Complete dominance a. Mendel ’ s work b. One allele overshadows/masks the other c. Homozygous dominant & heterozygous phenotypically the same

2. Incomplete dominance a. Offspring are phenotypically intermediate between 2 parents 1. Heterozygous flowers produce less red pigment than red homozygote

3. Codominance a. Both alleles of a gene are expressed phenotypically b. ABO blood grouping

B. Relationship between dominance & phenotype 1. How is dominance achieved a. Alleles=nucleotide sequence  proteins  function 1. Individual alleles do not interact 2. Dominance or recessive is achieved through allele expression b. Ex  Mendel ’ s peas 1. Round (dominant) & wrinkled (recessive) a. Round allele codes for enzyme b. Wrinkled allele codes for defective enzyme c. Tay-sachs disease 1. Disease manifests when enzymes cannot breakdown certain lipids in the brain a. Seizures, blindness, degeneration of motor & mental performance, & death 2. Homozygous dominant & heterozygous = no manifestation 3. Homozygous recessive = manifestation

2. Dominance/recessive a matter of viewpoint a. Tay-sachs disease 1. Organismal level dominant/recessive 2. 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 enzyme C. Frequency of dominance 1. Dominant allele not always the higher frequency a. Polydactyly D. Multiple alleles 1. Blood groups (ABO)

E. Pleiotropy 1. 1 gene affecting multiple phenotypes 2. Garden pea gene for flower color also influences seed color II. Extending Mendelian genetics for two or more genes A. 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 color a. However, a different gene for color (C) controls the release of the pigments needed for hair color 1. bb = brown but if ccbb will be albino

B. Polygenic inheritance 1. Multiple genes controlling a particular phenotype a. Phenotype exists as a continuum 1. Quantitative characters 2. Height & skin color

III. Nature and nurture: The environmental impact on phenotype