LECTURE 2 MENDEL S LAWS and EXTENSIONS TO MENDEL Reading Ch 2 p 13 28 Ch 3 p 43 53 Supplemental Reading Mendel s 1865 paper use the ESP link off the 140 website Problems Ch 2 solved problems I II 2 4 2 5 2 7 2 8 2 13 2 14 2 16 2 20 Ch 3 3 2 3 5 3 7 3 9 3 11 3 13 3 14 Website http mcb berkeley edu courses mcb140 In the late 1850 s when Mendel was making his discoveries no one had yet described meiosis or genes or even chromosomes Two theories of inheritance were prevalent at the time 1 blended inheritance in which traits from the parents become mixed and forever changed and 2 uni parental inheritance in which one parent contributes most to the inherited features of progeny Mendel performed a very careful controlled and quantitative analysis to examine the rules of inheritance In his own words he sought to deduce the law according to which traits appear in successive generations Why did Mendel succeed He used an experimentally tractable system Pisum sativum garden peas Peas have short generation time and can be self or cross fertilized He studied easily scorable unambiguous traits discrete vs continuous traits He used pure breeding lines He carefully controlled his matings so that he could be sure that the progeny observed were really a result of the cross he performed He realized that accidental pollinations would completely confuse his analyses Quantitative analysis Mendel worked with large numbers of plants and used statistics to analyze the data The monohybrid cross A cross between plants that exhibit differences in one distinct trait seed color seed shape flower color etc i e plants that produce yellow peas x plants that produce green peas Generation Parental P First filial F1 Second filial F2 yellow x green all yellow 3 yellow 1 green cross fertilize self fertilize What did Mendel deduce from these crosses Blending did not occur Green peas were recovered in the F2 generation Yellow was dominant to green There must be two types of yellow peas those that breed true like the yellow P generation plants and those that carry latent information for green peas like the F1 plants He confirmed this by selfing some of the F2 plants that gave yellow peas 1 3 of these breed true all yellow and 2 3 gave a 3 1 ratio of yellow to green To account for the 3 1 ratio Mendel deduced that each plant carries two differentiating characters genes one inherited from the maternal parent and one from the paternal parent Each gene can have alternative forms alleles if the two alleles are the same the parental type results and if the two alleles are different a hybrid type results LAW OF SEGREGATION The two alleles for each trait segregate during gamete formation then unite at random one from each parent at fertilization To represent the monohybrid cross a different way Genotype Phenotype P YY x yy yellow x green F1 Yy yellow F2 1 YY 2 Yy 1 yy 3 yellow 1 green The Punnett Square allows us to visualize a cross by examining the possible combinations of gametes from the parents A cross between two Yy heterozygotes Y y Y YY yellow yY yellow y Yy yellow yy green The Law of the Product states The probability of two or more independent events occurring together is the product of the probabilities that each event will occur by itself e g probability of yy progeny from a cross of two Yy heterozygotes 1 2 x 1 2 1 4 The Law of the Sum states The probability of either of two such mutually exclusive events occurring is the sum of their individual probabilities What s the probability of yellow progeny from a cross of two Yy heterozygotes Probability of YY Yy yY 1 4 1 4 1 4 3 4 THE LAW OF INDEPENDENT ASSORTMENT During gamete formation different pairs of alleles segregate independently of one another The dihybrid cross Generation P F1 F2 Phenotype Genotype yellow round x green wrinkled YYRR x yyrr all yellow round YyRr yellow round 9 parental type Y Ryellow wrinkled 3 recombinant type Y rr green round 3 recombinant type yyRgreen wrinkled 1 parental type yyrr The 9 3 3 1 ratio observed derives from two separate 3 1 phenotypic ratios the ratio of yellow to green is 12 4 or 3 1 and of round to wrinkled is 12 4 or 3 1 How could you determine the genotype of the yellow round plants You could perform a Test Cross by crossing an individual showing a dominant phenotype Y R to an individual with the recessive phenotype yyrr to reveal the genotype behind the dominant phenotype The test cross is a very important tool for the geneticist this is especially true when analyzing inheritance in organisms that cannot self fertilize If Y R is YYRR then all test cross progeny will be yellow round If Y R is YYRr then half will be yellow round and half will be yellow wrinkled If Y R is YyRR then half will be yellow round and half will be green round If Y R is YyRr then all four combinations occur with equal probability 1 1 1 1 Bottom line of Independent Assortment During Meiosis I different alleles of two genes on different chromosomes will move to opposite poles independently of one another Only true for genes that lie on different chromosomes or for genes that lie very far apart on the same chromosome Genes on the same chromosome exhibit linkage that is they tend to assort together We ll discuss linkage in more detail next week EXTENSIONS TO MENDEL In determining the laws of inheritance Mendel used a set of guidelines 1 one of the two alleles of a given gene showed complete dominance over the other 2 there are only two alleles of any given gene 3 genes determine one specific trait and 4 all genotypes are equally viable Let s look at some exceptions to these general rules What if the alleles show incomplete dominance If this is the case a novel phenotype unlike that of either parent shows up in the F1 Color in snapdragons is an excellent example P pure breeding red flowers x pure breeding white flowers F1 all pink NEW PHENOTYPE NOT LIKE EITHER PARENT F2 red pink white 1 2 1 This ratio is a good indication of the lack of complete dominance The phenotypic ratios are an exact reflection of the genotypic ratios Molecularly one can think of the combinations as follows CR CR gives 2 doses of gene expression and CW CW gives no dose CR CW has one dose of gene expression and this intermediate dose gives a novel phenotype CR CW CR CR CR red CW CR pink CW CR CW pink CW CW white What if the alleles show co dominance If this is the case then the F1 progeny display both parental traits The IA and IB blood types in humans are a good example These two