8 Nov Lecture Slides PDF

8 Nov Lecture Slides 11/8/24 2) The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offs...

8 Nov Lecture Slides 11/8/24 2) The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character A cross between such heterozygotes is called a monohybrid cross 47 Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently 48 1 8 Nov Lecture Slides 11/8/24 Figure 14.8 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of Hypothesis of dependent assortment independent assortment Sperm Predicted or 1 /4 1 /4 1 /4 1 /4 offspring of YR Yr yR yr Sperm F2 generation 1 1 /2 /2 YR yr 1 /4 YR YYRR YYRr YyRR YyRr 1 /2 YR YYRR YyRr 1 /4 Yr Eggs YYRr YYrr YyRr Yyrr 1 /2 Eggs yr YyRr yyrr 1 /4 yR YyRR YyRr yyRR yyRr 3 /4 1 /4 1 /4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 49 Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome Genes located near each other on the same chromosome tend to be inherited together 50 2 8 Nov Lecture Slides 11/8/24 The laws of probability govern Mendelian inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles 51 The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities Probability in an F1 monohybrid cross can be determined using the multiplication rule Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele 52 3 8 Nov Lecture Slides 11/8/24 Figure 14.9 Rr ´ Rr Segregation of Segregation of alleles into eggs alleles into sperm Sperm 1 /2 R 1 /2 r R R 1 /2 R R r 1 /4 1 /4 Eggs r r 1 /2 r R r 1 /4 1 /4 53 Exclusive vs. Independent Events The difference between mutually exclusive and independent events is: Mutually exclusive events can simply be defined as a situation when two events cannot occur at same time. Independent events occurs when one event remains unaffected by the occurrence of the other event. 54 4 8 Nov Lecture Slides 11/8/24 The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous 55 Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied 56 5 8 Nov Lecture Slides 11/8/24 EXPERIMENT P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of Hypothesis of dependent assortment independent assortment Sperm Predicted or 1 /4 1 /4 1 /4 1 /4 offspring of YR Yr yR yr Sperm F2 generation 1 1 /2 /2 YR yr 1 /4 YR YYRR YYRr YyRR YyRr 1 /2 YR YYRR YyRr 1 /4 Yr Eggs YYRr YYrr YyRr Yyrr 1 /2 Eggs yr YyRr yyrr 1 /4 yR YyRR YyRr yyRR yyRr 3 /4 1 /4 1 /4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/16 3/16 3/16 1/16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 57 The Multiplication Rule illustrated Given YyRr x YyRr Probability of YYRR = 1/4 (probability of YY) ´ 1/4 (RR) = 1/16 Probability of YyRR = 1/2 (Yy) ´ 1/4 (RR) = 1/8 For Heterozygous characters (Yy), the probability of the 1st allele is 1, because it doesn’t matter if it is dominant or recessive. (we need 1 of each) Based on the first allele, there is a definite probability for the 2nd allele being the opposite (1/2) 58 6 8 Nov Lecture Slides 11/8/24 More nuances of The Multiplication Rule For any individual, the probabilities of a given allele being provided to an offspring is as follows: AA, à 100% or 1.0 for A; 0% or 0 for a Aa à 50% or.5 or ½ for either A or a (but note other parent) aa à 100% or 1.0 for a; 0% or 0 for A. 59 The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment, what is the probability that F2 offspring will have an aabbccdd genotype? aa à ½x½=¼ bb à ½x½=¼ cc à ½x½=¼ dd à ½x½=¼ aa x bb x cc x dd = 1/256 60 7 8 Nov Lecture Slides 11/8/24 Extra Practice The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment, what is the probability that F2 offspring will have an _________ genotype? A) AaBbCcDd B) AABBCCDD C) AaBBccDd D) AaBBCCdd 61 What is the probability that the following pairs of parents will produce the indicated offspring? AABbCc x AaBbCc à AAbbCC AA à 1 x ½ = ½ bb à ½ x ½ = ¼ CC à ½ x ½ = ¼ aa x bb x cc = 1/32 62 8 8 Nov Lecture Slides 11/8/24 Extra Practice What is the probability that the following pairs of parents will produce the indicated offspring? A) AABBCC x aabbcc à AaBbCc B) AaBbCc x AaBbCc à AaBbCc C) aaBbCC x AABbcc à AaBbCc 63 Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Many heritable characters are not determined by only one gene with two alleles However, the principles of segregation and independent assortment apply even to more complex patterns of inheritance 64 9 8 Nov Lecture Slides 11/8/24 Extending Mendelian Genetics for a Single Gene Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: 1) When alleles are not completely dominant or recessive 2) When a gene has more than two alleles 3) When a gene produces multiple phenotypes 65 1) Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways 66 10 8 Nov Lecture Slides 11/8/24 Incomplete dominance in snapdragon color P Generation Red White CRCR CWCW Gametes CR CW F1 Generation Pink CRCW 1 1 Gametes /2 C R /2 CW Sperm F2 Generation 1/ 2 CR 1 /2 CW 1/ 2 CR CRCR CRCW Eggs 1/ 2 CW CRCW CWCW 67 Frequency of Dominant Alleles Dominant alleles are not necessarily more common in populations than recessive alleles For example, one baby out of 400 in the United States is born with extra fingers or toes The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage In this example, the recessive allele is far more prevalent than the population’s dominant allele 68 11 8 Nov Lecture Slides 11/8/24 Multiple Alleles ( > 2 alleles) Most genes exist in populations in more than two allelic forms For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither 69 Multiple alleles for the ABO blood groups. (a) The three alleles for the ABO blood groups and their carbohydrates Allele IA IB i Carbohydrate A B none (b) Blood group genotypes and phenotypes Genotype IAIA or IAi IBIB or IBi IAIB ii Red blood cell appearance Phenotype (blood group) A B AB O 70 12 8 Nov Lecture Slides 11/8/24 Pleiotropy Most genes have multiple phenotypic effects, a property called pleiotropy For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease 71 Pleiotrophic Effects in Sickle Cell Anemia 72 13 8 Nov Lecture Slides 11/8/24 Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes. Two mechanisms include: 1) Epistasis 2) Polygenic Inheritance 73 1) Epistasis In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus For example, in Labrador retrievers and many other mammals, coat color depends on two genes One gene determines the pigment color (with alleles B for black and b for brown) The other gene (with alleles E for color and e for no color) determines whether the pigment will be deposited in the hair 74 14 8 Nov Lecture Slides 11/8/24 BbEe BbEe Sperm 1 1 /4 BE 1 /4 bE 1 /4 Be /4 be Eggs 1/ 4 BE BBEE BbEE BBEe BbEe 1 /4 bE BbEE bbEE BbEe bbEe 1/ 4 Be BBEe BbEe BBee Bbee 1/ be 4 BbEe bbEe Bbee bbee 9 : 3 : 4 75 15