Lec 8 Outline- A01-2024 PDF

HOW IS GENETIC MATERIAL INHERITED? PART 2 Relevant reading: Morris, 4th edition, Chapter 14  TRANSMISSION GENETICS Each of us has his or her own personal genome. It differs from all that have existed before and from all that will come after. Transmission genetics = manner in whic...

HOW IS GENETIC MATERIAL INHERITED? PART 2 Relevant reading: Morris, 4th edition, Chapter 14  TRANSMISSION GENETICS Each of us has his or her own personal genome. It differs from all that have existed before and from all that will come after. Transmission genetics = manner in which genetic differences among individuals are passed from generation to generation. PEA PLANT TRAITS  PEA PLANT CROSSING Pea flowers have sperm- and egg-producing structures that allow for self-fertilization to occur. Mendel had to remove sperm-producing structures in order to ensure that only his intended cross would happen. PEA PLANT CROSSING  YELLOW VS. GREEN SEED TRAITS Plant grown from Plant grown from true-breeding strain with yellow seeds true-breeding strain with green seeds True-breeding strains that are crossed P1 generation constitute the P1 or parental generation. F1 generation The trait that appears in the F1 generation (in this case yellow seeds) is dominant, and the other trait (green seeds) is recessive.  YELLOW VS. GREEN SEED TRAITS The same outcome resulted from reciprocal crosses, in which Mendel interchanged which parent (male or female) exhibited each trait.  GENOTYPE à PHENOTYPE 1.Mendel explained his findings by supposing that there is a hereditary factor for seed color, shape etc. 2.We now know that the hereditary factors that result in contrasting traits are different forms of a gene that affect that trait. 3.The different forms of a gene are called alleles. 4.The combination of alleles in an individual is its genotype and the expression of the trait is its phenotype.  F2 GENERATION Seeds from F1 plants produced from a cross of true- breeding yellow-seed and green-seed plants are yellow because yellow is dominant and green is recessive in seed color. Peas are normally self-fertilizing and so, if they are left F1 generation alone, the pollen produced in each flower fertilizes the ovules. The F2 generation showed the reappearance of the recessive trait. Dominant: recessive ratio 3:1. F2 generation (3 yellow seeds:1 green seed)  THE PRINCIPLE OF SEGREGATION Each reproductive AA aa Homozygous cell (gamete) contains only one allele of each gene. P1 generation The fertilized egg cell, called the zygote, is formed from the random union of two Heterozygous Aa gametes, one from each parent F1 generation  The Principle of Segregation: the equal AA aa separation of alleles of a P1 generation gene into different gametes; half get one allele, the other half get the other allele. Aa F1 generation A a 1/2 1/2 A Expected ratio of AA:Aa:aa 1/2 AA Aa genotypes is 1:2:1. 1/4 1/4 Aa Expected ratio of F1 generation a Dominant : recessive 1/2 Aa aa phenotypes is 3:1. 1/4 1/4 F2 generation SEGREGATION OF ALLELES IN MEIOSIS Separating Maternal and Paternal Chromosomes in Meiosis I  INCOMPLETE DOMINANCE X In notating incomplete dominance, we use CRCR CWCW P1 generation superscripts to indicate the alleles, rather than upper-case and lower-case letters, because neither allele is dominant to the other. The phenotype of the CRCW heterozygous CRCW plant is intermediate, an F1 generation example of incomplete dominance. CR CW 1/ 1/ 2 2 CR 1/ 2 CRCR CRCW 1/ 4 1/ 4 The result of segregation can be observed directly, because the ratio of red:pink:white CRCW phenotypes is 1:2:1, which reflects the ratio CW 1/ 2 CRCW CWCW of CRCR:CRCW:CWCW genotypes. 1/ 1/ 4 4 F2 generation  CODOMINANCE In codominance, each allele produces a distinct phenotype that can be detected in heterozygous individuals.  PROBABILITY The possible outcomes of a cross are expressed as a likelihood or probability - the probability of occurrence of a genotype lies between 0 and 1 - Aa x AA à probability of the genotype aa is 0 - AA x aa à probability of the genotype Aa is 1 - usually intermediate value Sometimes it is necessary to combine the probabilities of 2 or more possible outcomes of a cross. 1. Multiplication rule: outcomes can occur simultaneously and the occurrence of one does not impact the likelihood of the other. 2. Addition rule: possible outcomes cannot occur simultaneously.  MULTIPLICATION RULE outcomes can occur simultaneously and the occurrence of one does not impact the likelihood of the other. The probability of rolling a double four – 1/6 x 1/6 = 1/36 http://il5.picdn.net/shutterstock/videos/7971196/thumb/8.jpg  ADDITION RULE possible outcomes cannot occur simultaneously (either of say 2 mutually exclusive events occurring). The probability of rolling a seven in any combination – 1/36 + 1/36 + 1/36 + 1/36 + 1/36 + 1/36 = 6/36 =1/6 https://bestcase.files.wordpress.com/2011/01/dicediagram.jpg  Addition/Multiplication Rules Mating of Aa x Aa (1/4) (3/4) (3/4) (3/4) = 27/256 Probability that the seed closest to the stem is green and the others are yellow is determined by the multiplication rule. (3/4) (1/4) (3/4) (3/4) = 27/256 Probability that any one seed among 4 seeds is green (and the others are yellow) is determined by first using the multiplication rule to determine the (3/4) (3/4) (1/4) (3/4) = 27/256 probability that a particular seed is green, then the addition rule to determine that any one seed is green. (3/4) (3/4) (3/4) (1/4) = 27/256 27/256 + 27/256 + 27/256 +27/256 = 108/256 = 42% Plant grown from true-breeding Plant grown from true- Independent strain with yellow and wrinkled breeding Assortment: segregation of seeds strain with green and round seeds one set of alleles of a gene pair is independent of the X segregation of another set of P1 generation alleles of a different gene pair. (genotype AA bb) (genotype aa BB) Because of dominance, the seeds in the F1 generation are yellow and round. F1 generation (genotype Aa Bb) The expected ratio of the four types of seeds is 9 yellow, round 3 green, round 3 yellow, wrinkled 1 green, wrinkled F2 generation INDEPENDENT ASSORTMENT Parental genotypes: Aa Bb ¾ yellow; ¼ green ¾ round; ¼ wrinkled So, yellow, wrinkled = ¾ x ¼ = 3/16 Independent assortment of genes in different chromosomes reflects the fact that non-homologous INDEPENDENT chromosomes can orient in either of two ways that are equally likely. ASSORTMENT A B A A b A B b a b a B a b a B Anaphase I Anaphase I Resulting gametes Resulting gametes A A A A B B b b a a a a b b B B  MENDEL’S LAWS Principle of Segregation Principle of Independent Assortment – Note: not all genes undergo independent assortment! – E.g. genes close together on the same chromosome = linked genes; do NOT assort independently The story is always more complicated… Epistasis: two genes interacting affect the same trait  Epistasis CI Ci cI ci 1/4 1/4 1/4 1/4 CI 1/4 CC II CC Ii Cc II Cc Ii Alleles Ci C Pigment 1/4 CC Ii CC ii Cc Ii Cc ii c No pigment cI I Inhibitor 1/4 Cc II Cc Ii cc II cc Ii i No inhibitor ci 1/4 Cc Ii Cc ii cc Ii Cc ii Genotypes of the form C– ii * have colored feathers, whereas all other genotypes have white feathers. The result is an F2 ratio of white:colored of 13:3, which is a modified form of the expected 9:3:3:1. *Note: C- indicates CC or Cc  PATTERNS OF INHERITANCE IN HUMANS Horizontal line between individuals represents mating. Generation I Vertical line leads to progeny. Progeny are arranged horizontally, II left to right in order of birth. III IV Identical twins Nonidentical twins Double line means mating between relatives. Diagram of family history Female = pedigree Male Open symbol means not affected. Darkened symbol means affected. PEDIGREE OF A DOMINANT ALLELE For a rare dominant trait, From matings in which one most matings that parent is affected, produce affected offspring approximately half the have only one affected offspring are affected. parent. Affected Affected individuals individuals are appear in each equally likely to successive be females or generation. males. PEDIGREE OF A RECESSIVE ALLELE  GENETIC TESTING Method of identifying the genotype of an individual, who might be at risk for a certain trait Benefits – Personalized medicine/treatment, informed decisions about healthcare – Better understanding of risks, behaviors – Feeling less anxious, leading to a better quality of life  GENETIC TESTING Risks – Limited answers – Physiological/emotional impact – Privacy concerns (life, health insurance) – Many studies look at entire genome – impact? HOW IS GENETIC MATERIAL INHERITED? PART 3 Relevant reading: Morris, 4th edition, Chapters 14 & 15  UNCOMMON INHERITANCE PATTERNS http://www.color-blindness.com/wp-content/images/inheritance-pattern.jpg https://ghr.nlm.nih.gov/art/large/mitochondrial.jpeg  HUMAN SEX CHROMOSOMES Almost none of the genes in the X chromosome have counterparts in the Y chromosome. ~ 1000 genes on X ~50 genes on Y  Meiosis in a female results in X-bearing eggs only. SEGREGATION OF THE SEX Meiosis in a male 1/2 1/2 CHROMOSOMES X X results in a 1:1 ratio of X-bearing and Y- X Eggs X bearing sperm. 1/2 X X X X X Female Female Sperm X Y 1/2 Y X Y X Y Male Male Random fertilization results in an expected ratio of 1/2 XX (female) and 1/2 XY (male) progeny.  Parental generation X-LINKED GENES x Red-eyed White-eyed female male F1 generation All of the progeny have red eyes. x = recessive mutation Red-eyed Red-eyed female male F2 generation White eyes reappear in the next generation, but only in males. All of the females have red eyes, and among the males, the ratio of red:white eyes is 1:1. Red-eyed Red-eyed White-eyed female male male 1:1 Most genes in the X X-LINKAGE chromosome have no counterparts in the Y chromosome. Therefore, a 1/2 1/2 “Crisscross” recessive mutation in an X- = X from Dad to XX linked gene is expressed in Homozygous daughter males. female à Then from that X Eggs X daughter to a son. Sperm A male with an 1/2 X-linked recessive trait XX XX X Heterozygous female Heterozygous female will have XY heterozygous Affected male (carrier) daughters and 1/2 unaffected sons. Y XY XY Normal male Normal male = All F1 are red- eyed  HETEROZYGOUS CROSS 1/2 1/2 XX Heterozygous Among progeny from carrier female a heterozygous X X Sperm Eggs carrier female, half of the daughters are expected to be 1/2 heterozygous carriers and half the sons are X XX XX expected to be Heterozygous female Homozygous female affected. XY Normal male = in F2 all females 1/2 are red eyed but males have 1:1 Y XY XY ratio of red : white Affected male Normal male eyes  MORGAN’S FRUIT x Red-eyed White- FLY CROSSES female eyed w+ w+ male w– Y x “w-” stands for the recessive Red-eyed Red-eyed white-eyed mutation in one X- female male chromosome w+ w– w+ Y Red-eyed Red-eyed White-eyed female male male w+ w+ or w+ w– w+ Y w– Y  MORGAN’S FRUIT FLY CROSSES The pattern of inheritance of the white-eyed mutation = The pattern of inheritance of the X-chromosome NORMAL CHROMOSOME SEPARATION x White-eyed Red-eyed female male w– w– w+ Y Normal chromosome separation results in each egg containing a single X X X chromosome. In a cross with white-eyed Most XY males receive females, normal chromosome their X chromosome separation results in female from their mother. progeny with red eyes and male progeny with white eyes. Red-eyed White-eyed female male w+ w– w– Y ABNORMAL CHROMOSOMAL SEPARATION: NONDISJUNCTION Note: XXX or OY were never observed = lethal NONDISJUNCTION: EVIDENCE THAT GENES RESIDE ON CHROMOSOMES NONDISJUNCTION OF SEX CHROMOSOMES IN HUMANS NONDISJUNCTION OF AUTOSOMES IN HUMANS https://upload.wikimedia.org/wikipedia/commons/3/31/Mitotic_nondisjunction.png Trisomy 21 = Down’s syndrome https://comd281-summerwiki.wikispaces.com/file/view/trisomy_21.gif/150343075/trisomy_21.gif