Chapter 1: Mendel s breakthrough: patterns, particles and principles of heredity

Transcription

1 Chapter 1: Mendel s breakthrough: patterns, particles and principles of heredity please read pages 10 through 13 Slide 1 of Chapter 1 One of Mendel s express aims was to understand how first generation traits (parents) would disappear in the second generation (F1 or hybrid) and reappear in the third generation (F2) Gregor Mendel conducted experiments with peas between 1857 and 1863 Presented results in 1865 Poorly received/understood: Karl Wilhelm von Nageli, a noted Swiss botanist. Nageli, whose thinking sometimes veered from science to mysticism, dismissed Mendel's work Note: Correns, one of three re-discoverers in 1900, was a nephew of Nageli! 1

2 Plant life cycles Endosperm is 3N 2 copies of egg chromosomes, 1 of sperm Maternal tissue: seed coat=ovule wall Pod/fruit wall= ovary wall From: Plant Life Cycles and Angiosperm Development: Susan R. Singer from Embryology: Constructing the Organism (S.F. Gilbert and A. M. Raunio, eds.) Sinauer Associates, Sunderland, MA. Seed First 2N cell=zygote Sperm and eggs= gametes (N) Slide 2 of Chapter 1 Review the life cycles of plants and be familiar with these terms: Sporophytic or maternal tissue Gametes (sperm and eggs) pollination and Fertilization Zygote 2

3 Mendel s Experimental Organism Slide 3 of Chapter 1 experimental organism was the garden pea: advantages- naturally self pollinated which generates true breeding plants (later we ll call these homozygous plants) easily cross fertilized or cross-pollinated readily available clear-cut alternative forms of traits seed traits (color, shape) allowed for easy generation of large data sets 3

4 Mendel s traits Slide 4 of Chapter 1 two of Mendel s traits exhibited Xenia XENIA: genetic differences among pollen grains can manifest as phenotypic differences in seed set on female parent in P generation. Xenia genes expressed in embryo (including cotyledons) or endosperm. But not seed coat or pod attributes genotypes of both egg and pollen determine phenotype phenotype of F1 generation manifest in F1 seeds produced on true breeding female parent. Phenotype could vary within a pod seed shape: round vs wrinkled embryo/cotyledon trait seed color: yellow vs green Mendel referred to this as albumen color actually is color of cotyledon, which is embyronic tissue five of Mendel s traits manifest in flowers, pods or stem morphology phenotype of F1 generation manifest in plants grown from seeds produced by cross pollination pod shape: inflated vs constricted Note: P generation pods contain F1 seed, and F1 pods contain F2 seeds, etc.! Therefore, parental generation pods have same genotype and phenotype irrespective of pollen genotype pod color: green vs yellow flower/pod position: axillary vs terminal stem length: standard vs dwarf 4

5 Fn seed vs. Fn plants pollen P plant P plant Male (pollen source) Female (emasculated) F1 F2 plants F1 seed P seed P seed F2 seed Slide 5 of Chapter 1 P generation fruit tissue (ovary walls=pericarp, ovule walls=seed coat) is derived from cells that are direct descendents of the parental mitotic divisions of the zygote/embryo in the seed from which the P generation plant arose. All such tissue is therefore genetically identical on a single plant, even if the P generation plant was pollinated/fertilized by a genetically different plant. Seeds on P generation plants used as females in a cross-fertilization carry F1 generation embryos. Seeds can have three distinct tissuesøembryo, including cotyledons (can be different from mother plant) Øseed coat (derived from maternal tissue just like the fruit wall or pericarp is) Øendosperm- derived from union of two clones of the egg and one clone of the sperm The embryo represents the progeny, so fruits and seeds are bi-generational. This is important when considering mendel s experiments since some traits are expressed in embryonic tissue which is enclosed within ovule walls (seed coats) and ovary walls (pericarp) 5

6 Xenia in Pea F1 seed all seed on female in the cross will have yellow seed (since seed color is an embryonic trait, and yellow is dominant to green) but pods are maternal tissue and not affected by the pollen genotype P1 male purple flower yellow seeds yellow pod P1 female white flower green seeds green pod true breeding (all progeny from self pollination identical to mother plant) F1 plant purple flower only green pod only Slide 6 of Chapter 1 segregation for xenia traits can be viewed among seeds within pods of F1 plants. segregation for non-xenia traits only manifest on plants derived from the seed on the F1 plants (i.e., F2 seed). 6

7 Gregor Mendel: context and tools Background: the historical puzzle of inheritance Genetics involves: careful observation of populations over generations analysis of carefully acquired data on individuals development and testing of theoretical frameworks Mendel was the first person to put these three elements together to form a theory of inheritance a great deal of observation took place in the time before Mendel -huge efforts at cataloging species was conducted (Linnaeus, Darwin) -cross fertilization within and among species was induced and observed Artificial selection was the first applied genetic practice plants and animals were domesticated through selection of individuals that exhibited favorable traits (e.g., nonaggressiveness in the progenitors of domesticated dogs, horses, cattle etc.; longer seed retention in cereals, etc.) The puzzle of passing on desirable traits by Mendel s birth (1822), plant and animal breeders were able to generate individuals with new and valued combinations of traits by controlled matings but the value of these progeny as parents was unpredictable (i.e., the value of an individuals own progeny was unpredictable) Sheep breeding was a particularly important activity in Moravia, and at a 1837 conference of the Moravian Sheep Breeders Society, the Abbott Cyril Napp proposed that breeders needed to discover three things: what is inherited how is it inherited, and what is the role of chance in inheritance? before Mendel, blending, preformation (e.g., the idea of a homunculus) and other erroneous theories were proposed to explain the contradiction between 1) the obvious reality that members of a species had progeny who were of the same species; and 2) the equally obvious reality that variation of individual phenotype (appearance) exists within and among families Mendel s work laid the foundation for our current view of heredity Key features of his approach include: focus on a single species, the naturally self-pollinated but easily cross-pollinated annual pea (Pisum sativum) focus on traits that had distinct, mutually exclusive ( antagoni -furthermore- Mendel worked with only two forms of each of the seven traits he studied selection and propagation of pure-breeding lines which upon self-pollination produce progeny that are collectively identical in form to their parents and to each other. Pure-breeding lines in a self-pollinated crop can be maintained in a pure state throughout multiple generations simply by isolating their progeny from other lines. careful control of matings between plants of different pure-lines, with attention to which parent was used as the female. generation of large datasets consisting of observations on individual progeny plants -note that the word plant here is inclusive of the embryo within a seed. comparison of such results with predictions based on hypothetical models Slide 7 of Chapter 1 Fig. 1.6 from Hartwell et al 7

8 Creating a monohybrid P1 male yellow seeds P1 female green seeds F1 seed all seed on female in the cross will have yellow seed (since seed color is an embryonic trait, and yellow is dominant to green) true breeding (all progeny from self pollination identical to mother plant) Slide 8 of Chapter 1 Genetic Analysis According to Mendel The embryos (i.e., progeny plants) that develop within the seeds of a plant used as a female in a cross (or hybridization) are deemed an F1 generation if the parents differed in at least one trait. analyses of the progeny of plants grown from the F1 seed was the basis of Mendel s work Mendel began his work by focusing on one trait at a time analysis of the frequencies of alternate forms of a single trait in the P, F1, subsequent generations of one cross is called Monohybrid analyses in reality, the parents used in Mendel s monohybrid crosses must have differed for other traits. the key is that Mendel ignored the other traits and focused on only one at a time (at first) 8

9 P, F1, and F2 generations Slide 9 of Chapter 1 Figure 1.9: Monohybrid analysis of seed color in pea (an embryonic trait) Mendel took pollen from a yellow seeded pure breeding line and placed it on stigmas of emasculated flowers of plants from a green seeded pure breeding line The F1 seeds (which matured in the pods on the female parent), were all yellow the reciprocal cross was also made (using the yellow seeded pure breeding line as the male parent) again, all of the F1 seeds were yellow Mendel planted the F1 seeds and allowed them to self pollinate and develop mature seed. The F2 seeds (which matured on the F1 plants) included both yellow and green forms of the seed color trait, in a ratio of approximately 3 yellow for every 1 green seed. These results refuted the concept of blending, since the green form of the trait in the P generation reappeared in the F2 generation after disappearing in the F1 generation. Mendel observed that there are two kinds of behavior of yellow pea derived plants: some breed true (have only yellow seed progeny upon self pollination) some generate both yellow and green seed upon self pollination Mendel hypothesized that 1) for each trait, each plant carries two discrete copies of a unit of inheritance (which we now call genes) 2) these genes come in two forms, which we now call alleles 3) one allele is dominant and the other is recessive -the yellow allele of the seed color gene in pea is dominant to the green allele of the same gene. The P generation plants in Fig. 1.9 either carry two dominant alleles (the yellow parent), or two recessive alleles (the green parent) F1 generation seeds in Figure 1.9 carry one yellow and one green allele. end end 9

10 The Law of Segregation Slide 10 of Chapter 1 Figure 1.10: The law of segregation is two part 1) gametes (sperm and eggs) contain only one allele of a given gene; -pure breeding yellow peas form gametes that all have the yellow allele, and -pure breeding green peas form gametes that all have the green allele -F1 generation plants form both kinds of gametes in equal quantities ( ½ with the yellow allele, and ½ with the green allele). -in other words, both alleles can reside in and pass through an F1 generation without any change in their fundamental behavior -so alleles are SEGREGATED from each other in gamete formation 2) Individual progeny are created from the random union of one male gamete and one female gamete. 10

11 Self pollination of the F1 Generation Slide 11 of Chapter 1 gametes have only one allele (Y or y) the two alleles of an F1 appear in equal frequencies in that F1 s gametes since the frequency of Y equals the frequency of y in both eggs and pollen of an F1, the probability of a random male gamete carrying Y = 0.5, which equals the probability of it carrying the y allele gametic union in self fertilization is random, leading to four possible ways to form a zygote: (egg-derived written first): Yy, YY, yy, and yy each of those outcomes has a probability of ¼, by two lines of reasoning: none of the four is more likely than the other, so they are equally probable the Law of the Product states that the probability of two (or more) independent events occurring simultaneously ( or sequentially) equals the product of the probabilities of the individual events. 11

12 The outcomes of two events are independent if they satisfy two constraints: 1) the outcome of one event doesn t influence the outcome of the other event; and 2) they can occur together (i.e., they are not mutually exclusive) examples: two consecutive coin tosses- one coin doesn t influence the other; and it s possible to do a Y egg combining with a y sperm- the probability of the egg being Y is not affected by the identity of the sperm; and zygotes always derive from one sperm and one egg Slide 12 of Chapter 1 12

13 Law of the Sum Slide 13 of Chapter 1 events are in the eye of the beholder possible events in the context of genetics: the genotype of a gamete randomly selected from one plant the phenotype of a plant randomly selected from a population of plants a zygote is formed by union of an egg carrying Y and a sperm c a zygote is formed by union of an egg carrying Y and a sperm c 13

14 just one way to form YY or yy two mutually exclusive ways to achieve Yy Slide 14 of Chapter 1 14

15 Frequencies and ratios Frequencies are used in genetics to represent the proportion that one class of individuals represents of a whole population. A list of the frequencies of all classes is a frequency distribution. A complete frequency distribution must sum to 1.0. We often speak of ratios in genetics. Frequency distributions are converted to ratios by using the numerator of the fractional frequencies after application of a common denominator. Slide 15 of Chapter 1 15

16 Frequencies and corresponding ratios: examples Below are examples of frequency distributions and their corresponding ratios Frequency distribution Ratio 0.75 yellow, 0.25 green 3 yellow : 1 green 0.25 YY, 0.5Yy, 0.25 yy 1 YY : 2 Yy : 1 yy 0.8 tall, 0.2 short 4 tall : 1 short Slide 16 of Chapter 1 16

17 Inter-converting frequencies and ratios Assume you have 25 blue, 50 green, and 25 black marbles for a total of 100 marbles. The Frequency distribution of marble colors is 1/4 blue : 1/2 green : 1/4 black. The ratio of marble colors is 1 : 2 : 1 (blue/green/black) To convert from fractional frequencies to ratios, apply a common denominator to each frequency, (in this case the common denominator is 4, so the freq. dist. becomes 1/4 blue : 2/4 green : 1/4 black). Then use the numerators to represent the ratios: 1 blue : 2 green : 1 black. To convert a ratio to frequencies, sum each of the values in the ratio expression (in this case 1, 2 and 1, which sums to 4) ; and use that as the denominator under each of the ratio values. So a ratio of 1 : 2 :1 translates into a frequency distribution of 1/4 : 2/4 : 1/4. Slide 17 of Chapter 1 17

18 Ratios and Frequencies of a subset of classes Ratios among classes remain constant even if one or more classes is discarded If we discard all of the black marbles from the slide above, the ratio of blue to green marbles is unchanged and remains: 1 blue : 2 green But the frequencies in an original population do not equate with the correct frequencies in a subset of classes. We had 1/4 blue and 1/2 green when we had the black marbles included, but that cannot be a complete frequency distribution since it does not sum to 1.0. To derive the correct frequencies of a subset of classes, first write down the ratios (1 : 2), and then convert that to a frequency distribution. The sum of ratio values is 1+2=3, so using 3 as the denominator, the ratio of 1 : 2 converts to a frequency distribution of 1/3 and 2/3. Slide 18 of Chapter 1 18

19 Frequencies and Ratios: an application (1 of 2) ¼ ¼ ¼ ¼ 1/4 YY 2/4 Yy 1/4 yy Ratio of homozygous to heterozygous Yellow F2 Progeny: 1 YY : 2 Yy; Which is equivalent to a frequency distribution of 1/3 YY and 2/3 Yy Slide 19 of Chapter 1 From fig. 1.12, Hartwell When we look only at the yellow progeny we are If you know the ratio of the occurrences of two events (for instance 1 YY for every 2 Yy), you can convert the ratio into frequency distribution (frequency of each type of event- 1/4, 1/2, etc.) by summing the values in the ratio ( in this case 2+1) and using that sum as a denominator for each events ratio value. 19

20 Frequencies and Ratios: an application (2 of 2) Yellow F2 s YY 1/3 YY Yy 2/3 Yy Self pollination F3 1/3 * [1/1 YY] =4/12 YY 2/3 * [1/4 YY 2/4 Yy 1/4 yy] = 2/12 YY 4/12 Yy 2/12 yy Yellow F3 s Ratio 6(=4+2) YY : 4 Yy :2 yy Ratio 6 YY : 4 Yy Frequencies 6/10 YY, 4/10 Yy; or 3/5 YY, 2/5 Yy Slide 20 of Chapter 1 Why multiply the frequencies of genotypes derived from Yy by 2/3? Because two independent events are required for a given genotype to occur in the progeny of a Yy F2 plant. first, the F2 had to be Yy. That probability is 2/3 second, the probability of, say, YY is 1/4 in the progeny of a Yy F2 Since these two events are independent, we multiply the two probabilities: 2/3 * 1/4 = 2/12 20

21 Some Definitions Slide 21 of Chapter 1 Phenotype- appearance Genotype- Alleles present in cells of a plant for one or more genes Homozygous- both alleles of a gene are identical Heterozygous- the two alleles are different 21

22 Slide 22 of Chapter 1 22