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Gregor MendelVersuche über Pflanzenhybriden

The First Law

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The starting material

“In all, 34 more or less distinct varieties of Peaswere obtained from several seedsmen andsubjected to a two year's trial. All the …varieties yielded perfectly constant and similaroffspring; at any rate, no essential differencewas observed during two trial years. Forfertilization 22 of these were selected andcultivated during the whole period of theexperiments. They remained constant withoutany exception.”

http://www.mendelweb.org/CollText/homepage.html

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“Pure-breeding line”An awkward phrase that is best

retired, but never will be.It refers to an organism that

exhibits a particular trait (e.g.,seed color), and all progeny ofthat organism (whether it isselfed, or outcrossed to anothersuch organism) also exhibit thattrait.

Pure-breeding lines are best madeby selfing, or brother-sistercrosses (like Nefertiti).

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“An SCN9A channelopathy causescongenital inability to experience pain”

Nature Dec. 14, 2006“The index case for thepresent study was a ten-year-old child, well knownto the medical service afterregularly performing 'streettheatre'. He placed knivesthrough his arms andwalked on burning coals,but experienced no pain.He died before being seenon his fourteenth birthday,after jumping off a houseroof.”

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William Ernest Castle – founder ofmouse genetics (UCB 1936-1962)

1. Inbreeding as a tool for making geneticallyuniform strains of mice that are homozygous forevery allele in the genome.2. Brother-sister matings – makes 12.5% of all lociin the genome homozygous (Clarence Little).

Why? – homework!!After 40 generations of brother-sister mating,>99.98% of genome is homozygous. By F60, miceare considered genetically identical to one another.

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Mendel picked the pea as the system.What traits to pick?

“Experiments which in previous years were madewith ornamental plants have already providedevidence that the hybrids, as a rule, are notexactly intermediate between the parentalspecies. With some of the more strikingcharacters, those, for instance, which relate to theform and size of the leaves, the pubescence ofthe several parts, etc., the intermediate, indeed, isnearly always to be seen; in other cases,however, one of the two parental characters isso preponderant that it is difficult, or quiteimpossible, to detect the other in the hybrid.”

http://www.mendelweb.org/CollText/homepage.html

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The reaffirmation of a knownphenomenon

Mendel is pointing out the distinction between two “types”of traits.

1. The hybrid plant is “intermediate” in phenotypebetween two parents. For instance, the offspring of atall and a short plant would be intermediate in height.

2. The hybrid plant has the phenotype like one of theparents. For instance a green x yellow cross yieldsonly yellow-seeded plants.

Mendel chose to study “type 2 traits” – a judiciousdecision. We now know that the laws he discovered indoing so also apply to “type 1” traits, but that fact isconsiderably more difficult to observe.

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yellow green

Qualitative (“simple”) trait

aa AA

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short tall

Quantitative (“complex”) trait

AA aa

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Not Mendel’s faultIn retrospect, we see that the overwhelming majority of traits in humans, other

animals, and plants – traits that are of most interest and importance from apublic health, and other societally relevant perspectives (height, weight,body plan, facial appearance, skin color) – are quantitative.

Mendel – wisely – chose to study a set of “qualitative” traits because he was askilled reductionist. As a consequence, he discovered two fundamentalfacts about the functioning of the genetic material. The teaching ofgenetics, however, always begins with Mendel’s work, and this createstwo erroneous impressions:

1. … that the traits he studied are “controlled by a single gene.” That, ofcourse, is not true (all traits are controlled by multiple genes) – he simplyworked with plants that were genetically different from each at only onelocus out of the many required for the development of the trait. We shallreturn to that point shortly.

2. … that simple Mendelian relationships of recessivity and dominancebetween alleles, and “one gene-one trait” correlations he – supposedly –observed are ubiquitous in Nature. “She has her mother’s eyes.” “He getshis brains from his Dad.” Neither trait – eye color nor “intelligence” – exhibitsimple Mendelian inheritance, yet most people assume otherwise.

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The genesis of the famous term“… in other cases, however, one of the two parental characters is so

preponderant that it is difficult, or quite impossible, to detect the other inthe hybrid.

This is precisely the case with the Pea hybrids. In the case of each of the7 crosses the hybrid-character resembles that of one of the parental forms soclosely that the other either escapes observation completely or cannot bedetected with certainty. This circumstance is of great importance in thedetermination and classification of the forms under which the offspring of thehybrids appear. Henceforth in this paper those characters which aretransmitted entire, or almost unchanged in the hybridization, and therefore inthemselves constitute the characters of the hybrid, are termed the dominant,and those which become latent in the process recessive. The expression"recessive" has been chosen because the characters thereby designatedrecede or entirely disappear in the hybrids, but nevertheless reappearunchanged in their progeny, as will be demonstrated later on.”

“In der weiteren Besprechung werden jene Merkmale, welche ganz oderfast unverändert in die Hybride-Verbindung übergehen, somit selbst dieHybriden-Merkmale repräsentiren, als dominirende, und jene, welche in derVerbindung latent werden, als recessive bezeichnet. Der Ausdruck "recessiv"wurde desshalb gewählt, weil die damit benannten Merkmale an den Hybridenzurücktreten oder ganz verschwinden, jedoch unter den Nachkommenderselben, wie später gezeigt wird, wieder unverändert zum Vorscheinekommen.”

http://www.mendelweb.org/CollText/homepage.html

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Inverting the direction of the crossdoes not alter the phenotype of the

hybrid“All experiments proved further that it is entirely

immaterial whether the dominating trait belongsto the seed plant or to the pollen plant; the formof the hybrid remains identical in both cases.”

Recall Leeuwenhoek’s “proof” using grey rabbitsthat sperm provides all the genetic material, andthe egg solely provides nourishment.

Eeeh, what’s up with THAT, doc?

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What to do with the hybrid (i.e., theF1 plants)?

Mendel decided to let them self (which Pisumsativum does naturally, thank you very much).He then grew the progeny (the F2) and didprecisely what he promised: he counted thenumber of phenotypic classes in this F2, andmeasured the ratio.

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The dataExpt. 1. Form of seed. -- From 253 hybrids 7324 seeds were obtained in the second trial

year. Among them were 5474 round or roundish ones and 1850 angular wrinkledones. Therefrom the ratio 2.96:1 is deduced.

Expt. 2. Color of albumen. -- 258 plants yielded 8023 seeds, 6022 yellow, and 2001green; their ratio, therefore, is as 3.01:1.

Expt. 3. Color of the seed-coats. -- Among 929 plants, 705 bore violet-red flowers andgray-brown seed-coats; 224 had white flowers and white seed-coats, giving theproportion 3.15:1.

Expt. 4. Form of pods. -- Of 1181 plants, 882 had them simply inflated, and in 299 theywere constricted. Resulting ratio, 2.95:1.

Expt. 5. Color of the unripe pods. -- The number of trial plants was 580, of which 428 hadgreen pods and 152 yellow ones. Consequently these stand in the ratio of 2.82:1.

Expt. 6. Position of flowers. -- Among 858 cases 651 had inflorescences axial and 207terminal. Ratio, 3.14:1.

Expt. 7. Length of stem. -- Out of 1064 plants, in 787 cases the stem was long, and in277 short. Hence a mutual ratio of 2.84:1. In this experiment the dwarfed plants werecarefully lifted and transferred to a special bed. This precaution was necessary, asotherwise they would have perished through being overgrown by their tall relatives.Even in their quite young state they can be easily picked out by their compact growthand thick dark-green foliage.

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A bit of metaanalysis“In this generation there reappear, together with the

dominant characters, also the recessive ones with theirpeculiarities fully developed, and this occurs in thedefinitely expressed average proportion of 3:1, so thatamong each 4 plants of this generation 3 display thedominant character and one the recessive. This relateswithout exception to all the characters which wereinvestigated in the experiments. The angular wrinkledform of the seed, the green color of the albumen, thewhile color of the seed-coats and the flowers, theconstrictions of the pods, the yellow color of the unripepod, of the stalk, of the calyx, and of the leaf venation,the umbel-like form of the inflorescence, and thedwarfed stem, all reappear in the numerical proportiongiven, without any essential alteration. Transitionalforms were not observed in any experiment.”

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Brilliant in Brünn“The dominant character canhave here double meaning;namely, that of a parentalcharacter, or a hybrid-character.In which of the two meanings itappears in each separate casecan only be determined by thefollowing generation. As aparental character it must passover unchanged to the whole ofthe offspring; as a hybrid-character, on the other hand, itmust maintain the samebehavior as in the firstgeneration.”

Mendel’s papers were burnedshortly after his death, and wedon’t have access to his labnotebooks.

We shall therefore never know,whether this was brilliantforesight, or whether hegenerated the F3, and then letthe data tell him, what’s goingon.

He says here that an F2 plantthat looks like one of theparents can, in fact, behomozygous for the dominantallele, OR heterozygous.

He went ahead to prove that, bygenerating that F3.

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Analysis of the grandchildren

Fig. 2.12MCB140, 29-08-07 20

“Those forms which in the first generation exhibitthe recessive character do not further vary inthe second generation as regards thischaracter; they remain constant in theiroffspring.

It is otherwise with those which possess thedominant character in the first generation. Ofthese two-thirds yield offspring which displaythe dominant and recessive characters in theproportion of 3:1, and thereby show exactly thesame ratio as the hybrid forms, while only one-third remains with the dominant characterconstant.”

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Mendel, humble son of a peasant family,pollinating textbooks all over the world for

the rest of time with his nomenclature“The ratio of 3:1, in accordance with which the

distribution of the dominant and recessivecharacters results in the first generation,resolves itself therefore in all experiments intothe ratio of 2:1:1. …

If A be taken as denoting one of the two constantcharacters, for instance the dominant, a therecessive, and Aa the hybrid form in which bothare conjoined, the expression

A + 2Aa + ashows the terms in the series for the progeny of

the hybrids of two differentiating characters.”

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And now (drum roll) – the first law

“Experimentally, therefore,the theory is confirmed thatthe pea hybrids form eggand pollen cells which, intheir constitution, representin equal numbers allconstant forms which resultfrom the combination of thecharacters united infertilization. “

“Es ist daher auch aufexperimentellem Wege dieAnnahme gerechtfertigt,dass die Erbsen-HybridenKeim- und Pollenzellenbilden, welche ihrerBeschaffenheit nach ingleicher Anzahl allenconstanten Formenentsprechen, welche ausder Combinirung der durchBefruchtung vereinigtenMerkmale hervorgehen.”

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“I always feared I would die young of stomachcancer, as my mother had, and the fear worsenedafter my three children were born. When Iconsider that each of our children has a 50%chance of having this mutation, I know they atleast have the same option I did, and I hope toshow them what a livable solution it is.”

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MCB140, 29-08-07 25Fig. 2.22

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Brilliant in Brünn, part II“With Pisum it was shown by experiment that the hybrids form egg and pollen cells of different kinds,

and that herein lies the reason of the variability of their offspring.If it chance that an egg cell unites with a dissimilar pollen cell, we must then assume that between

those elements of both cells, which determine opposite characters some sort of compromise iseffected. The resulting compound cell becomes the foundation of the hybrid organism thedevelopment of which necessarily follows a different scheme from that obtaining in each of thetwo original species.

With regard to those hybrids whose progeny is variable we may perhaps assume that between thedifferentiating elements of the egg and pollen cells there also occurs a compromise, in so far thatthe formation of a cell as the foundation of the hybrid becomes possible; but, nevertheless, thearrangement between the conflicting elements is only temporary and does not endurethroughout the life of the hybrid plant. Since in the habit of the plant no changes areperceptible during the whole period of vegetation, we must further assume that it is only possiblefor the differentiating elements to liberate themselves from the enforced union when the fertilizingcells are developed. In the formation of these cells all existing elements participate in an entirelyfree and equal arrangement, by which it is only the differentiating ones which mutually separatethemselves. In this way the production would be rendered possible of as many sorts of egg andpollen cells as there are combinations possible of the formative elements.”

This is stunning in a way that the history of biology has not seen before. What Mendel says is thefollowing:

An organism that happens to be heterozygous for a locus carries two distinct alleles of that gene. Thetwo alleles do not change each other’s nature – for the time while they are stuck in the samenucleus. Then, when their carrier – the organism – makes gametes, the two alleles becomeseparated again.

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“Brilliant” isn’t strong enoughThis is “swish central”

Nothing but net.

Mendel’s data showed – to him – that in a heterozygote,the two alleles – A and a – remain DISTINCT andSEPARATE.

They reach a compromise for the life of the plant, but then,during gametogenesis, they go their separate ways,unchanged.

To describe this incredibly simple idea as influential wouldbe akin to calling Michael Jordan a “pretty goodshooting guard.”

Sadly, this idea sat on the bench for the entire 1865-1900season. More on why that happened – shortly.

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A useful term

If a trait follows in its inheritance Mendel’sfirst law (i.e., Mendelian ratios ofphenotypes are observed in pedigrees),that phenomenon is described as “simpleMendelian inheritance” (SMI).

Examples: cystic fibrosis; sickle cellanemia; hemophilia A.

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An awful, awful term:“monogenic trait”

If a trait follows SMI – what does that sayabout its genetic architecture?

A highly pernicious school of lack of thoughtin biological instruction uses the term“monogenic trait.” It is most unfortunate.

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X

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In what sense is blood clotting a“monogenic” trait?

In no sense at all.What is “simple” is the genetic DIFFERENCE between an unaffected

individual and an individual who has hemophilia. It can be as smallas a single base pair change.

In other words, what the phenotype tracking allows us to do is look atthe genetics of the difference between the genotypes of theorganisms with respect to the trait under study.

If a particular phenotype follows SMI, then all that says is:With respect to the trait under study, the difference in genotype

between an organism with that phenotype and without it is due toa genetic difference at a single position in the genome (=“a singlelocus”).

Nothing can be learned from this analysis about the number of genesthat are required for that phenotype to develop, or about the rolethis particular gene plays in having this phenotype develop.

PKU, for example, has to do with the breakdown of aromatic aminoacids, and its lack causes mental retardation. What is the specificrole of phenylalanine breakdown in cognition?

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Koopman et al. (1991) Nature 351: 117.

XY XX+Sry transgene

Manhood – a monogenic traitNote: SRY directly causes the conversion of a female embryo to a male one

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Motherhood – a monogenic trait?!

A Defect in Nurturing in Mice Lackingthe Immediate Early Gene fosBBrown et al.Cell, Vol. 86, 297–309, July, 1996

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In other words“Mendelian inheritance” of traits (note: OF TRAITS) is largely the exception,

not the rule in Nature.Mendel made two titanic contributions to science:1. From an epistemological perspective, he created a fundamentally novel,

enormously powerful experimental paradigm, one that replaces allprevious heuristic, trial-and-error-based efforts in this field with amethodical, hypothesis-driven, rigorous quantitation-based approach, inwhich one sets up controlled crosses, determines the number ofphenotypic classes that results from those crosses, analyses theirincidence in each generation separately, and determines the ratios oforganisms that fall into each class. This – to the letter – remains the coreof the genetic research paradigm to this day. It is astonishingly powerful,as you shall see.

2. From an ontological perspective, he discovered two of the three mostimportant principles of the behavior of genes (the equal segregation intogametes of two alleles of a gene during gametogenesis; and theindependence of the behavior of two distinct genes in this process). Thethird – linkage – was discovered in 1906 by Bateson and Punnett, and wewill discuss it at great length shortly. All autosomal genes obeyMendel’s first law, and all unlinked genes obey Mendel’s second law.

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One more thingMendel was the first to realize that heredity can be studied

quite successfully without any understanding of themolecular mechanisms that underlie genetic processes.

As you will see in Prof. Garriga’s section and in Prof.Brem’s section, this allows geneticists to study andunderstand highly complex phenomena (e.g., embryonicdevelopment; gene regulation; the control of celldivision) without significant (or, frequently, any) regardto the physicochemical processes that underlie thosephenomena.

More on that in parts II and III of this class.

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“What are the genes? What is the nature of theelements of heredity that Mendel postulated aspurely theoretical units? … Frankly, these arequestions with which the working geneticist hasnot much concern himself…

If the gene is a material unit, it is a piece of achromosome; if it is a fictitious unit, it must bereferred to a definite location in a chromosome.… Therefore, it makes no difference in theactual work in genetics which point of view istaken.”

T.H. MorganThe Relation of Geneticsto Physiology and MedicineNobel Lecture, June 4, 1934

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Next time

1. Mendel’s 2nd law2. The danger of respecting authority3. Enter the chromosome