DOCUMENT RESUME ED 190 357 Sr 031 301 Bonar, John P., Ed .

called dominant and indicated with n capital letter (for example, B); he usedthe matching letter in lower case (for example, b) for thc masked feature,whiCh he termed recessive. He explained how hidden features were transmittedusing tbe concept that each individual contains a pair 'of thesc factors (twobits) for a given feature, and that either of the two bits of information maybe independently distributed to the offspring.Although Mendel began witil erosses involving only onc feature at a time,he did not' limit his studies to such crosses. Gradually Mendel produced plantsthat SVC/r pure smith -Thr Iwo fawn. -E3tafriples were gThooth, yellow andwrinkled, green seeds, described in Excursion 6-2.When plants that were pure strain for the seed features just described werecrossed, all the .first-generation seeds were smooth and yellow. Mendel thenplanted thc seeds and allowed the resulting plants to pollinate themselves.At this point there was a problem in predicting the features of the offspring.Did the features of seed color and seed shape always remain together, or were,they assorted independently in thc two gametes? Here Mendel called on abranch of mathematics for help.Probability is the branch of matkemativhat deals with the prediction ofchance events. Some knowledge of probability is yery helpful in understandinginheritance. Probability is measured on a scale that runs from 0 (impossibility)to I (certainty). For example, the probability of a coin's being flipped andcoming up heads is .5 (or ij. That is, we cxpect about half of a number ofcoin tosses to come up heads. Thc probability of tails is also i, Similarly, thetprobability of a die's turning up 3 is iOf course, several events may occur at the same time. For example, wemay flip two pennies together. What is the probability of both pennies comingTtTtTtTtPossiblespermPossibleeggs.,Figure 1TtTTItTtttFigure 2T44up tails on the same throw? In such a case, the probability of the doubleevent is the product of the separate probabilities of the single events. Therefore, because the probability of tails on each coin is i, the probability ottailson both coins is i x i, or 4.PossiblespermttPossible Teggs.One of Mendel's important findings was thc independent assortment of twoor more sets of bits Thus, the ideas of probabili 'ust discussed- apply togenetic combinations just as they do to tosses of coi s or dice.The "Punneu square"-iS a convenient way of predicting the features andprobabilities of offspring from various crosses. In this.pethod the bits fromeach parent are listed on the sites of a square or rectangle. The possible unionsof the various types of bits are shown in the boxes of the square.Figure, 1 illustrates the use of this method with the cross of a single traittallness. The sierm are pure strain for dwarfness (tt) and the eggs are purestrain for tallness (TT).The first generation shows a probability of 4 It in each square. This isbecause i of the bits for each parent arc t and T. 3 x 3 is 4 for each offspring.Of course, all four squares arc alike, -sb the. probability of a Tt offspring is4 x 4, oi- 1.Figure 2 shows the cross of two first-generation offspring, each Tt. Eachpareni is thus i 1-4ind i t.14.,i,,,.; .iv,?es,,,-4',:,-, .-., q.,.3, I.'. ' ,1i-kari. !NI;;W.'s::r.),'''',3 1'10," ' ' t-1. 1.0:',Y 4 , ' i44'; - 100i N 3 i''''1 t-\ .' ' ''"." .-c.d i.a.'.S':,Wt'''''*:. .4' '4) . #.' 46,4 .,z.,. , \ -,,,, ,-;," --.- .io. .404 Iv,*t'4.; \.I4404106.- itX,.iHtitictiM1Y;tilot*ht2.42,4itS; gi.} 4.3,.-,.!,",::,,.'f.''. ----.et;

The second generation shows a 1 probability for TT (/ T x T) and 1probability for tt. There aie two ways that Tt can result, so the prdbabilityis 2 x X 4, or 4. This means that the probabilitieS are for I out of 4 tobe TT, 1 out of 4 t be tt, and 2 out of 4 (or 1 out of 2) to be.Tt. The "bit"makeup (called genotype) of a IT. tt, or Tt is different But in appearance(caned phenotype) the TT and Tt are le same,use of the masking ofthe recessive trait, t, by the dominant tr it, T So.ph notypically, the probability is for 3 to be tall r n d I to be dwarf.When Mendel followed two features at the sa-mt time, fis in Excursion 6-2,tre-tbundprobabilities worked-out in a manner siMilar tothat for a single feature.The parents of the first generation were pure strain for smooth, yellow peas(SSYY) and wfinkle'd, green peas (ssyy). In Figure 3, two of the first generation were crossed (remember, all first generation were the. samesmooth,yellow seeds, SaYy).4Possible spermSYSysYsySSYYSSYySsYYSs ssyySYPossibleS99sSY,,Figure 3'1The probability of each bit (S, s, V. or y) iS. Therefore, the probabilityof any one square is x or A. When the phenotypes of all the squaresare totaled, we see that there are 9 cha!nces in 16 for smooth, yellow seeds,3 in 16 for smooth, green. 3 in 16 for wrinkled, yellow, and Igreen peas.6 for wrinkled,Modern views show that Mendel's conclusions were basically correct, buttoo simple to explain all observations. For example, crossing-over may complicate application of the simple two-bit model. The bits of information fromthe parent for one feature may be exchanged with those for another feature.Or one bit may not be dominant over another, but instead the two differenttraits %ay, blend. The morning glory example in Chapter 7 and Excursion7-1 illstrAQ this. Alio, research since Mendel's time indicates that there nutybe more than two bits for any given feature.GENETICS SINCE MENDELA greal deal of research has been done and countless books written on thesubject since Mendel',F time. Much of the terminology has changed.01n thesttident text, however,ierriiinology has been kept to a minimintt. You probably-. s;.,.ettAt:*:.etwit4.t.4tsriT5

-4.11tV, ,4yiaauliii(d rcmind yourself that this is not a course in genetics. Thc text shouldbc allowed to carry the student &rough the, development of a simple modelfor inheritance. FuriOntentally, Mis is the two-bit model proposed by Mendel.e Some teachers may Iptve a definite orientation and extensiv training ingenetics. Others may have varying amounts of expertise in the subject. There- may bc some whO would welcome an introduction to the modern terminologyand concepts:not for the student's use, but for their own usc.The twentieth century has seen the development of the concept of chromo,shosomes and genes. Chromosomes, soMetimes referred to aS the '. threads of life,'T I.are located in the cell nucleuS. Their number varies with different plant andanimal species. They are paired, and in a. process called mitosis, each ncwcell that results from cell division receives a complete complement of chromosomes, called the diploid number. However, in the process of formation ofgametes, or reproductive cells, he chroMosome number is halved, with eachgamete receiving only one of ach pair, called the monoploid numbcr. Thisprocess is called meiosis. Whet the male and female gametes (sperm and egg)unite, the chromosomes pair up again to the. full number.Genes, which correspond to the factors or bits of information-in the Mendelian model, are located on the chromosomes. If two genes affect the samecharacteristic of an organism in contrasting ways, and ila gamete can containeither one but not both of the genes, then the genes are said to be the allelesof each other. 'Alleles are situated at the same location (locus) on the sametype of chromosome.Gene names are usually abbreviated, commonly with a letter of the alphabet. If the gene seems to mask its allele in the first generation, it is spokenof as yominant. The masked allele is recessive to the dominant gene. Byconvefition, the dominant gene is indicated with a capital (B), while the recessive is expressed by the same letter in lower case (b).It is not always possible to tell from the appearance of an organism whatits genes are. Therefore, it is-often helpfulto distinguish between the pheno.type of ap organism (its appearance) and its, genotype (its genetic makeup).In general, BB and Bb organisms would be phenotypically indistinguishable,,.,.hut woule be geneti9lly different.If bot genes of a pair are the same allele (BB or bb), the organism issaid to be homozygou for that feature. If the paired genes are different (Bb);the organism is described as heterozygous for the feature. Hybrid is anotherterm for heterozygous, and pure strain represents the homozygous condition.Because the genotype of an individual cannot always be determined by astudy of its phenotype, thl test cross (sometimes called backcross) is oftenhelpful. In a,test Cross, thv individual is crossed with another individual thatis homozygously recessive for the feature that is .of interest.The nature ofthe offspring allows one to infer whetherthe original individual was homozygous or heterozygous for the feature.Mendel did not postulate a connection between his bits of information andsex determinatiOn. Nor did he investigate certain characteristics that are linkedwith the sex ofj)te individual. Normally the members of a pair of chromosomes are .similar in appearance: HoweVer, those in one pair, the X- and jj4;} "krZirk:,

,J.:S.or sex, chromosomes, arc easily distinguishable. In humans an XX indiVidualis a female, while a male is XY, In other organimns this arrangement maybe reversed. Various abnormalities may be associated with unusual arrange-As.NOments of sex chromosomes (XXY. XYY, etc.)X bb ? orIn most cases it makes no ditkrence whether the mating is, Bbbb X Bb ?. However, there arc genes that do,not Fehaye identically inreciprocal mathtgs. Among these are the sex-linked gerkes.-Sex-linked genes are located on the X-chromosome. Like most other chro.mosomes, the X-chromosome is made up of many genes. However, the Ychromosome seems to contain feW if any genes.ifi-us,bylts-coturterparrirtthe same locus on the other X-chromosome. However, because the Y-chromosome is a genetic blank, there is no allele to interact with the sex-linkedgene on the X-chromosome.Several human disorders are known to be caused by sex-linked genes.Red-green color-blindness and hemophilia are two comnion examples. Bothof these distrders are much more frequent in males than they are in females.For example, in the United States the frequency of red-green color-blind menis about I in 41. The comparable figure for women is in 144.Because A of United States males are red-sreen color-blind, we can assuTethat about III of the X-chromosomes bear the gene. For a female to be colorblind, she must Rave two X-chromosomes with genes for color-blindness. (Ifshe is heterozygous for the gene, she is merely a carrier.) Thus, for womenof -6 (or Th) will have X-chromosomes bearing the recessive gene for colorIblindness.Another thing that Mendel did not investigate was the details of the effectof environment upon the expression of inherited features. We now know thatan individual's gsnes determine what he.may become, but it is the interactionbetween genes And environment that determines what he does become.AN OVERVIEWThe,study of heredity, especially as it relates to human features, holds afascinati

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