.. ..... CONCEPTUAL LIFE SCIENCE GeDetia INTRODUcnON Living organisms resemble their parents. This is due to the 1raDsmission oftraits or genetic chtzracters from one generation to the next. We know from OlD' study of reproduction that sexual reproduction involves receiving genetic material from each parent. In humans, halfofYOlD' genetic material comes from YOlD' mother and halffrom YOlD' father. We know that the genetic material is DNA. The DNA is orpni%ai into genes that are located on the chromosomes in thenucleusofthecell. Historically,theundelStaDding ofthetransmission ofthe 1raits came before the undemanding ofthe mechanism ofthe tnmsmission. Gregor Mendel worked on traits with his peas in the mid-19th century. The cluomosome was Dot discovered until the 18905. Once biologists put the traits and the chromosomes together, we began to understandthe science ofgenetics. CLASSICAL MENDELIAN INHERITANCE Gregor Mendel was tbe abbot ofa Catholic monastery in amtra1 Europe in the 19th century. One ofbis duties in the monastery involved tending the garden where he became interested in the traits ofthe pea plants. Table 19-1 lists some ofthe characters ofbispeas. . Table 19-1. Characters in Mendel's gardenneas. Cbaracter Flowers -color A Red or white Flowers -location Axial or terminal Pods-color GreeD or Yellow Pods -structure lDf1ated or constricted Seeds-Seeds -color Round or wrinkled Green or yellow PIIIDtS -heil!bt Tall or short Some notes about Mendel's peas. In Mendel's day there was much more variation in plBDts that are used for food than you find today. Go to a store where they sell prden seeds ad see what they have to offer. Yau will probably be able to buy seeds for peas that give round, green peas; in inflated, green pods; with white flowers that 1ft termiDaI (on the ends oftbe branches). There are yellow string beans, the so-called '\ax beaDs." People generally expect that peas are green and are found in P=lpods. Mendel also had yellow peas and yellow pods. Just about the only variation found in modem days is the height ofthe plants. 19-1. Filament --+-; ~----O.,aay ·. 19-2 MeDdeliaD iDheritance We begin studying Mendelian inheritance with red and white flower color. Mendel noted that the peas in his garden from year to year had red and white flowers. Some plants with red flowers came from plants with red flowers and made plants next year with red flowers. These were pure-breeding red-floweredplants. Similarly he noticed that he had pure-breeding white-floweredplants. Mendel kept very detailed, meticulous Dotes OD his work with his peas. However, he had DO clue as to the mechanism by which the characters were transmitted. He might have thought that it was like mixing cans ofpainttogether. The flower is the reproductive structure ofthe plant. Figure 19-1 shows the parts ofthe flower. The female component ofthe flower is called the pistil. The male parts are called the sttmlens. Male Reproductive PIUts-Th••t ....... Femal. Reproductive Perts-The Pistil 'tlgma Anther Style Figure19-1. Reproductivestructuresoftheflower. The male stamens produce the pollen. The pollen grains are the male sexual units ofthe plant. They are produced in theantherofthe flowerthat is supported by a filament. In the female pistil is an ovary, which contains ovules. The ovules lie the female sexual units ofthe plant. Each ovule contains an egg that will become fertilized by the pollen. After fertilization, each ovule will become a seed while the ovary becomes a fi'uit.~ .. 19-3 When plants IepI'Oduce, pollen from 1he an1her ofone flower is transfeued to the stigma ofanother flower. The pollen grains digest their way through the"style to the ovary. In the ovary, chromosomes from one ofthe po)]en grains fertilize each ovule. Sometimes, flowers can self-pollinate by transferring pollen from the anthers to the stigma in the same flower. Crossingplants involvestransferofpollen. A cross involves 1ransfer ofpollen . fromthestamensofoneflowertothepistilofanother. Forexample,pollenfroma plant producing mi flowers might be placed on1he stigmaofa plant with white flowers. Or, pollen from a white-flowered plant could be used to inoculate the stigma of a Jed-flowered plant. In either case, this constitutes a cross between a Jed-flowered plant and a white-flowered plant. Mendel did buDcheds ofthese crosses over aperiod ofmany years. Once the cross has been performed, you need to wait for the peas to develop in their pods. Then you must ·barvest the peas and put them away in storage over the Winter. Then, next Spring, you plant the seeds and see wbat·grows. So, as we discUss the types ofcrosses that Mendel performed with his peas, keep in mind that each stage in the cross takes one year. Sample mating to illustrate simple Mendelian inheritllllCe Mendel took red-flowered peas that were pure-breeding and crossed them with pure-breeding white-flowered plants. These plants were the pan:ntalgencration, represented by P. The following year, the plants that came up all bad red Dowers. NODe ofthe plantshad whiteflowerseventhough one ofthe patentshadwhite tlOWeJS. This is the generation ofoffspring aDd is called the FI generation. The letter HF" comes Dum the adjective "filiaJ" derived from a Latin word refcrriDg to cbildren (offspring) and their· relation to each other (the "sons" oftbe parental generation). This cross and its result is illustrated in Figure 19-2. p x Figure 19-2. pm:entaJ cross in simple Mendelian iDberi1ance.19-4 Crossing ofthe Fl generation The foJJowing year, Mendel crossed the Fired-flowered p18111s from the previous year. Recall that all ofthe FI plants had red flowers even though ODe ofthe original parents had white flowers. The result ofcrossing the red-flowered FI plants the production ofan F2 generation. In the F2 generation, the white trait returned. Mendel also noticed that there were about three times as many plants with red flowers as there were with white flowers. See Figure 19-3. Figure 19-3. F. cross and F2 results. After not being expressed in the FI generation, white was expressed in the F2 generation. Pod color exhibits simple Mendelill1l inherittmce For another example we can consider pod color. Mendel crossed PIR-bR:eding plants having green pods with pure-breeding plants havin& yellowpods. The FI generation all had green pods as shown in Figure 19-4. p x Figure 19-4. Parental cross iDvolviDg greeD 8Dd ye]Jow pods.'.' ".' 19-5 Once again, one ofthe traits was