Biology, Chapter Nine, Notes

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UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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Chapter Nine (Fundamentals of Genetics)
SECTION ONE: MENDEL’S LEGACY Genetics is the field of biology devoted to understanding how characteristics are transmitted from parents to offspring. GREGOR MENDEL In 1843, at the age of 21, Gregor Mendel entered an Austrian monastery. In tending the garden there, he observed the growth of many plants. In 1851, he entered the University of Vienna to study science and mathematics. His mathematics courses included training in statistics. His knowledge of this subject later proved valuable in his research on heredity – the transmission of characteristics from parents to offspring. Mendel is remembered most for his experiments with Pisum sativum, a species of garden peas. Mendel’s Garden Peas Mendel observed seven characteristics of pea plants. A characteristic is a heritable feature – a feature that can be passed on from parents to offspring through DNA. Each characteristic occurred in two contrasting traits genetically determined variants of a traits, characteristic. The pea characteristics Mendel observed were Plant height (traits: long and short) Flower position along stem (traits: axial and terminal) Pod color (traits: green and yellow) Pod appearance (traits: inflated and constricted) Seed texture (traits: round and wrinkled) Seed color (traits: yellow and green) Flower color (traits: purple and white) Gregor Mendel collected seeds from his pea plants and recorded each plant’s traits and seeds. The next year, he planted the seeds and made observations. For example, he saw that purple-flowering plants grew from most of the seeds that came from purple-flowering plants. However, white-flowering plants also grew from the seeds that came from purple-flowering plants. He wanted to find an explanation for such variations.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) Mendel’s Methods Through controlling how pea plants were pollinated, Mendel was able to observe how traits were passed from one generation to the next. Pollination occurs when pollen grains produced in the male reproductive parts of a flower, called the anthers, are transferred to the female reproductive part of a flower, called the stigma. SelfSelf-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant. Cross-pollination occurs Crossbetween flowers from two different plants. To prevent self-pollination, all of the anthers of a plant are removed. Crosspollination can then be performed by manually transferring pollen from the flower of a second plant to the stigma of the antherless plant. Mendel selected parent plants with specific traits and observed the traits that appeared in the offspring. MENDEL’S EXPERIMENTS

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At first, Mendel studied each characteristic and its contrasting traits individually. He began by growing plants that were true-breeding for each trait. Plants that are true-breeding or pure, for a trait true-breeding, always produce offspring with that trait when they self-pollinate. For example, pea plants that are true-breeding for the trait of purple flowers self-pollinate to produce offspring with purple flowers. Mendel produced truebreeding plants by self-pollinating the pea plants for several generations. He eventually obtained 14 true-breeding plant types. Mendel cross-pollinated pairs of plants that were true-breeding for

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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contrasting traits of a single characteristic, and named these parents the P generation. generation When the plants matured, Mendel recorded the number of each number of each type of offspring produced by each cross. He called the offspring of the P generation the first filial generation, or F1 generation He then allowed the flowers from the F1 generation. generation to self-pollinate, and then collected the seeds. Mendel called the plants in this generation the F2 generation With this process, Mendel performed hundreds of generation. crosses and documented the results by counting and recording the observed traits of every cross.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) MENDEL’S RESULTS AND CONCLUSIONS

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In one of his experiments, Mendel crossed a plant true-breeding for purple flowers with one true-breeding for white flowers. The resulting seeds produced a F1 generation with only purple flowers. Only one of the two traits found in the P generation had appeared. Next, Mendel allowed the F1 plants to selfpollinate and planted the resulting seeds. When the F2 generation plants grew, he observed that about one-fourth of the plants had white flowers and three-fourths of the plants had purple flowers. His observations and records led Mendel to hypothesize that something within the pea plants controlled these characteristics. He called these controls factors. Mendel hypothesized that each trait was inherited by means of a separate factor, and reasoned that a pair of factors must control each trait since there were two alternative forms, such as in the trait of flower color, with purple and white. Recessive and Dominant Traits Whenever Mendel crossed strains, one of the P traits failed to appear in the F1 plants. In every case, that trait reappeared in a ratio of about 3:1 in the F2 generation. Since this pattern showed up in thousands of crosses, Mendel concluded that one factor in a pair may prevent the other from having an effect. He hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it dominated the other factor for the other trait in the pair. He thought the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor. Thus, a trait controlled by a recessive factor has no effect when paired with a trait controlled by a dominant factor. The Law of Segregation Mendel concluded that the paired factors separate during the formation of reproductive cells, and each reproductive cell receives one factor of each pair. When the two gametes combine during fertilization, the offspring will then have two factors

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) for each characteristic. The law of segregation states that a pair of factors is segregated during the formation of gametes.

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The Law of Independent Assortment Mendel also crossed plants that differed in two characteristics, such as flower color and seed color. The data from these more-complex crosses showed that the traits produced by dominant factors do not necessarily appear together. A green seed pod produced by a dominant factor could appear in a white-flowering pea plant. Mendel concluded that the factors for individual characteristics are not connected. The law of independent assortment states that the factors separate independently of one another during the formation of gametes. SUPPORT FOR MENDEL’S CONCLUSIONS Most of Mendel’s findings agree with what biologists now know about molecular genetics, genetics the study of the structure and function of chromosomes and genes. A chromosome is a threadlike structure made up of DNA. A gene is a segment of DNA on a chromosome that controls a particular hereditary trait. Since chromosomes occur in pairs, genes do as well. Each of two or more alternative forms of a gene is called an allele, allele which Mendel referred to as factors. Letters are used to represent alleles. Capital letters represent the dominant alleles and lowercase letters represent recessive alleles. The actual letter selected to represent a trait is usually the first letter of the dominant trait. During meiosis, gametes receive one chromosome from each homologous pair of chromosomes. Later, when gametes combine in fertilization, the offspring receives one allele for a given trait from each parent. The law of independent assortment is observed only for genes located on separate chromosomes or located far apart on the same chromosome.
SECTION 1 REVIEW 1. Describe what a true-breeding plant is. 2. Outline how Mendel produced plants that had genes for both contrasting traits of a characteristic. 3. Define the terms dominant and recessive. 4. State in modern terminology the two laws of heredity that resulted from Mendel’s work. 5. Differentiate genes from alleles. CRITICAL THINKING 6. How did Mendel’s F1 generation plants differ from his F2 generation plants? 7. Many inherited disorders of humans appear in children of parents who do not have the disorder. How can you explain this? 8. During meiosis, what allows genes located on the same chromosome to separate independently of one another?

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) SECTION TWO: GENETIC CROSSES GENOTYPE AND PHENOTYPE

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An organism’s genetic makeup is its genotype which consists of the alleles that genotype, the organism inherits from its parents. The genotype of a purple-flowering pea plant may either be PP or Pp and the genotype of a white-flowering pea plant is pp. An organism’s appearance is its phenotype The phenotype of a PP or Pp pea phenotype. plant is purple flowers, whereas the phenotype of a pp plant is white flowers. Phenotype does not always indicate genotype, and certain environmental factors can affect phenotype. For example, lack of nutrition may cause a genetically tall plant to remain short. When both alleles of a pair are alike, the organism is said to be homozygous for that characteristic, and may be homozygous dominant or homozygous recessive. When the two alleles in the pair are different, the organism is heterozygous. heterozygous PROBABILITY Probability Probability is the likelihood that a specific event will be occur, and may be expressed as a decimal, a percentage, or a fraction. Probability is determined by the following equation: Probability = Number of times an event is expected to happen Number of times an event could happen

The results predicted by probability are more likely to occur when there are many trials. For example, many coin tosses should yield a result of heads 50 percent of the time and tails 50 percent of the time. However, tossing a coin only a few times might not end in this result. After many tries, it would be likely to get the percentage predicted based on probability. PREDICTING RESULTS OF MONOHYBRID CROSSES A cross in which one characteristic is tracked is a monohybrid cross The cross. offspring of a monohybrid cross are called monohybrids. A cross between a pea plant that is true-breeding for producing purple flowers and one that is true-breeding for producing white flowers is an example of a monohybrid cross. A diagram called a Punnett square is used by biologists to predict the probable distribution of inherited traits in offspring. The following examples show how a Punnett square can be used to predict outcomes of different types of crosses.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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(PP)

Example 1: Homozygous × Homozygous A cross between a pea plant homozygous for a (pp) purple flower color (PP) and a pea plant homozygous for white flower color (pp) is shown. The alleles carried in p p gametes for the homozygous dominant parent are represented by P’s and the alleles carried by the homozygous recessive parent are represented by p’s. Each box in the Punnett square is filled in with the allele P above it and left of it outside the square. The Pp Pp combinations of alleles indicate the possible genotypes that may result from the cross. The predicted genotype P is Pp in every case, so there is a 100% probability that the offspring will have the genotype Pp and the Pp Pp phenotype purple flower color. Example 2: Homozygous × Heterozygous (Pp) P p This Punnett square shows a cross between a pea plant that is homozygous dominant for the trait of purple flower color and a pea plant that is heterozygous for the same trait. Two possible genotypes can result from this cross: PP or Pp. There is a 50% chance the offspring will have a genotype of PP and a 50% chance the offspring will have genotype of Pp. There is a 100% chance the offspring will have the phenotype of purple flower color.

P Pp P Pp Pp Pp

(PP) (PP)

Example 3: Heterozygous × Heterozygous An example of this type of cross is self pollination in the F1 generation for flower color. See page 4 for the Punnett square. The ratio of the genotypes that appear in offspring is called the genotypic ratio. ratio For the monohybrid cross shown, the genotypic ratio is 1 PP: 2 Pp: 1 pp. The ratio of the offspring’s phenotypes is called the phenotypic ratio The probable ratio. phenotypic ratio of the cross represented is 3 purple: 1 white.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) Example 4: Testcross Remember that a pea plant with purple flowers may have the genotype PP or Pp. To determine whether a pea plant with purple flowers is homozygous dominant or heterozygous, you could perform a testcross testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual. By looking at the offspring, you can determine the unknown genotype. If the genotype of the unknown individual were homozygous dominant, then all the offspring would most likely be purple. If the genotype were heterozygous, then half the offspring are predicted to be purple and the other half of the offspring are predicted to be white. See the image to the right for a visual aid. Example 5: Incomplete Dominance

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In Mendel’s pea-plant crosses, one allele was completely dominant over another, a relationship called complete dominance In dominance. complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype. Sometimes, the F1 generation will have a phenotype that is a mixture of the parents, a relationship called incomplete dominance This dominance. occurs when the phenotype of a heterozygote is intermediate between the phenotypes determined by the dominant and recessive traits. In four o’clock flowers, both the allele for red flowers (CRCR) and the allele for white flowers (CWCW) affect the offspring. When a homozygous red four o’clock flower is crossed with a homozygous white four o’clock, all the F1 offspring will have pink flowers (CRCW). When two pink flowers are crossed, the probable genotypic ratio is 1 CRCR: CRCW: 1 CWCW. The probable phenotypic ratio is 1 red: 2 pink: 1 white.

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston) Example 6: Codominance Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. In codominance, neither allele is dominant or recessive, nor do the alleles blend in with the genotype. Roan horses and cattle are an example of codominance. When a brown horse and a white horse are crossed, their offspring are brown splotched and white splotched.

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You may have trouble seeing the difference between incomplete dominance and codominance. Remember that inn incomplete dominance, the alleles blend, while in codominance, both alleles are expressed. For example, you know that in incomplete dominance, when red flowers are crossed with white flowers, pink flowers are the result. If this was a situation with codominance, when red and white flowers were crossed, flowers with red and white splotches would be the result. PREDICTING RESULTS OF DIHYBRID CROSSES A dihybrid cross is a cross in which two characteristics are tracked. The offspring of dihybrid crosses are called dihybrids. These are more complicated than monohybrid crosses because more combinations are possible. Homozygous × Homozygous Suppose you want to predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds. In pea plants, the allele for round seeds (R) is dominant and the allele for yellow seeds (Y) is dominant. When setting up a Punnett square, you first independently sort the alleles from one parent. For the parent with round, yellow seeds (RRYY), list RY, RY, RY, and RY along one of the sides. For the parent with wrinkled, green seeds (rryy), list ry, ry, ry, and ry along the other side of the Punnett square. This is to include independent assortment. After that, cross as usual – each box is filled with the alleles above it and to the left of it outside the square. The genotype for all of the offspring will be

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)

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heterozygous for both traits (RrYy); therefore, all of the offspring will have round, yellow seed phenotypes. Heterozygous × Heterozygous The same procedure is used to determine the results of crossing two pea plants heterozygous for round, yellow seeds. The offspring are likely to have nine different genotypes, which would result in pea plants with the following four phenotypes: 9/16 with round, yellow seeds (RRYY, RRYy, RrYY, and RrYy) 3/16 with round, green seeds (RRyy and Rryy) 3/16 with wrinkled, yellow seeds (rrYY and rrYy) 1/16 with wrinkled, green seeds (rryy)

SECTION 2 REVIEW 1. Explain why a phenotype might not always indicate genotype. 2. Identify the equation used to determine probability. 3. Explain how you might go about determining the genotype of a purple-flowering plant. 4. Illustrate in the form of a Punnett square the result of crossing a pink flower four o’clock with a white-flowering four o’clock. 5. Explain the difference between a monohybrid cross and a dihybrid cross and give an example of each. CRITICAL THINKING 6. The offspring of two short-tailed cats have a 25% chance of having no tail, a 25% chance of having a long tail, and a 50% chance of having a short tail. Using this information, what can you hypothesize about the genotypes of the parents and the way in which tail length is inherited? 7. If you crossed two purple-flowering pea plants and all the F1 offspring were purple-flowering, what could you say about the genotypes of the parents? If some of the F1 offspring were white-flowering, what could you say about the genotypes of the parents?

UNIT THREE: GENETICS AND BIOTECHNOLOGY (Text from Modern Biology, Holt, Rinehart, and Winston)
HIGHLIGHTS CHAPTER HIGHLIGHTS

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Section 1: Mendel’s Legacy The study of how characteristics are transmitted from parents to offspring is called genetics. Mendel observed seven characteristics of pea plants. Each characteristic occurred in two contrasting traits. Self-pollination, in which pollen is transferred from the anthers of a flower to either the stigma of the same flower to either the stigma of another flower on the same plant, normally occurs in pea plants. Cross-pollination occurs when pollen is transferred between flowers of two different plants. Mendel concluded that inherited characteristics are controlled by factors that occur in pairs. In his experiments on pea plants, one factor in a pair masked the other. The trait that masked the other was called the dominant trait. The trait that was masked was called the recessive trait. The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes. Two factors fro a characteristic are then combined when fertilization occurs and a new offspring is produced. The law of independent assortments states that factors for individual characteristics are distributed to gametes independently. The law of independent assortment is observed only for genes that are located on separate chromosomes or are fare apart on the same chromosome. We now know that the factors that Mendel studied are alleles, or alternative forms of a gene. Each of two or more alternative forms of a gene is called an allele. One allele for each trait is passed from each parent to the offspring. Section 2: Genetic Crosses The genotype is the genetic makeup of an organism. The phenotype is the appearance of an organism. Probability is the likelihood that specific event will occur. A probability may be expressed as a decimal, a percentage, or a fraction. A Punnett square can be used to predict the outcome of genetic crosses. A cross in which one characteristic is tracked is a monohybrid cross. The offspring of a monohybrid cross are called monohybrids. A testcross, in which an individual of unknown genotype is crossed with a homozygous recessive individual, can be used to determine the genotype of an individual whose phenotype expresses the dominant trait. Complete dominance occurs when heterozygous individuals and dominant homozygous individuals are indistinguishable in phenotype. Incomplete dominance occurs when two or more alleles influence the phenotype and results in a phenotype intermediate between the dominant trait and the recessive trait. Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive, nor do the alleles blend in the phenotype as they do in incomplete dominance. A cross in which two characteristics are tracked is a dihybrid cross. The offspring of a dihybrid cross are called dihybrids.

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