11-1 The Work of Gregor Mendel

11-1 The Work of Gregor Mendel
photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

Copyright Pearson Prentice Hall
Gregor Mendel’s Peas Gregor Mendel’s Peas Genetics is the scientific study of heredity. Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity. Mendel carried out his work with ordinary garden peas. Copyright Pearson Prentice Hall

Gregor Mendel’s Peas Mendel knew that the male part of each flower produces pollen, (containing sperm). the female part of the flower produces egg cells. To cross-pollinate pea plants, Mendel cut off the male parts of one flower and then dusted it with pollen from another flower. Copyright Pearson Prentice Hall

Gregor Mendel’s Peas During sexual reproduction, sperm and egg cells join in a process called fertilization. Fertilization produces a new cell. Pea flowers are self-pollinating. Copyright Pearson Prentice Hall

Gregor Mendel’s Peas Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. Cross-pollination Mendel was able to produce seeds that had two different parents. Copyright Pearson Prentice Hall

Genes and Dominance Genes and Dominance A trait is a specific characteristic that varies from one individual to another. Mendel studied seven pea plant traits, each with two contrasting characters. He crossed plants with each of the seven contrasting characters and studied their offspring. Copyright Pearson Prentice Hall

Genes and Dominance Each original pair of plants is the P (parental) generation. The offspring are called the F1, or “first filial,” generation. The offspring of crosses between parents with different traits are called hybrids. Copyright Pearson Prentice Hall

Mendel’s F1 Crosses on Pea Plants
Genes and Dominance Mendel’s F1 Crosses on Pea Plants When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive. Copyright Pearson Prentice Hall

Genes and Dominance Mendel’s Seven F1 Crosses on Pea Plants Mendel’s F1 Crosses on Pea Plants When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive. Copyright Pearson Prentice Hall

Genes and Dominance Mendel's first conclusion was that biological inheritance is determined by factors that are passed from one generation to the next. Today, scientists call the factors that determine traits genes. Copyright Pearson Prentice Hall

Genes and Dominance Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. The different forms of a gene are called alleles. Mendel’s second conclusion is called the principle of dominance. Copyright Pearson Prentice Hall

Genes and Dominance The principle of dominance states that some alleles are dominant and others are recessive. Copyright Pearson Prentice Hall

Genes and Dominance Mendel’s F1 Crosses on Pea Plants When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive. Copyright Pearson Prentice Hall

Segregation Segregation Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F2 plants. Copyright Pearson Prentice Hall

Segregation Mendel's F2 Generation F1 Generation F2 Generation P Generation When Mendel allowed the F1 plants to reproduce by self-pollination, the traits controlled by recessive alleles reappeared in about one fourth of the F2 plants in each cross. Tall Short Tall Tall Tall Tall Tall Short Copyright Pearson Prentice Hall

Segregation The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness. Copyright Pearson Prentice Hall

Segregation Mendel suggested that the alleles for tallness and shortness in the F1 plants segregated from each other during the formation of the sex cells, or gametes. Copyright Pearson Prentice Hall

Segregation Alleles separate during gamete formation. During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant. Copyright Pearson Prentice Hall

11-2 Probability and Punnett Squares
photo credit: W. Perry Conway/CORBIS Copyright Pearson Prentice Hall

Genetics and Probability
The likelihood that a particular event will occur is called probability. The principles of probability can be used to predict the outcomes of genetic crosses. Copyright Pearson Prentice Hall

Punnett Squares Punnett Squares The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. Punnett squares can be used to predict and compare the genetic variations that will result from a cross. Copyright Pearson Prentice Hall

Punnett Squares A capital letter represents the dominant allele for tall. A lowercase letter represents the recessive allele for short. In this example, T = tall t = short The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Punnett Squares Gametes produced by each F1 parent are shown along the top and left side. The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Punnett Squares Organisms that have two identical alleles for a particular trait are said to be homozygous. Organisms that have two different alleles for the same trait are heterozygous. Homozygous organisms are true-breeding for a particular trait. Heterozygous organisms are hybrid for a particular trait. Copyright Pearson Prentice Hall

Punnett Squares All of the tall plants have the same phenotype, or physical characteristics. The tall plants do not have the same genotype, or genetic makeup. One third of the tall plants are TT, while two thirds of the tall plants are Tt. Copyright Pearson Prentice Hall

Punnett Squares The plants have different genotypes (TT and Tt), but they have the same phenotype (tall). Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous Copyright Pearson Prentice Hall

Probability and Segregation
One fourth (1/4) of the F2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F2 have two alleles for short (tt). The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants. Copyright Pearson Prentice Hall

Probabilities Predict Averages
Probabilities predict the average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values. Copyright Pearson Prentice Hall

11-3 Exploring Mendelian Genetics
Copyright Pearson Prentice Hall

Independent Assortment
To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross. Copyright Pearson Prentice Hall

The Two-Factor Cross: F1 Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). RRYY x rryy All of the F1 offspring produced round yellow peas (RrYy). Copyright Pearson Prentice Hall

The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation. Copyright Pearson Prentice Hall

The Two-Factor Cross: F2 Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation. RrYy × RrYy Copyright Pearson Prentice Hall

The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation. Represents: Independent Assortment When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation. Copyright Pearson Prentice Hall

The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. Genes that segregate independently do not influence each other's inheritance. Copyright Pearson Prentice Hall

The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. Copyright Pearson Prentice Hall

A Summary of Mendel's Principles
Genes are passed from parents to their offspring. If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. Copyright Pearson Prentice Hall

A Summary of Mendel's Principles
In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. Copyright Pearson Prentice Hall

Beyond Dominant and Recessive Alleles
Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes. Copyright Pearson Prentice Hall

Incomplete Dominance When one allele is not completely dominant over another it is called incomplete dominance. In incomplete dominance, the heterozygous phenotype is between the two homozygous phenotypes. Copyright Pearson Prentice Hall

RR A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). WW Some alleles are neither dominant nor recessive. In four o’clock plants, for example, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. Copyright Pearson Prentice Hall

Codominance In codominance, both alleles contribute to the phenotype. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. Copyright Pearson Prentice Hall

Multiple Alleles Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. Copyright Pearson Prentice Hall

Different combinations of alleles result in the colors shown here. KEY C = full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele c = albino; no color; recessive to all other alleles Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here. photo credits: 1. ©John Gerlach/Visuals Unlimited 2.Animals Animals/©Richard Kolar 3. ©Jane Burton/Bruce Coleman, Inc. 4. ©Hans Reinhard/Bruce Coleman, Inc. AIbino: cc Chinchilla: cchch, cchcch, or cchc Himalayan: chc, or chch Full color: CC, Ccch, Cch, or Cc Copyright Pearson Prentice Hall

Polygenic Traits Traits controlled by two or more genes are said to be polygenic traits. Skin color in humans is a polygenic trait controlled by more than four different genes. Copyright Pearson Prentice Hall

LE 14-12 20/64 15/64 6/64 1/64 AaBbCc AaBbCc aabbcc Aabbcc AaBbcc
Fraction of progeny 6/64 1/64

Applying Mendel's Principles
Thomas Hunt Morgan used fruit flies to advance the study of genetics. Morgan and others tested Mendel’s principles and learned that they applied to other organisms as well as plants. Copyright Pearson Prentice Hall

11-4 Meiosis 11-4 Meiosis Copyright Pearson Prentice Hall

Each organism must inherit a single copy of every gene from each of its “parents.” Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. Copyright Pearson Prentice Hall

Chromosome Number Chromosome Number All organisms have different numbers of chromosomes. A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent. These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes. Copyright Pearson Prentice Hall

Chromosome Number These sets of chromosomes are homologous. Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. Copyright Pearson Prentice Hall

Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. For Drosophila, the diploid number is 8, which can be written as 2N=8. Copyright Pearson Prentice Hall

Chromosome Number The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes. These cells are haploid. Haploid cells are represented by the symbol N. For Drosophila, the haploid number is 4, which can be written as N=4. Copyright Pearson Prentice Hall

Phases of Meiosis Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell. Copyright Pearson Prentice Hall

Phases of Meiosis Meiosis involves two divisions, meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Copyright Pearson Prentice Hall

Phases of Meiosis Meiosis I Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Copyright Pearson Prentice Hall

Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes. Copyright Pearson Prentice Hall

Phases of Meiosis Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. MEIOSIS I Prophase I MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Copyright Pearson Prentice Hall

Phases of Meiosis When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles. Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged. Copyright Pearson Prentice Hall

Phases of Meiosis Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes. Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell. MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell. Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells. Copyright Pearson Prentice Hall

Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids. Copyright Pearson Prentice Hall

Phases of Meiosis Meiosis II During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Meiosis II Telophase I and Cytokinesis I Metaphase II Anaphase II Telophase II and Cytokinesis Prophase II Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS II Prophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS II Metaphase II The chromosomes line up in the center of cell. MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis. Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS II Anaphase II The sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell. Copyright Pearson Prentice Hall

Phases of Meiosis MEIOSIS II Telophase II and Cytokinesis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells. Copyright Pearson Prentice Hall

Gamete Formation Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm. Copyright Pearson Prentice Hall

Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction. Copyright Pearson Prentice Hall

Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. Copyright Pearson Prentice Hall

Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis. Copyright Pearson Prentice Hall

Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes. Copyright Pearson Prentice Hall

11-1 The Work of Gregor Mendel

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