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INTRODUCTION TO GENETICS: THE WORK OF GREGOR MENDEL Ch. 11 Ms. Haut
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.
Modern Theory of Heredity Based on Gregor Mendel’s fundamental principles of heredity Parents pass on discrete inheritable factors (genes) to their offspring These factors remain as separate factors from one generation to the next
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. 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.
Gregor Mendel’s Peas Pea flowers are self- pollinating. Sperm cells in pollen fertilize the egg cells in the same flower. The seeds that are produced by self- pollination inherit all of their characteristics from the single plant that bore them.
Gregor Mendel’s Peas Copyright Pearson Prentice Hall Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants. He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.
Gregor Mendel’s Peas This process is called cross-pollination. Mendel was able to produce seeds that had two different parents. Copyright Pearson Prentice Hall
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.
Genes and Dominance Each original pair of plants is the P (parental) generation. The offspring are called the F 1, or “first filial,” generation. The offspring of crosses between parents with different traits are called hybrids. The F 1 hybrid plants all had the character of only one of the parents. 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
Principal of Dominance Copyright Pearson Prentice Hall The principle of dominance states that some alleles are dominant and others are recessive. For each characteristic, an organism inherits two alleles, one from each parent. If 2 alleles differ, one is fully expressed (dominant allele); the other is completely masked (recessive allele)
Useful Genetic Vocabulary Homozygous—having 2 identical alleles for a given trait (PP or pp) Heterozygous—having 2 different alleles for a trait (Pp); ½ gametes carry one allele (P) and ½ gametes carry the other allele (p) Phenotype—an organism’s expressed traits (purple or white flowers) Genotype—an organism’s genetic makeup (PP, Pp, or pp)
Law of Segregation M endel crossed the F 1 generation with itself to produce the F 2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F 2 plants. Copyright Pearson Prentice Hall
Law of Segregation 1 st Law of genetics The two members of an allele pair segregate (separate) from each other during the production of gametes.
Segregation When each F 1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F 1 plant produces two types of gametes— those with the allele for tallness, and those with the allele for shortness. 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 Copyright Pearson Prentice Hall 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.
Punnett Square Cross a heterozygous tall pea plant with a dwarf pea plant. T = tall, t = dwarf Tt x tt Tt T t tt tt Tt tt
Monohybrid Cross Genetic cross between individuals that are identical in all traits but one.
Practice Monohybrid Cross In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross between two plants that are heterozygous for the seed shape trait, what are the genotypic and phenotypic ratios? S= spherical seed s = dented seeds SsSs X SsSs SSSsSs SsSsss S s Ss Genotypic Ratio= 1 SS: 2 Ss: 1 ss Phenotypic Ratio= 3 spherical seed plants: 1dented seed plant Genotypic Ratio= Phenotypic Ratio=
Probability and Segregation One fourth (1/4) of the F 2 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 F 2 have two alleles for short (tt). Copyright Pearson Prentice Hall
Copyright Pearson Prentice Hall Gametes are also known as a) genes. b) sex cells. c) alleles. d) hybrids.
Copyright Pearson Prentice Hall The offspring of crosses between parents with different traits are called a) alleles. b) hybrids. c) gametes. d) dominant.
Copyright Pearson Prentice Hall In Mendel’s pea experiments, the male gametes are the a) eggs. b) seeds. c) pollen. d) sperm.
Copyright Pearson Prentice Hall In a cross of a true-breeding tall pea plant with a true- breeding short pea plant, the F 1 generation consists of a) all short plants. b) all tall plants. c) half tall plants and half short plants. d) all plants of intermediate height.
Copyright Pearson Prentice Hall If a particular form of a trait is always present when the allele controlling it is present, then the allele must be a) mixed. b) recessive. c) hybrid. d) dominant.
Copyright Pearson Prentice Hall Organisms that have two different alleles for a particular trait are said to be a) hybrid. b) heterozygous. c) homozygous. d) recessive. 6
Copyright Pearson Prentice Hall Two F 1 plants that are homozygous for shortness are crossed. What percentage of the offspring will be tall? a) 100% b) 50% c) 0% d) 25% 7
Copyright Pearson Prentice Hall The Punnett square allows you to predict a) only the phenotypes of the offspring from a cross. b) only the genotypes of the offspring from a cross. c) both the genotypes and the phenotypes from a cross. d) neither the genotypes nor the phenotypes from a cross. 8
Independent Assortment Copyright Pearson Prentice Hall To determine if the segregation of one pair of alleles affects the segregation of another pair of alleles, Mendel performed a two-factor cross.
Independent Assortment Copyright Pearson Prentice Hall The Dihybrid Cross: Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). All of the F 1 offspring produced round yellow peas (RrYy).
Independent Assortment Copyright Pearson Prentice Hall The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).
Independent Assortment Copyright Pearson Prentice Hall Mendel crossed the heterozygous F 1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F 2 generation. RrYy × RrYy
Gamete formation AaBb AB Ab aB ab AABb AB Ab AB Ab AaBbCc ABC ABc AbC Abc aBC aBc abC abc
Independent Assortment The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F 2 generation. Copyright Pearson Prentice Hall
Law of Independent Assortment 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
Independent Assortment 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.
A Summary of Mendel‘s Principles Copyright Pearson Prentice Hall 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.
A Summary of Mendel‘s Principles Copyright Pearson Prentice Hall 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.
Beyond Dominant and Recessive Alleles Copyright Pearson Prentice Hall Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.
Beyond Dominant and Recessive Alleles 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.
Beyond Dominant and Recessive Alleles A cross between red (RR) and white (WW) four o’clock plants produces pink-colored flowers (RW). Copyright Pearson Prentice Hall
Beyond Dominant and Recessive Alleles 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.
Codominance In chickens, black feather color (BB) is codominant to white feather color (WW). Both feather colors show up in a checkered pattern in the heterozygous individual (BW). Cross a checkered hen with a checkered rooster. What are the genotypic and phenotypic ratios?
Feather colors in chickens Black (BB) x White (WW) = Black and White checkered Chicken BW B W BB WWBW Codominance
Multiple Alleles Some genes may have more than just 2 alternate forms of a gene. Example: ABO blood groups A and B refer to 2 genetically determined polysaccharides (A and B antigens) which are found on the surface of red blood cells (different from MN blood groups) A and B are codominant; O is recessive to A and B
Multiple Alleles for the ABO Blood Groups 3 alleles: I A, I B, i Figure 9.18
Blood Types The immune system produces blood proteins That may cause clotting when blood cells of a different type enter the body. Figure 9.19
Example of ABO Blood Groups A man with type A blood whose father had type O marries a woman with type AB. What are the possible blood types of their children? IAIA IBIB IAIA i I A I B iI A i I A I B IAiIAiIAIBIAIB X ½ type A : ¼ type AB : ¼ type B
Beyond Dominant and Recessive Alleles 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.
Applying Mendel's Principles Copyright Pearson Prentice Hall 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.
Applying Mendel‘s Principles Copyright Pearson Prentice Hall Mendel’s principles can be used to study inheritance of human traits and to calculate the probability of certain traits appearing in the next generation. Genetics and the Environment Characteristics of any organism are determined by the interaction between genes and the environment.
Copyright Pearson Prentice Hall In a cross involving two pea plant traits, observation of a 9 : 3 : 3 : 1 ratio in the F 2 generation is evidence for a) the two traits being inherited together. b) an outcome that depends on the sex of the parent plants. c) the two traits being inherited independently of each other. d) multiple genes being responsible for each trait.
Copyright Pearson Prentice Hall Traits controlled by two or more genes are called a) multiple-allele traits. b) polygenic traits. c) codominant traits. d) hybrid traits.
Copyright Pearson Prentice Hall In four o'clock flowers, the alleles for red flowers and white flowers show incomplete dominance. Heterozygous four o'clock plants have a) pink flowers. b) white flowers. c) half white flowers and half red flowers. d) red flowers.
Copyright Pearson Prentice Hall A white male horse and a tan female horse produce an offspring that has large areas of white coat and large areas of tan coat. This is an example of a) incomplete dominance. b) multiple alleles. c) codominance. d) a polygenic trait.
Copyright Pearson Prentice Hall Mendel's principles apply to a) pea plants only. b) fruit flies only. c) all organisms. d) only plants and animals.