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Chapter 13

Chapter 13 Textbook Notes - Mendel and the Gene

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Biology
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BIO152H5
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Fiona Rawle

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Notes From Reading CHAPTER 13:M ENDEL AND THE GENE (PGS.279-308) Chapter 13 – Mendel and the Gene Key Concepts - Mendel discovered that in garden peas, individuals have two alleles, or versions, of each gene. Prior to the formation of eggs and sperm, the two alleles of each gene separate so that one allele is transmitted to each egg or sperm cell. - Genes are located on chromosomes. The separation of homologous chromosomes during anaphase of meiosis I explains why alleles of the same gene segregate to different gametes. - If genes are located on different chromosomes, then the alleles of each gene are transmitted to egg cells and sperm cells independently of each other. This occurs because chromosomes line up randomly in metaphase of meiosis I. - There are important exceptions and extensions to the basic patterns of inheritance that Mendel discovered. Introduction - The cell theory describes the basic structure of organisms - The theory of evolution by natural selection clarifies why species change through time - These theories explain fundamental features of the natural world and answer some of our most profound questions about the nature of life: o What are organisms made of? o Where did species come from? - 1865- Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas - Early 20 century, Walter Sutton and Theodor Boveri formulated the chromosome theory of inheritance which proposes that meiosis causes the patterns of inheritance that Mendel observed 13.1 Mendel’s Experiments with a Single Trait th - Mendel was a 19 century monk who lived and worked not far from Vienna, Austria (now known as the city of Brno in the Czech Republic) - Mendel questioned about heredity o Heredity – is the inheritance or transmissions of traits from parents to their offspring o Trait – is any characteristic of an individual ranging from overall height to the primary structure of a particular membrane protein What questions was Mendel trying to answer? - Mendel wanted to know why offspring resemble their parents and how transmission of traits occurs - There were two hypotheses formulated to answer this questions Notes From Reading CHAPTER 13:M ENDEL AND THE GENE (PGS.279-308) o Blending inheritance – claimed that the traits observed in a mother and father blend together to form the traits observed in their offspring an offspring’s traits are intermediate between the mother’s and father’s trait o Inheritance of acquired characteristics – claimed that traits present in parents are modified, through use, and passed on to their offspring in the modified form Garden Pease Serve as the First Model Organism in Genetics - Model organism – (for genetic studies) is a species where individuals are small, short-lived, inexpensive to care for, able to produce large numbers of offspring, and easy to manipulate experimentally - Mendel chose the common garden pea as his model organism because it is easy to grow, has a short reproductive cycle, and produces large number of seeds - Genetics – is the branch of biology that focuses on the inheritance of traits o Uses model of organisms because the conclusions drawn from them turn out to apply to many other species as well - Two additional feature of the pea made it possible for Mendel to design his experiments o He could control mating and the traits he studied were easily recognizable How Did Mendel Arrange Matings? - Figure 13.1a shows a garden pea flower, including its male and female reproductive organs - Sperm cells are produced in pollen grains, which are small sacs that mature in the male reproductive structure of the plant; eggs are produced in the female reproductive structure - Fertilization begins when pollen grains are deposited on the tube like section of the female reproductive organs o Sperm cells travel down this tube to the egg cells, where fertilization takes place - Peas normally pollinate themselves, a process called self-fertilization o Takes place when pollen from one flowers falls on the female reproductive organ of that same flower - Mendel could prevent this by removing the male reproductive organs containing pollen from each flower - Used this pollen to fertilize the female reproductive organs of flowers on different plants  performing cross-pollination - Using this technique, Mendel could control the matings of his model organisms What Traits Did Mendel Study? - Mendel conducted his experiments on varieties of peas that differed in seven traits: seed shape, seed colour, pod shape, pod colour flower colour, flower and pod position and stem length - These observable features of an individual is referred to as phenotype (“show type”) - Mendel studied two distinct phenotypes existed for all seven traits - Mendel began his work with breeders called pure lines or true-breeding lines Notes From Reading CHAPTER 13:M ENDEL AND THE G ENE (PGS.279-308) o Pure line consists of individuals that produce offspring identical to themselves when they are self-pollinated or crossed to another member of the pure-line population - He used these plants to create hybrids o Hybrid – offspring from matings between two different pure lines that differed in one or more traits Inheritance of a Single Trait - Mendel’s first set of experiments involved crossing pure lines that differed in just one trait (ie. Seed shape) - Mendel began his single-trait crosses by crossing individuals from round and wrinkled seeded pure lines - Adults used in an initial experimental cross such as this represent the parental generation, their offspring are called F1generations – for “first filial” Certain Traits “Recede” - When Mendel crossed plants with round seeds and plants with wrinkled seed, all of the F 1 offspring had round seeds - The traits didn’t blend together to form an intermediate phenotype  the round-seeded form appeared intact - The genetic determinant for wrinkled seeds seemed to have disappeared - Mendel performed a second set of crosses called a reciprocal cross – a et of matings in which the mother's phenotype in the first cross is the father's phenotype in the second cross, and the father's phenotype in the first cross is the mother's phenotype in the second cross - The results of the two crosses were identical - Reciprocal cross established that it does not matter whether the genetic determinants for seed shape are located in the male or female parent Dominant and Recessive Traits - Mendel allowed the F pr1geny to self-pollinate when they matured - The wrinkled seed trait reappeared in the next F gen2ration in ¼ of the plants - He called the genetic determinant for wrinkled seeds recessive and the determinant for round seeds dominant - It was appropriate to use recessive as none of the F1 individuals had wrinkled seeds - Dominant for round seeds was appropriate as the round-seed determinant appeared to dominate over the wrinkled-seed determinant when bother were present - In genetics, the terms dominance and recessiveness identify only which phenotype is observed in individuals carrying two different genetic determinants - Mendel repeated these experiments with each of the other six traits - In each case, he saw that the dominant trait was present in a 3:1 ratio over the recessive trait in the F2generation. Notes From Reading CHAPTER 13:M ENDEL AND THE G ENE(PGS .279-308) The Nature and Behaviour of the Hereditary Determinants - To explain these results, Mendel proposed a hypothesis called particulate inheritance. - In it he suggested that hereditary determinants maintain their integrity from generation to generation, directly contradicting the blending inheritance hypothesis. - Instead of blending together, they act like discrete entities or particles What are Genes, Alleles and Genotypes? - Gene – indicate the hereditary determinant for a trait - Mendel proposed that each individual has two versions of each gene and are now known as alleles o Different alleles are responsible for the variation in the traits that Mendel studied - The alleles that are found in a particular individual are called its genotype o An individual’s genotype has a profound effect on its phenotype The Principle of Segregation - Principle of segregation – alleles segregate into different gametes during gamete formation, then come back together when an egg is fertilized by a sperm to form a zygote - Mendel used a letter to indicate the gene for a particular trait o He would us an uppercase letter (R) to show a dominant allele and lowercase ® for a recessive allele o Individuals have two alleles of each gene - Homologous – individuals that have two copies of the same allele (RR or rr) for a gene - Heterozygous – has two different alleles for the same gene (Rr) - Pure-line individuals always produce offspring with the same phenotype because they are homozygous—no other allele is present. - A mating between two pure lines that differ in one trait (RR and rr) results in offspring that are all heterozygous for that gene (Rr) and all the dominant phenotype (figure 13.4a) - A mating of parents that are both heterozygous for a trait (Rr and Rr) is called a monohybrid cross. o Cross results in offspring are ¼ RR, ½ Rr, and ¼ rr, which produces a 3:1 ratio of phenotypes - Years after Mendel published his work, R.C Punnett invented a straightforward technique for predicting the genotypes and phenotypes that should appear in the resulting offspring - A punnet square is used to predict the genotypes and phenotypes of the offspring from a cross. 1. Write each unique gamete genotype produced by one parent to make a new row 2. Write each unique gamete genotype produced by the other parent in a column 3. Create boxes in the square or rectangle composed of the rows and columns 4. Fill in the boxes with the offspring genotypes that result from fusion of the gamete genotypes in the appropriate row and column Notes From Reading CHAPTER 13:M ENDEL AND THE GENE (PGS .279-308) 5. Predict the proportions of each offspring genotype and phenotype by tallying the offspring genotypes and phenotypes present in the boxes - Mendel's genetic model—a set of hypotheses that explains how a particular trait is inherited— explains the results of these crosses. Testing the Model - Mendel then used self-crosses with the F of2spring to test his model - The results were as predicted - Plants with wrinkled seeds (the recessive trait, rr) produced only offspring with wrinkled seeds (rr) - Plants with round seeds (the dominant trait, RR or Rr) produced offspring with round or wrinkled seeds in the expected ratio. 13.2 Mendel’s Experiments with Two Traits - Mendel crossed a pure-line parent that produces wrinkle, green seeds - The F1 offspring of this cross should be heterozygous at both genes - Mendel used dihybrid crosses—matings between parents that are both heterozygous for two traits—to determine whether the principle of segregation holds true if parents differ in more than one trait. - Mendel’s results supported the principle of independent assortment, in which alleles of different genes are transmitted independently of each other. - Hypothesis of independent assortment: alleles of different genes don’t stay together when gametes form (figure 13.5a) - Hypothesis of dependant assortment: alleles of different genes stay together when gametes form (figure 13.5b) Using a Testcross to Confirm Predictions - Mendel’s goal was to test the predication that an RrYy plant produces four different types of gametes in equal proportions - Mendel invented a technique called a testcross – a parent that is homozygous recessive for a particular trait is mated with a parent that has the dominant phenotype but an unknown genotype o Because the genetic contribution of the homozygous recessive parent is known, the genotype of the other parent can be inferred from the results. - Mendel performed a testcross between parents that were RrYy and rryy - Mendel used the testcross to further confirm the principle of independent assortment and found that the offspring showed the expected 1:1:1:1 ratio of phenotypes. Notes From Reading CHAPTER 13:M ENDEL AND THE G ENE PGS .279-308) 13.3 The Chromosome Theory of Inheritance - The chromosome theory of inheritance arose out of Sutton and Boveri’s careful observations of meiosis. - It states that chromosomes are composed of Mendel’s hereditary determinants, or what we now call genes. - Locus – physical location of a gene - The physical separation of alleles during anaphase of meiosis 1 is responsible for Mendel’s principle of segregation - Mendel's principle of segregation can be explained by the independent alignment and separation of homologous chromosomes containing alleles for a gene at meiosis I - The chromosome theory states that Mendel’s rules can be explained by the independent alignment and separation of homologous chromosomes at meiosis 1 13.4 Testing and Extending the Chromosome Theory - Fruit flies (Drosophila melanogaster) became a model organism for genetic research early in the 20th century. - In order to study the genetics of Drosophila, Thomas Hunt Morgan needed different phenotypes - Individuals with the most common phenotypes were called wild-type - What they needed were individuals with mutations, in which some gene had changed - Luckily, Morgan found a fly that had white eyes instead of the wild-type red eyes to study. The Discovery of Sex Chromosomes - Nettie Stevens began studying the karyotypes of insects, beetles, about the time that Morgan began his work with fruit fly - Her analysis showed that in females, diploi
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