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Chapter 13

Textbook and Class Notes Collaborated - Unit 3 - Chapter 13 Bio 1A03

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Xudong Zhu

Bio 1A03 Unit Three: Gene Structure and Expression 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 cells, the two alleles of each gene separate so that one allele is transmitted to each egg or sperm cell  The separation of homologous chromosomes during anaphase of meiosis I explains why alleles of the same gene segregate to different gametes  Genes are located on different chromosomes, then the alleles of each gene are transmitted to egg cells and sperm cells independently of each other – 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  In 1865, Gregor Mendel worked out the rules of inheritance through a series of brilliant experiments on garden peas  Early in the 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  Gregor Mendel o Was a 19 century monk o Lived and worked not far from Vienna, Austria, in what is today the city of Brno in the Czech Republic o Educated at the University of Vienna o Interested in heredity o In 1865 – worked out the rules of inheritance through experiments with garden peas  Heredity – the transmission of traits from parents to their offspring o Trait – any characteristic of an individual What Question Was Mendel Trying to Answer?  Mendel was addressing the basic question of why offspring resemble their parents and how transmission of traits occurs  Two hypotheses o Blending inheritance – claimed that the traits observed in a mother and father blend together to form the traits observed in their offspring  Thus, offspring traits are intermediate between the mothers and fathers traits  Eg/ Black sheep and white sheep would produce grey sheep 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  Eg/ Giraffes have long necks because they strain to reach leaves high in the trees and they produce longer necked offspring Garden Peas Serve as the First Model Organism in Genetics  Genetics – the branch of biology that focuses on the inheritance of traits o Uses model organisms because the conclusions drawn from them turn out to apply to many other species as well  Worked with the common garden pea (Pizom sativum) as his model organism Bio 1A03 o Easy to grow o Short reproductive cycle o Produces large number of seeds o Could control mating’s o Traits he studied were easily recognizable  Prevented peas from self-fertilization (pollinating themselves) by removing the male reproductive organs containing pollen from each flower  He then performed cross-pollination – using the pollen to fertilize the female reproductive organs of flowers on different plants  Seven recognizable traits o Seed shape o Seed color o Pod shape o Pod color o Flower color o Flower and pods position o Stem length  Phenotype – observable features of an individual o Pea population had two distinct phenotypes for each of the seven traits  Pure lines produced identical offspring when self-pollinated  Created hybrids by mating two different pure lines that differed in one or more traits First Experiment – Inheritance of a Single Trait  Involved crossing pure lines that differed in just one trait  Parental generation – adults in the cross  F g1neration – offspring (“first filial”)  When Mendel crossed plants with round seeds and plants with wrinkled seeds  All F1 offspring had round seeds  Reciprocal cross – a set of mating’s in which the mothers phenotype in the first cross is the fathers phenotype in the second cross, and the fathers phenotype in the first cross is the mothers phenotype in the second cross o Results of the two crosses were identical – established that it does not matter whether the genetic determinants for seed shape are located in the male or female parent o Figure 13.3, page 282; Reciprocal Cross  Dominant and Recessive Traits o Allowed the F1 progeny to self-pollinate  The wrinkled seed trait reappeared in the next F2 generation in ¼ of the plants  Recessive (in this case) – genetic determinant for wrinkled seeds  Dominant (in this case) – genetic determinant for round seeds  Terms dominant and recessive identify only which phenotype is observed in individuals carrying two different genetic determinants  Dominant does not necessarily have higher fitness and does not mean “superior” o Repeated these experiments with each of the other six traits – saw that the dominant trait was present in a 3:1 ratio over the recessive trait in t2e F generation Bio 1A03 The Nature and Behavior of the Hereditary Determinants  Particular Inheritance – hereditary determinants maintain their integrity from generation to generation, directly contradicting the blending inheritance hypothesis  Genes – hereditary determinants for a trait  Alleles – different versions of a gene o Different alleles are responsible for the variation in the traits that Mendel studied o Eg/ Shape of seed (gene); round and wrinkled (alleles)  Genotype – the alleles found in an individual o Has a profound effect on its phenotype o Alleles are dominant or recessive  Used letters to indicate the gene for a particular trait o Eg/ Seed shape – R dominant allele, r recessive allele  Homozygous – individuals with two copies of the same allele (RR or rr)  Heterozygous – individuals with different alleles (Rr)  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 o Pure line individuals always produce offspring with the same phenotype because they are homozygous o Mating between two pure lines that differ in one trait (RR and rr) result in offspring that are all heterozygous for that gene and all the dominant phenotype  Figure 13.4, page 284; Offspring of a Cross between pure Lines o Monohybrid Cross – a mating of parents that are both heterozygous for a trait (Rr and Rr)  Results in offspring that are ¼ RR, ½ Rr and ¼ rr – produces a 3:1 ratio of phenotypes  Mendel’s genetic model – a set of hypothesis that explains how a particular trait is inherited – explains the results of these crosses o Summary Table 13.1, page 285; Mendel’s Genetic Model for a Cross between Pure Lines  Mendel then used self-crosses with the F2 offspring to test his model – the results were as predicted o Plants with wrinkled seeds (rr) produced only offspring with wrinkled seeds (rr) o Plants with round seeds (RR or Rr) produced offspring with round or wrinkled seeds in the expected ratio  Punnet square – used to predict the genotypes and phenotypes of the offspring from a cross 1. Write each gamete genotype produced by one pa
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