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Biology Chapter 13.docx

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Lovaye Kajiura

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Biology Chapter 13: Mendel and the Gene Bernard Ho November 6, 2010 Mendel’s Experiments with a Single Trait − Genetics is the branch of biology that focuses on the inheritance of traits − Heredity is the transmission of traits from parents to offspring − A trait is any characteristic of an individual, ranging from overall height to the primary structure of a protein (category) − Mendel set out to address the most fundamental of all issues concerning heredity o What are the basic patterns in the transmission of traits from parents to offspring? − At that time, two hypotheses had been formulated o The first was blending inheritance, which claimed that the traits observed in a mother and father blend together to form the traits observed in their offspring  As a result, the offspring’s traits are intermediate between the mother’s and father’s traits o The second was called the inheritance of acquired characters, which claimed that traits present in parents are modified, through use and passed on to their offspring in the modified form  The idea is that the genetic determinants present in an individual are modified through use and passed on to offspring in the modified form − A 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 to study the pea plant Pismum sativum − Under normal conditions, garden peas pollinate themselves rather than requiring pollen from other pea plants for fertilization to occur − Self-fertilization (selfing) takes place when pollen from one flower falls on the female reproductive organ of that same flower − Selfing is common because pollen from other plants rarely reaches the flower since its petals form a compartment that encloses the male and female reproductive organs − However, Mendel circumvented this arrangement by removing the male reproductive organs from a flower before any pollen formed − Later, he could transfer pollen from another flower to that flower’s female reproductive organs with a brush − This type of mating is referred to as cross-pollination and using this technique, Mendel could control the matings of his model organism − Traits Mendel studied o 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 o Phenotype is the observable features of an individual (round seed) o In the pea populations Mendel studied, two distinct phenotypes existed for all seven traits o Mendel began his work by obtaining individuals from pure lines o A pure line consist of individuals that produce offspring identical to themselves when they are self-pollinated or crossed with another member of the pure-line population o This result was important because he knew what the offspring from the matings between pure-lines would look like o He could then compare these results with outcomes of crosses between individuals from different pure-lines o Hybrids are offspring from matings between true-breeding parents that differ in one or more traits − Inheritance of a Single Trait o Mendel began his single-trait crosses by crossing individuals from round and wrinkled seeded pure lines o Adults used in an initial experimental cross represent the parental generation o Their progeny are called the F ge1eration (first filial) o Subsequent generations from the F 1 parents are called the F , F 2 3 generation etc. o In his first set of crosses, Mendel took pollen from round seeded plants and placed it on the female reproductive organs of plants from the wrinkle seeded line  All the seeds produced by progeny from this cross were round  The traits did not blend together to form an intermediate phenotype, instead the round seeded form appeared intact  This result was in stark contrast to the predictions of the blending- inheritance hypothesis  Also, the genetic determinant for wrinkled seeds seemed to have disappeared o Mendel performed a second set of crosses, this time with pollen taken from an individual germinated with a wrinkle seeded pea o These crosses represent a reciprocal cross, a set of matings where the mother’s phenotype in the first cross is the father’s phenotype in the second cross and vice versa  In this case, the results of the reciprocal crosses were identical  All of the 1 progeny in the second cross had round seeds, just like the first cross  This second 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 o Mendel planted the F se1ds and allowed the individuals to self-pollinate when they matured o He collected the resulting F g2neration and observed that some seeds were round, while others were wrinkled o The wrinkled shape reappeared in the F gener2tion after disappearing completely in the F 1eneration o Mendel invented some important terms to describe the result o He designated the genetic determinant for wrinkled shape as recessive o In contrast, he referred to the genetic determinant for round seeds as dominant  Important to note that dominance has nothing to do with fitness of individuals o In genetics, the term dominance and recessiveness identify only which phenotype is observed in individuals carrying two different genetic determinants o Mendel also noticed that the round and wrinkled seeds of the F 2 generation were present in a ratio of 3:1 − The Nature and Behaviour of Hereditary Determinants o To explain the patterns that he observed, Mendel proposed a competing hypothesis called particulate inheritance o He maintained that hereditary determinants for traits do not blend together or acquire new or modified characteristics through use o They maintain their integrity from generation to generation o Instead of blending together, they act like discrete entities or particles o Gene is now used to indicate the hereditary determinant for a trait o Mendel also proposed that each individual has two versions of each gene, called alleles o Different alleles are responsible for the variation in the traits that Mendel studied o 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, its physical traits o Mendel proposed that some alleles are dominant and others are recessive o The Principle of Segregation  To explain the 3:1 ratio of phenotypes in F ind2viduals, Mendel reasoned that the two alleles of each gene must segregate into different gamete cells during the formation of eggs and sperm in the parents  As a result, each gamete contains one allele of each gene, an idea called the principle of segregation  Individuals with two copies of the same allele are said to be homozygous  Individuals with two different alleles for the same gene are said to be heterozygous − Mendel’s Claims o Peas have two alleles of each gene (true for many other organisms) o Alleles do not blend together (hereditary determinants maintain their integrity from generation to generation) o Each gamete contains one allele of each gene (due to principle of segregation) o Males and females contribute equally to the genotype of their offspring o Some alleles are dominant to other alleles Mendel’s Experiment with Two Traits − Mendel wanted to know if the principle of segregation holds true if parental lines differ with respect to two traits − To explore this issue, Mendel crossed a pure-line parent that produced round, yellow seeds with a pure-line parent that produced wrinkled, green seeds − A mating between parents that are both heterozygous for two traits is called a dihybrid cross − Mendel’s earlier experiments had established that the allele for yellow seeds was dominant to the allele for green seeds − The allele for seed shape and allele for seed colour present in each parent would separate from one another and be transmitted independently, a hypothesis called independent assortment − Mendel crossed the RRYY seed with the rryy seed and produced all F 1 generation of RrYy (yellow, round) − However, in the F ge2eration, four phenotypes were present in a ratio if 9:3:3:1 (9 yellow round, 3 green round, 3 yellow wrinkled, 1 green wrinkled) − Based on these data, he accepted the hypothesis that alleles of different genes are transmitted independently of one another − This result became known as the principle of independent assortment − Using a Testcross to confirm predictions o A testcross uses a parent that contributes only recessive alleles to its offspring to help determine the unknown genotype of the second parent o Testcrosses are useful because the genetic contributions of homozygous recessive parents are easy to predict and analyze The Chromosome Theory of Inheritance − Sutton and Boveri discovered that chromosomes are composed of genes − Ex. Gene for seed shape is at a particular position along a chromosome, called a locus − A genetic locus is the physical location of a gene − The physical separation of alleles during anaphase of meiosis I is responsible for Mendel’s’ principle of segregation − For alleles of two different genes, they assort independently of one another at meiosis I since these genes are located on different non-homologous chromosomes − Four types of gametes are produced in equal proportions − This is the physical basis of Mendel’s principle of independent assortment Testing and Extending the Chromosome Theory − The fruit fly (Drosophila melanogaster) became a model organism for testing the chromosome theory of inheritance − The fruit fly was adopted as a model organism by Thomas Hunt Morgan and his students − Morgan’s group referred to individuals with the most common phenotype as wild type − While examining his cultures, Morgan discovered a male fly that had white eyes rather than the wild-type red eyes − Morgan inferred that the white-eyed phenotype resulted from a mutation, a change in gene − To explore how the white-eye trait is inherited in fruit flies, Morgan mated a red- eyed female with the mutant white-eyed male o All the F 1rogeny had red eyes − When Morgan did the reciprocal cross, by mating white-eyed females to red- eyed males, he got a different result o All F 1emales had red eyes, but all F 1ales had white eyes − This experiment suggested that a definite relationship between the sex of the progeny and the inheritance of eye colour − Discovery of Sex chromosomes o Nettie Stevens discovered the X and Y chromosomes through studying beetles o She proposed that an individual’s genotype at the sex chromosomes determines its gender o XX  female, XY  male − X-Linked Inheritance and Chromosome Theory o Morgan realized that the transmission pattern of the X chromosome in males and females could account for the results of his reciprocal crosses o He proposed that the gene for white-eye colour in fruit flies is located on the X chromosome and that the Y chromosome does not carry an allele of this gene o This hypothesis is called the X-linked inheritance or X-linkage o The key observation is that even though the X and Y chromosomes synapse during prophase of meiosis I, they differ in size, shape and gene content o According to the hypothesis of X-linkage, a female has two copes if the gene that specifies eye colour because she has two X chromosomes (one from her mom and one from her dad) o A male has only one copy of the eye-colour gene because he only has one X chromosome, which comes from his mother o Morgan’s experimental results can be explained if the gene for eye colour is located on the X chromosome and if the allele for red colour is dominant to the allele for white colour − When reciprocal crosses give different results, it is likely that the gene in question is located on a sex chromosome − Genes on non-sex chromosomes are said to show autosomal inheritance − The correlation between white eye colour and inheritance of X chromosome was important evidence in support of the hypothesis that chromosomes contain genes − The discovery of X-linked inheritance convinced most biologists that the chromosome theory of inheritance was correct − Genes located on the same chromosome o The physical association of genes that are found on the same chromosome is called linkage o After Morgan established that the white eye gene was located on the X chromosome, he established that one of several genes that affects body colour is also located on the X o Red eyes and grey body are the wild-type phenotype in this species, white eyes and yellow body occur as rare m+ta+t phenotypes o The alleles for red eyes and grey body (w , y ) are dominant to the alleles for white eyes and yellow body (w,y) o Thus it seemed logical to predict that the linked genes would always be transmitted together during gamete formation o Linked genes should violate the principle of independent assortment  Independent assortment is observed when genes are on different chromosomes because the alleles of unlinked genes segregate to gametes independently of one another during meiosis I  When genes are on the same chromosome, their alleles are carried to gametes together  In metaphase I, independent chromosomes can line up randomly (Ex. dad chromosomes can line up on same side or different side)  Genes located
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