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BIOL 205 (111)
Kenton Ko (10)


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Queen's University
BIOL 205
Kenton Ko

CH2: SINGLE-GENE INHERITANCE Sept/9/11 - - There are many types of analytical approaches to gene discovery, the most common being detection of single-gene inheritance patterns. - Inheritance patterns may be recognized in the progeny of controlled matings or crosses - In this case, we mate a mutated form of the property with a wild type form and the ratio of wild to mutant reveals whether a single gene controls that difference in property under study. - The use of mutants in this way is called genetic dissection - How do we obtain mutants? We visually screen a large number of individuals, looking for a chance occurrence of mutants in that population - Single gene inheritance patterns are useful in human genetics o Cystic fibrosis and Tay-Sachs disease are inherited as a single mutant gene. 2.1 Genes and Chromosomes - An organism’s unique and complete set of genetic information is its genome - In eukaryotes, the bulk of the DNA of a genome is found in the nucleus of each cell and this Chromosomes nuclear DNA is divided into units called chromosomes - Through using chromosome paints as a probe to stain identical chromosomes the same colour, Diploid we now know that nuclear genetic material of most animals and plants are diploid - contain two complete genomes and 2 identical chromosome sets. Haploid - The number of chromosomes in the basic genomic set is called the haploid number (n) o Ex. Molds, yeasts, mushrooms, and algae ar familiar haploid organisms. - Each chromosome contains 1 molecule of DNA wrapped around molecular spools called Nucleosomes, histones nucleosomescomposed of an octamer of proteins called histones Chromatin - DNA and associated nucleosomes constitute chromatin - Chromatin is more compact near the centromere Heterochromatin o Heterochromatin: dense chromatin Euchromatin o Euchromatin: less dense chromatin Parts of a Chromosome: Nucleolar organize - The nucleolar organizer contains multiple repeats of genes encoding ribosomal RNA nucleolus - A spherical body containing ribosomal RNA, the nucleolus is often visible attached to the nucleolus organizer Telomeres - Telomeres are the tips of chromosomes and have a unique replication mechanism different from that for the rest of the chromosome - There are untranscribed regions between genes that are variable and a large part of this intergenic space has resulted from the accumulation of a mobile type of DNA called Transposable element transposable elementwhich are known to have an effect on genome rearrangement in the course of evolution. Introns - Introns are noncoding regions that interrupt the coding segments of a gene, most coding regions are 1-3kb but large introns can cause a gene to seem enormous. 2.2 Single-Gene Inheritance Patterns Mendel’s Law of Equal - Mendel investigated the inheritance of 7 properties in peas: pea color, pea shape, pod color, pod Segregation shape, flower color, plant height and position of the flowering shoot. o Character and trait are used synonymously with property - For each character, he studied 2 contrasting phenotypes and pure lines – all offspring produced by matings within the members of that line were identical o Ex. All progeny of any mating were yellow seeded in the yellow-seeded line. - The lines that were part of the parental generation (P) produced the first filial generation or F1 and the crossed descendants of F1 produced the second filial generation or F2. - - Mendel concluded the following from his experiments: 1. A gene is a hereditary factor producing pea colour 2. Each plant has a pair of this type of gene 3. The gene comes in 2 forms called alleles 4. A plant can be either Y/Y, y/y or Y/y. 5. The Y allele is dominant thus, the phenotype will be yellow and the y allele is recessive 6. In meiosis, the members of a gene pair separate equally into eggs and sperm, this equal Mendel’s first law separation is Mendel’s first law or the law of equal segregation 7. Hence, a single gamete contains only one member of the gene pair 8. At fertilization, gametes fuse randomly, regardless of which of the alleles they bear. Zygote - A zygote is a fertilized egg, that develops into a progeny individual. Homozygote o Homozygote : same pair of identical alleles Heterozygote o Heterozygote: different pair of alleles Monohybrid o Monohybrid: a heterozygote for one gene Genotypes - Genotypes are allelic combinations underlying phenotypes. Homozygous dominant o Homozygous dominant: Y/Y Heterozygous dominant o Heterozygous dominant: Y/y Heterozygous recessive o Heterozygous recessive: y/y Monohybrid cross - Monohybrid cross is a cross of the same type of genotype i.e. Y/y x Y/y 2.3 The Chromosomal Basis of Single Gene Inheritance Patterns - When somatic (body) cells divide to increase their number, the accompanying nuclear division mitosis is called mitosis , which can take place in diploid or haploid cells and produces daughter cells with the same genomic content - Results in one progenitor cell to become 2. 2n 2n +2n Meiocytes - Most eukaryotes have a sexual cycle and in these organisms, specialized diploid cells called meiosis meiocytes are set aside to divide to produce sex cells. - In meiosis, two cell divisions take place and has two nuclear divisions occurring in only diploid cells. - Results in 2 divisions with 4 cells produced. 2n  n+n+n+n - The difference between mitosis and meiosis is that in meiosis, homologous pairs of sister chromatids unite to form a bundle of four homologous chromatids, called synapsis, helped by the Synaptonemal complex (SC) - Replicate sister chromosomes are called a dyad (Greek word for 2) - Bivalent: the unit comprising a pair of synapsed dyads - Tetrad : the four chromatids that make up a bivalent - At the tetrad stage, crossing over changes the combinations of alleles of several different genes but doesn’t directly affect single gene inheritance patterns. o Ex. Start: one homolog carries A and one carries a o Replication: one dyad is AA and one is aa o Pairing: Tetrad is A/A/a/a o First division products: one cell AA, the other cell aa o Second division products: four cells, two of type A and two of type a. Single gene inheritance in haploids - In fungi, there are 2 simple forms of sexes called mating types : MATa and MATα determined by the alleles of one gene. - Normal wild type yeast colonies are white, but occasionally, red mutants arise - Replication of r+ (wild type) and r would give a tetrad of two meiotic products of genotype r+ and two of r, all contained in a membranous sac called an ascus. - Haploid genetics is simple because a cross requires only one meiosis. Molecular Basis of single- gene segregation and expression Demonstrating chromosome - Restriction fragment length polymorphism (RFLP) uses restriction enzymes that cut DNA at segregation at the molecular specific base sequences in the genome, followed by a Southern blot. level - If at a specific location a target site is missing or there is an extra site, and such a site flanks the sequence hybridized by a probe, then this reveals a RFLP. The Nature of Alleles and their products - PKU phenotype is inherited as a Mendelian recessive and is caused by a defective allele of the gene that encodes the liver enzyme phenylalanine hydroxylase (PAH)
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