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Chapter 2

Chapter 2: Single Gene Inheritance Chapter 2 summary (Intro to Genetics 5ed by Alberts). 1. chromosomes and genes 2. Mendel's experiment, law and results 3. Single Gene Inheritance 4. Mitosis and Meiosis 5. DNA segregation Detection Methods (i.e.

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BIOL 2040
Joel Shore

10/01/11 Chapter 2: Single Gene Inheritance - To study how organisms became the way they are, biological properties must be broken down o i.e. animal locomotion, nutrient uptake, colour etc. - gene discovery: how genes of a genome influence properties - practice of a wild type X mutant type and check for SGI (ratio observations) in the offspring Brief History - 1900s, rediscovery of Mendel’s Law – genes are responsible for inheritance - Galton worked on inheritance in humans which were not discrete - Darwin’s theory of evolution was missing the correct mechanism of inheritance o At the time he believed that blood was responsible for inheritance - Galton thus did blood transfusion to test Darwin’s idea and discovered that blood was not responsible Genes and Chromosomes - genome: complete set of genetic info (DNA) o eukaryotes have a nucleus that contain chromosomes that can be:  diploid (2n): contain 2 complete genomes (2 chromosome sets)  haploid (n): one set of a genome (X); spp. are usually haploids - Chromosomal Structure (notes) - Genes are the transcribed segments on the DNA of a chromo (aka exons) o Exons encode for AA (after splicing of introns, exons are joined) o Untranscribed region (introns) on genes vary in size and DNA content  Introns resulted from an accumulation of mobile DNA called transposable elements  Intron vary to great lengths, unlike exons that remain fairly constant  Are spliced out through signals at the ends of the introns - Genes are made into mRNA  AA  proteins o Genes are regulated by promoter region Single Gene Inheritance Patterns (SGI) Mendel’s Law of Equal Segregation (notes) - Selfed: self-mating (i.e. a plants own pollen falling on its own stigma to produce a progeny) - Zygote: the 1 cell that develops into an progeny (aka a fertilized egg) - genotypes = Homozygous dominant (Y/Y), heterozygous dominant (Y/y) or homozygous recessive (y/y) - Pure bred: Y/Y or y/y – each line produces only Y gametes or only y gametes - Monohybrid (heterozygote) cross: Y/y X Y/y o [?] Produce gametes that are both male and female, some of which are ½ Y and the other ½ y o Male and female gametes fuse randomly - Equal segregation only detectable by meiosis on heterozygote All 1:1, 3:1 and 1:2:1 ratios are diagnostics of single gene inheritance and based on equal segregation in a heterozygote Significance of Mendel’s Model 1. Genes were the hereditary factor for producing a property 2. Gene comes in 2 forms called alleles 3. Possible allele match ups: Y/Y, y/y or Y/y 4. Y dominant allele and y is recessive allele 5. Mendel’s First Law: Law of equal segregation; during gamete formation, gene pairs separate equally to pass on 1 of its alleles per gametes 6. Gametes fuse randomly during fertilization Chromosomal Basis of Single Gene Inheritance Patterns Mitosis & Meiosis (notes) Single gene inheritance in haploids - Animals and plants  indirection allele segregation  > 1 individual provides meiocytes - Haploid organisms  direction allele segregation within 1 meiocytes o Ascus sac holds meiosis together - i.e. Yeast (notes) o has simple sec forms called mating types: MATa and MATα o + sign often combined with the wild type allele Molecular basis of Single Gene Segregation (SGS) and Expression Replication - DNA replication occurs in both meiosis and mitosis - Duplication of genetic info regardless of wild type or mutant type - Semi conservative DNA [?] Chromosome Segregation - Purpose is to show DNA segregation in meiotic products as one-half contain “A” DNA and the other half “a” DNA through various methods such as, 1. RFLP (notes) o Examines variation in DNA sequence between samples of homologous DNA o Genetic variation can create or abolish endonuclease sites  Restriction enzymes cuts at specific sites  These sites vary between individuals and are hybridized with a probe  Southern Blot done 2. CAPS (notes) o Analyse genetic markers o Similar to RFLP but requires PCR amplification to DNA first 3. SSCP (notes) o Identify sequence changes in amplified DNA.  Amplified DNA  Denature DNA. Heat then cool rapidly to maintain ssDNA.  Denaturing to a ssDNA allows for it to fold on itself so it can be ran on a gel  Non-denaturing gel.  Stain DNA.  ssDNA folds up on itself.  Run gel. Nature of Alleles and their products - Observation of how mutant alleles cause structural or functional changed different to the WT = gene function:  Null allele: proteins encoded lack gene function  Leaky mutation: reduce function of protein encoded (Lec 18)  If mutant on protein active site = null  If mutant on intron = null  If mutant on exon = silent  If mutant on exon and protein active site = leaky - Example: PKU o Recessive inherited allele that is defective in the gene that encodes for the enzyme PAH to break down Phe into Tyr o Instead of Tyr being produced, phenylpyruvic acid is made instead and casues mental retardation; cure = babies feed on low Phe diet o Many small mutation son the exon region to encode for PAH; these region are essential Dominance and Recessiveness - Speaking of dominance and recessive on a molecular basis: - Recessive – haplosufficient mutations in genes still functions o i.e. +/m where m = null but the + is enough for gene function - null dominant – Haploinsufficient  no function o mutation can lead to new functions that are dominant and mask WT Force of Segregation - spindle fibres are // to cell axis and extend form poles to arrange chromo at the equator - spindle fibres are polymers of the protein tubulin that attach to kinetochore which attaché to centromeres - during cytokinesis, SF pull chromatids into daughter cells Discovering Genes by Observing Segregation Ratios - Mutant phenotypes affect the biological property of interest and if SGI applies, the mutant can be either dominant or recessive. - Nomenclature: letters are used to represent mutant allele  recessive mutant – lower case letters  dominant mutant – upper case letters o The wild type is represent by + (i.e. +/alb) - Standard procedure: mutant X wild type - 3 possible outcomes of the cross: 1. Discovering gene active in flower colour (white vs. red) o Results (notes): mutant is a recessive allele (so, mutant parent is homo [?])  + masks alb and flower colour is white only when the genotype is alb/alb (homo) 2. Discovering gene for wing development (short vs. long) o Results (notes): mutant is a dominant allele (so, parent mutant is hetero [?]) 3. Discovery gen
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