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Chapter 8 Mendel & Inheritance Genes, alleles & inheritance • Genes – Nucleotide sequence that code for a specific trait. – The expressed trait is called a character or Phenotype • Trait – Manifestation of genes • Genome – A complete set of ALL the genes in an organism 2 Genes, alleles & inheritance • Allele – Copy of a gene – Copies carried by an single individual called genotype • Locus – Physical location of a gene on a strand of DNA • Chromosome – Organized group of genes Gene A B C D – Part of a genome 3 Terminology • Homozygous – homologous copies of a gene are the same • Heterozygous – homologous copies of a gene are different 4 5 Gene mutations & traits • How do new alleles arise? • Mutation: • Different allele different trait 6 Trait Inheritance • Heredity – the transmission of traits from parents to children – Inherit ½ from each parents • Genes program specific traits 7 Who is Gregor Mendel? • German monk 1822-1884 • Most monks were committed to theology and the study of nature • Mendel started on his family farm studying bees and physics • Interested in how characteristics formed 8 Mendel’s pea plants Why peas? • Fast Growing – Easy to grow in greenhouse or garden – Easy to maintain large numbers • Easy to control mating • Many clearly identified traits 9 Mendel’s experiments • Parental (P) Generation – Collected stamen (male organs) – Used to fertilize a stigma (female organ) • Collected offspring (seeds) – Grew plants from 10 Mendel’s experiments • Collected two of his new plants (F1 Generation) • Repeated fertilization process – looked at the resulting plants (F2 Generation) • Repeated experiment a Thousands of times 11 12 Predicting outcomes • Mendel found there were two types of traits: • Dominant • Recessive 13 Punnett Squares 14 Coin Toss • Segregation pattern of alleles is a chance event – Like a coin toss • We can predict the probability of – a coin landing heads up – a coin landing tails up • Flip two coins and you get a 1:2:1 ratio 15 16 Phenotypic & Genotypic Results 17 Mendel’s law #1 • Segregation of Chromosomes • The two homologus copies of a gene end up in different gametes – Meiosis • Therefore, possible offspring genotypes predicted using mother and fathers genotypes 18 For Example • Can you Roll your tongue? • T= Yes; t= No TT? Tt? Tt? • Dominant genes: • Recessive genes: 19 For Example • You have ____ copies of the tongue rolling gene. • Your Genotype: • Homozygous: • Heterozygous: Alleles T= Yes; t= No 20 Diagramming inheritance • Draw a punnett square to predict the phenotypes TT tt tt Tt Tt Tt t t Tt Tt Tt T T t T Tt t T Tt TT 21 Mendel’s pea plants • 3:1 Pattern of inheritance found in a variety of traits – Called Mendelian Genetics • Parent pea plants pass on characteristics to offspring • Published his results in 1866 22 Multi-trait cross • Dihybrid cross • Used to determine the ration of two genes • Can determine if genes are connected in some way 23 Di-hybrid cross • Human eye color 24 Mendel’s law #2 • Independent Assortment • Chromosomes are randomly & independently assigned to gametes • Therefore traits on different chromosomes are inherited independently 25 Independent Assortment 26 Incomplete Dominance • Two alleles produce different phenotypes – Homozygous genotype • When these two alleles are together produce a third phenotype – intermediate Heterozygous genotype Intermediate phenotype 27 Incomplete dominance Heterozygous genotype Intermediate phenotype HcHc http://images47.fotki.com/v1402/photos/1/138 4902/6613115/IMG_0643-vi.jpg HcHs http://www.cosmogirl.com/cm/cosmogirl/images/coral -eyeshadow-gtl0606-240x312.jpg HsHs http://www.annabelleswigs.co.uk/ima ges/clips_STW_24_col130.JPG 28 Human Blood Types (Co-dominance) 29 Continuous variation • Mendelian trait – Each trait controlled by a single gene • Continuous variation (polygenic trait) – Multiple genes of small effect 30 Continuous variation • Human Skin Color 31 Epigenetics • Environment can affect phenotype 32 Questions: • Why do some conditions “run in the family”? • Why are some children or some generations not affected? • What are mendelian traits? 33 Questions: • Dominant traits • Genetic recombination (be able to explain this) • Mendelian Traits show typical dominant/recessive expression, based on the combination of two alleles 34 Questions: • Be able to do simple mendelian crosses – PP x pp – Pp x pp – Pp x Pp • What is: – Incomplete dominance? – Co-dominance? – Polygenic Traits? 35 36