Chapter 13 Mendelian Genetics The theory of chromosome inheritance Cell Bio Fair The Cell Bio Fair is a final project for the class in which groups of 1 4 students will construct and demonstrate models of cellular functions that were discussed in class These models will be presented the last two days of class These models can take virtually any form from a solid object to a play or song to a computer simulation The group will provide a one page written description of the how the model works and a description of the biological process being demonstrated on the day of the presentation The Cell Bio Fair project will contribute 5 to the final grade Learning Goals By the end of class you should be able to 1 Explain how chromosomes can cross over 2 Describe how crossing over leads to genetic variance 3 Comprehend what makes an allele dominant or recessive 4 Explain how Mendel developed the concept chromosomal adherence 5 Complete a pundit square Crossing Over When homologs synapse two pairs of chromatids are held by a network of proteins called the synaptonemal complex Crossing over occurs when a complex of proteins the chromosomes and then reattaches the pieces so that the segments are swapped Crossing Over Crossing over produces new combinations of within a chromosome combinations that did not exist in either parent This is known as genetic recombination Independent Assortment Produces Genetic Variation As a result of and crossing over the chromosomal complement of the offspring is a random combination of genetic material from each parent and unlike that of either parent Independent assortment produces different combinations of due to the separation of homologous chromosomes during meiosis I Genetic Variation combination of alleles 1 Crossing over varies the within each chromosome 2 Random assortment of homologous chromosomes varies the combination of chromosomes present in gametes different As a result the number of genetically gametes that you can produce is virtually limitless Genetic Variation Chapter 13 Mendelian Genetics The theory of chromosome inheritance Mendel Defined basic laws of inheritance for all eukaryotes Why did he succeed Nope it was not just by accident 1 It was easy to cross defined strains with his chosen organism pea plants 2 He chose and traits 3 He used pure breeding lines so that genetic constitutions were reproducible Important concept Gametes sex cells and somatic cells must have different numbers of heritable units Otherwise the number of heritable units per cell would double with every generation The theory of genetic inheritance Used the garden pea as the model organism due to the relatively generation time and numbers from seed but also because he could choose which traits to mate Phenotypes Trait Seed shape Round Wrinkled Yellow Green Inflated Constricted Green Yellow Purple White Axial on stem Terminal at tip Seed color Obvious and distinct traits Pure breeding line exactly the same genomes and alleles Pod shape Pod color Flower color Flower and pod position Stem length Tall Dwarf Pure breeding lines If I cross these lines they will always give the same and genotype unless a random mutation occurs X offspring Experiments with a single trait His first set of experiments consisted of crossing pure bred lines that differed in just one trait Crossed pure lines of round seeds and wrinkled seeds Would the offspring be wrinkled round or a blended combination Experiments with a single trait Mendel then did something very interesting He planted the F1 seeds and allowed the individuals to when they matured Mendel repeated this experiment with each of the other 6 traits In each case he obtained similar results one form was always and the trait didn t show up until the F2 generation How could these patterns be explained Dominant versus recessive Remember we get one from each parent Dominant if one copy of the allele displays the phenotype Recessive if two copies of the same allele are required for the phenotype to show The Principle of Segregation To explain the 3 1 ratio of phenotypes in the F2 generation Mendel reasoned that each gene pair must during the formation of gamete cells He called this the principle of segregation For pure lines he drew diagrams called Punnett squares to represent the different alleles such as RR for dominate round and rr for recessive wrinkled Homozygotes are individuals with two of the same alleles for a given gene The Principle of Segregation This could then explain how the F2 generation would yield a 3 1 ratio if the parents were each heterozygotes which means individuals with two different alleles for each gene
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