Lecture 2 BIO 311D 1st Edition Outline of Last Lecture I Vocabulary II Stages of Meiosis III A Comparison of Meiosis and Mitosis IV Characteristics Unique to Meiosis V Genetic Variation Contributes to Evolution Outline of Current Lecture I Mendel s Experimental Quantitative Approach II The Law of Segregation III Mendel s Model IV The Test Cross V The Law of Independent Assortment Current Lecture Mendel s Experimental Quantitative Approach His particulate hypothesis was the idea that parents pass on genes o To prove this he conducted an experiment with garden peas Advantages of pea plants 1 Many varieties with distinct heritable features 2 Mating can be controlled 3 Each flower has sperm producing organs stamens and egg producing organs carpel 4 Cross pollination fertilization between different plants involves dusting one plant with pollen from another True breeding plants that produce offspring of the same variety when they self pollinate Mendel mated two contrasting true breeding varieties called hybridization P generation 1st generation or parent generation o Mendel mated purple and white flowers for the first generation F1 generation 2nd generation or the offspring o Produced phenotypes of all purple flowers F2 generation 3rd generation or the second offspring o Produced a 3 1 ratio of purple to white flowers The Law of Segregation Two alleles for a heritable character segregate during gamete formation and end up in different gametes Discovered by using monohybrids crosses between one character The white color trait did not disappear but rather it was recessive Mendel called the purple flower the dominant trait The white colored flower became the recessive trait Mendel s Model His model was used to explain the 3 1 inheritance pattern Model stated four things 1 Alternative versions of genes account for variations in inherited characters Alternative versions of a gene are called alleles Each gene resides at a specific locus 2 Two alleles are inherited one from each parent 3 If the two alleles differ then one is dominant and the other is recessive 4 The Law of Segregation Two alleles separate during gamete formation and end up in different gametes An egg or sperm gets only one of the two alleles present in the organism The Testcross A testcross is used to determine an unknown genotype To do a testcross breed the unknown species with a homozygous recessive individual If the offspring display the recessive phenotype then the mystery parent must be heterozygous The Law of Independent Assortment Dihybrid cross determines whether two characters are transmitted to offspring as a package or independently Dihybrid cross was used to discover the law of independent assortment Found that each pair of alleles segregates independently of each other during gamete formation However genes located near each other on the same chromosome tend to be inherited together
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