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Introduction to Genetics How are traits passed on? S Gregor Mendel S Austrian monk who studied pea plants S He chose these plants to study because they were: S Easy to grow S Mature quickly S Have sharply contrasting traits Traits of Pea Plants Pea Plants S Naturally, pea plants self-pollinate. S Mendel observed that some plants, when they were allowed to self-pollinate, always “bred” true. S This means that these plants were “true-breeding” (Ex. Tall plants always produced tall plants.) S Mendel cross-pollinated some plants to observe the results. S When he crossed true-breeding tall plants and true breeding short plants, he found that he always got tall plants S Then, he crossed the resulting tall plants, and found that the short plant reappeared. S P = Parental Generation S F1 = first filial generation S F2 = Second filial generation Why did Mendel get these results? S All living things have genes, or sections of chromosomes that have instructions for making different characteristics S There are always two versions of these genes, one from each parent S Called Alleles S Represented by letters Alleles S Alleles, or the two forms of each gene, can be S Dominant – S this trait will always show up if the dominant allele is present S Represented by capital letters S Recessive – S this trait will only show up in the absence of the dominant allele S Represented by lowercase letters S Examples of S Dominant traits – tongue rollers, brown eyes, dark hair, left thumb hand clasping S Recessive traits – non-tongue rollers, light eyes, light hair, right thumb hand clasping Allele Combinations S Depending on the two alleles present, an organism can be considered homozygous or heterozygous for a trait S Homozygous – two alleles are the same, either both dominant or both recessive (TT or tt) S Heterozygous/Hybrid – two alleles are different, one dominant and one recessive (Tt) S The different arrangements of alleles is called an organism’s genotype Phenotype S An organism’s genotype will determine its phenotype, or physical characteristic S For example: S A plant with the alleles TT is Homozygous Dominant & one with the alleles Tt is heterozygous, and tt is read Homozygous Recessive S Both plants have the dominant gene for tallness (T) and will therefore have a tall phenotype Back to Mendel’s Plants S Why did the trait for short plants disappear in the F1 generation? S Why did it reappear in the F2 generation? S Punnett squares can be used to demonstrate the different possible combinations of alleles….. Punnett Squares t T T Tt Tt T T t TT Tt F1 results F2 results crossed t Tt Tt t Tt tt Mendel’s Laws S Law of Dominance S When there are two forms of a gene for one trait, one form may be dominant over the other Law of Segregation S Two forms of each gene are segregated during the formation of reproductive cells Law of Independent Assortment S Genes for different traits may assort independently of one another