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Fundamentals of Genetics Part I Gregor Mendel • Tended garden in monastery in Austria and saw many plants grow • Later entered college studying math and science • Is considered to be the Father of Heredity: transmission of characteristics from parents to offspring Mendel’s Peas • Observed characteristics (a heritable feature) of pea plants – Height, flower position, pod appearance, seed texture, seed color (yellow, green), flower color (purple, white) – Each characteristic occurred in two contrasting traits: a genetically determined variant (variety) of a characteristic • Used his knowledge of statistics to analyze observations of characteristics • Collected seeds from pea plants, recording characteristics of plant from which each seed was collected • The following year he planted these seeds – Saw purple flowered plants grew from the seeds he collected from purple flowered plants – Also saw some white flowered plants grew from the seeds from purple flowered plants Mendel’s Methods • Able to document traits of each generation’s parent by controlling how pea plants were pollinated – Pollination: occurs when pollen grains produced in the male reproductive part (anther) of a flower are transferred to the female reproductive part of a flower (stigma) • Self-pollination: occurs when pollen is transferred from the anthers of a flower to the stigma of either the same flower or a flower on the same plant – Pea plants usually do this • Cross-pollination: involves flowers of two separate plants • Self-pollination can be stopped and cross-pollination performed (remove anther and manually transfer pollen to stigma of another plant) – By doing this Mendel was able to protect his flowers from receiving any other pollen via wind or insects controlling the experiment Mendel’s Experiments • Grew plants “pure”, or true-breeding, for each trait – Pure: always produce offspring with that trait – Ex. Pea plants pure for trait of purple flowers self-pollinate to produce offspring with all purple flowers • Mendel cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristic. – He called the true-breeding parents the P generation. – Transferred pollen from the anthers of one plant to the stigma of another plant. • For example, to cross a plant that was true-breeding for the trait of yellow pods with one that was true-breeding for the trait of green pods: – – 1. remove the anthers from the plant that produced green pod 2. Dust the pollen from a yellow-podded plant onto the stigma of a green-podded plant and allowed the seeds to develop. • When the plants matured, Mendel recorded the number of each type of offspring produced by each cross. – He called the offspring of the P generation the first filial generation, or F1 generation. • He then allowed the flowers from the F1 generation to self-pollinate and collected the seeds, and grew plants from the seeds. – Mendel called the plants in this generation the second filial generation, or F2 generation. Following this process, Mendel performed hundreds of crosses and documented the results of each by counting and recording the observed traits of every cross. Mendel’s Experiment in Picture Mendel’s Results and Conclusions • In one experiment Mendel crossed a plant pure for green pods with a plant pure for yellow pods – F1 generation was all green • Next, Mendel allowed F1 to self-pollinate and planted the resulting seeds – F2 generation plants grew and ¾ were green, ¼ yellow pods 3:1 • He hypothesized that something within the pea plants controlled the characteristics he observed (called these controls, “factors”) • He reasoned that there must be a pair of factors controlling each trait (not just one factor) We Now Know • Mendel’s “factors” = genes • What do genes do?! • There are a pair of factors controlling each trait because peas (like humans!) are diploid. • They have TWO of each type of chromosome. Each chromosome carries one copy of the gene in question. SO: The pea plant has TWO genes for flower (or pod) color – one on each homologous chromosome. The genes may be the same, or different. In the example below, they are different. – We call the alternate versions of the genes alleles. Dominant and Recessive Trait • Whenever Mendel crossed plants that were pure for a characteristic, one of the P traits failed to appear in the F1 plants but then reappeared in F2 generation. – The F2 generation always had a 3:1 ratio. • Concluded that one factor in a pair may prevent the other from having effect • Hypothesized that trait appearing in F1 generation was controlled by dominant factor because it masked (dominated) the other factor for specific trait. • Trait not appearing in F1 generation but reappeared in F2 was controlled by recessive factor Law of Segregation • Mendel concluded that paired factors separate during formation of reproductive cells. What process forms reproductive cells?! – Each gamete receives only one factor of each pair – When two gametes combine to form the offspring they then have two factors controlling specific trait • Law of Segregation : a pair of factors is segregated, or separated, during the formation of gametes Law of Independent Assortment • Mendel crossed plants that differed in two characteristics (ex. Flower color and seed color) – proved that traits produced by dominant factors do not necessarily appear together – Factors for different characteristics are not connected • Law of Independent Assortment: factors for different characteristics are distributed to gametes independently