Traits+and+Factors

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Traits and Factors Mendel hypothesized that something within the pea plants controlled the characteristics observed. He called these controls factors. Mendel hypothesized that each trait was inherited by means of a separate factor. Because the characteristics that he observed had two alternative forms, he reasoned that a pair of factors must control each trait. Recessive and Dominant Traits: Whenever Mendel crossed P-generation plants with different forms of a trait, one form of the trait failed to appear in the F1 plants. In every case, the trait reappeared in a ratio of about 3:1 in the F2 generation. This pattern emerged in thousands of crosses and led Mendel to conclude that one factor in a pair may prevent the other from having an effect. Mendel hypothesized that the trait appearing in the F1 generation was controlled by a dominant factor because it masked, or dominated, the factor for the other trait in the pair. He thought that the trait that did not appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor. Thus, a trait controlled by a recessive factor had no observable effect on and organism's appearance when that trait was paired with a trait controlled by a dominant factor. The Law of Segregation: Mendel concluded that the paired factors separate during the formation of reproductive cells. That is, each reproductive cell, or gamete, receives one factor of each pair. When two gametes combine during fertilization, the offspring have two factors for each characteristic. The law of segregation states that a pair of factors is segregated, or separated, during the formation of gametes. This happens because homologous pairs of chromosomes separate in meiosis I. The Law of Independent Assortment: Mendel also crossed plants that differed in two characteristics, such as flower color and see color. The data from these more complex crosses showed the traits produced by dominant factors do not necessarily Appear together. A green seed pod produced by a dominant factor could appear with a recessive trait like white flowers. Mendel concluded that the factors for individual characteristics are not connected. The law of independent assortment states that factors separate independently of one another during the formation of gametes. Recall that crossing-over occurs in prophase I. Also that the pairs of homologous chromosomes separate randomly in meiosis I and chromatids separate in Meiosis II. Mendel's factors go into gametes randomly because of these events. Since crossing over involves segments of chromosomes, factors for different traits that are close together on a chromosome are not always separated. Therefore, the law independent assortment is observed only when genes are located on separate chromosomes or are located far apart on the same chromosome. Supporting Mendel's Observations: Chromosomes had not been yet been discovered in Mendel's time, but how they functions supports Mendel's observations. A chromosome is a threadlike structure made up of DNA.A gene is the segment of DNA on a chromosome that controls a particular hereditary trait. Because chromosomes occur in pairs, genes also occur in pairs. Each of two or more alternative forms of a gene is called an allele. Mendel's factors are now called alleles. Letters are used to represent alleles. Capital letters refer to dominant alleles, and lowercase letters referred to recessive alleles. For example, the dominant allele for the trait of purple flowers may be represented by P, and the recessive allele for the trait of white flower color may be represented by p. Whether a letter is capitalized or lowercase is important. The actual letter selected to represent an allele is typically the first letter of the dominant trait. During meiosis, gametes received one chromosome from each homologous pair of chromosomes. Thus, when the gametes combine in fertilization, the offspring receives from each parent one allele for a given trait. 1. Mendel called the something that controlled a characteristic a .......... . 2. Mendel reasoned that .......... of factors must control each trait. 3. The F1 generation all had a ........... factor because they all showed the same characteristic. 4. The trait that reappeared in the F2 generation was controlled by a .......... factor. 5. Paired factors ......... during the formation of gametes. 6. The .......... states that a pair of factors is segregated during the formation of gametes. 7. Crosses of two or more traits show that .......... factors are not necessarily passed together. 8. The .......... states that factors separate independently of one another when gametes are made. 9. Factors for different traits don't always separate if they are .......... on a chromosome. 10. A .......... is a small part of a chromosome that controls a particular hereditary trait. 11. Alternate forms of a gene is called an .......... . 12. Capital letters are used to refer to dominant .........., and lower case letters refer to recessive.......... . 13. When the gametes combine in .........., the offspring receives from each parent ........ allele for a given trait.