Chapter 9 Fundamentals of Genetics

1. Chapter 9 Fundamentals of Genetics Section 1 Mendel’s Legacy

2. What is Genetics? • Genetics is the field of biology devoted to understanding how characteristics are transmitted from parents to offspring. • Genetics was founded with the work of Gregor Johann Mendel.

3. Gregor Mendel • Entered into a monastery, went to University of Vienna to study science and mathematics, returned to monastery to teach. • While at the University, Mendel learned how statistics works. This knowledge later proved valuable in his research on heredity (the transmission of characteristics from parents to offspring) • Mendel used Pisumsativum, a species of garden peas to carry out his research.

4. Mendel’s Garden Peas • Mendel observed seven characteristics of pea plants. • A characteristic is a heritable feature. • Each characteristic occurred in two contrasting traits. • A trait is a genetically determined variant of a characteristic.

5. Pea Characteristics Mendel Observed • Plant height (traits: long and short) • Flower position on stem (traits: axial and terminal) • Pod color (traits: green and yellow) • Pod appearance (traits: inflated and constricted) • Seed texture (traits: round and wrinkled) • Seed color (traits: yellow and green) • Flower color (traits: purple and white)

6. While Observing the Plants Mendel Noticed: • Most purple-flowering plants produced other purple-flowering plants. However, some white-flowering plants grew from some of the seeds of purple-flowering plants. • While most tall plants grew from most seeds obtained from tall plants, short plants grew from some of the seeds obtained from tall plants. • Mendel wanted to find an explanation for these varitations.

7. Mendel’s Methods • Mendel was able to observe how traits were passed from one generation to the next by carefully controlling how pea plants were pollinated. • Pollinations occurs when pollen grains produced in the male reproductive parts of a flower (the anthers) are transferred to the female reproductive part of a flower (the sigma) • Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant.

8. Self-pollination vs. Cross-pollination • Self-pollination occurs when pollen is transferred from the anthers of a flower to the stigma of either that flower or another flower on the same plant. • Cross-pollination occurs between flowers of two plants. • Pea plants normally reproduce through self-pollination. • This can be prevented by removing all of the anthers from the flowers of a plant. • Then cross-pollination can be performed by manually transferring pollen from the flower of a second plant to the stimga of the antherless plant.

9. Mendel’s Experiments • Mendel began by growing plants that were true-breeding for each trait. • True-breeding or purebred for a trait always produce offspring with that trait when self-pollinated. • Mendel obtained 14 true-breeding plant types, one for each of the 14 traits.

10. Mendel’s Experiments (next) • Mendel cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristic. • The true-breeding parents were called the P generation • 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.

11. Mendel’s Experiments (next) • He then allowed the flowers from the F1 generation to self-pollinate and collected the seeds. • Mendel called the plants in this generation the second filial generation, or F2 generation. • Following this process, Mendel performed hundreds to crosses and documented the results of each by counting and recording the observed traits of every cross.

12. Mendel’s Results and Conclusions • Mendel noticed that when he would take a true-breeding plant and crossed it with the true-breeding plant with the opposite trait, only one of the two traits found in the P generation appeared in the F1 generation. • Mendel allowed the F1 generation to self-pollinate and planted the resulting seeds. When the F2 generation plants grew, he noticed that three-fourths of the F2 generation plants had the F1 trait and about one-fourth had the trait which did not show in the F1 generation.

13. Mendel’s Results and Conclusions • Mendel hypothesized that something within the pea plants controlled the characteristics observed. • He called these controls factors. • He also hypothesized that each trait was inherited by means of a separate factor.

14. Recessive and Dominant Traits • Mendel concluded that one factor in a pair may prevent the other form having an effect. • He hypothesized that the trait appearing in the F1 generation was controlled by the dominate factor because it masked, or dominated, the factor of 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.

15. Law of Segregation • The law of segregation states that a pair of factors is segregated, or separated during the formation of gametes. Two factors for a characteristic are then combined when fertilization occurs and a new offspring is produced.

16. The Law of Independent Assortment • The law of independent assortment states that factors for individual characteristics are distributed to gametes independently. • This law is observed only for genes that are located on separate chromosomes or are far apart on the same chromosome. • This law is supported by the independent segregation of chromosomes to gametes during meosis.

17. Support for Mendel’s Conclusions • Most of Mendel’s findings agree with what biologists now know about molecular genetics. • Molecular genetics is the study of the structure and function of chromosomes and genes. • We now know that the factors that Mendel studied are alleles, or alternative forms of a gene. • Letters are now used to represent alleles. • Uppercase letters refer to the dominant alleles, while lowercase letters refer to the recessive alleles.