Introduction to Genetics

1. Introduction to Genetics • For thousands of years farmers and herders have been selectively breeding their plants and animals to produce more useful hybrids. • It was somewhat of a hit or miss process since the actual mechanisms governing inheritance were unknown. • Knowledge of these genetic mechanisms finally came as a result of careful laboratory breeding experiments carried out over the last century and a half.

2. Intro. To Genetics • By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts of cell division and sexual reproduction. • The focus of genetics research then shifted to understanding what really happens in the transmission of hereditary traits from parents to children.

3. Gregor Mendel • A number of hypotheses were suggested to explain heredity, but Gregor Mendel, a little known Central European monk, was the only one who got it more or less right. • His ideas had been published in 1866 but largely went unrecognized until 1900, which was long after his death

4. Gregor Mendel • His early adult life was spent in relative obscurity doing basic genetics research and teaching high school mathematics, physics, and Greek in Brünn (now in the Czech Republic). • In his later years, he became the abbot (friar) of his monastery and put aside his scientific work.

5. Gregor Mendel • While Mendel's research was with plants, the basic underlying principles of heredity that he discovered also apply to people and other animals because the mechanisms of heredity are essentially the same for all complex life forms.

6. Mendel and Peas • Through the selective cross-breeding of common pea plants over many generations, Mendel discovered that certain traits show up in offspring without any blending of parent characteristics. • For instance, the pea flowers are either purple or white--intermediate colors do not appear in the offspring of cross-pollinated pea plants.

7. Mendel and Peas • Mendel observed seven traits (specific characteristic) that are easily recognized and apparently only occur in one of two forms: • flower color is purple or white • flower position is axial or terminal • stem length is long or short • seed shape is round or wrinkled • seed color is yellow or green • pod shape is inflated or constricted • pod color is yellow or green

9. Mendel’s Peas • The pea plant was favorable organism for these studies because it was self-fertilizing • When he made crosses, he followed only 1 or 2 (out of his 7) traits (characters) at a time • He employed a very consistent method: - Opened flower & placed pollen from one type onto the stigma

10. Mendel’s methods • Mendel covered each flower with little bag • When pods were ripe harvested them and planted seeds • He counted the number of each type of offspring and carefully recorded all of his data.

11. Mendel’s First Experiment • Crossed (P1): Pure breeding Tall x Pure breeding Short (Dwarf) • (P1) = parental generation (pure breed) • Pure (true) breeding means that if the plants were allowed to self-pollinate, they’re offspring would be identical to themselves. • Predictions: The offspring would be: All tall All short All intermediate Some would be tall and some short

12. Mendel’s 1st and 2nd Experiment • 1st Exp (P1): Crossed Pure Tall x Pure Short • All offspring (F1): All Tall • These offspring of the parental generation are calledhybrids, which are offspring with different traits than parents. • (F1) = first filia (son or daugther) • 2nd Exp: Bred F1 • Results: Ratio of 787 tall to 277 short (3:1)

13. Mendel’s Principle of Segregation • Mendel assumed that the two “Factors” for each trait must exist in the parental germ cells producing the gametes (pollen / egg) • These “factors” are called alleles. • Each allele came from the parents and were united in fertilization • In forming pollen and egg, the two alleles for any trait must separate and go into different gametes • This became known as Mendel’s “Principle of Segregation”

14. Mendel’s Third Experiment • Crossed one of the F1tall plants with its dwarf parent: F1 Tall x Dwarf (P1) • Possible Outcomes: All would be tall Mixture of Tall & Dwarf All would be intermediate • Experimental results —> 50% 50%

15. Mendel’s Notation • After the third experiment, Mendel formulated his “Principles of Dominance” which states that some alleles are dominant and some are recessive. • Used capital letter to denote what he called the dominant form of the trait: T = tall • Used lower case letter to denote what he called the recessive trait: t = short (dwarf) • Thus for the Tall and Dwarf crosses: TT = Original pure-breeding tall parent tt = Original pure-breeding short parent Tt = Hybrid F1 offspring • Pure-breeding forms later called homozygous • Hybrids later called heterozygous

16. Mendel’s Experiment 3 • Mendel recognized that it is not always possible to tell what offspring will be like by inspecting the parent • Mendel could test if tall plants were pure-breeds (homozygotes) or hybrid (heterozygotes) by the “back-cross” or “test-cross” • Test Cross: Crossing a organism with the dominant phenotype with the same organism with the recessive phenotype to determine the genotype of the dominant organism. Tt TT tt tt Not in your notes

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18. Writing Mendel’s Crosses Using His Notational System • It was now possible to account for the 3:1 ratio in the F1 in Mendel’s second experiment: • This method of calculating traits of offspring in each generation is called a Punnett square.

19. Punnett Squares • The gene combination that might result from a genetic cross can be determined by drawing a Punnett square. • Punnett squares help to determine phenotypes and genotypes. • Phenotype – Physical characteristics (Purple) • Genotype – Genetic make-up (BB or Bb)

20. Independent Assortment • After showing alleles segregated during the formation of gametes, Mendel wondered if they did so independently. • In other words, does the segregation of one pair of alleles affect the segregation of another set of alleles? • Example: Does the gene for seed color have anything to do with the gene for seed shape?

21. Independent Assortment • To answer these questions, Mendel performed an experiment to follow two different genes as they passed from one generation to the next. • This experiment is known as the Two-Factor Cross or Dihybrid cross.

22. Mendel Extended His Analysis • Mendel chose two different characteristics of the pea: -Seed Coat: Smooth Wrinkled -Seed color: Yellow Green

23. Two-Factor (dihybrid) Cross • First, Mendel crossed pure-breeding plants that produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy). • All of the F1 offspring produced round yellow peas (RrYy). • This proved round and yellow must be dominant alleles. • Try doing the Punnett square.

24. RRYY r r y y

25. Independent assortment • This cross does not indicate whether genes assorted independently. • This cross only provided hybrids plants (F1) needed to produce an F2 generation that would provide the answers to Mendel's question.

26. Results • Mendel knew the F1 had a genotype of RrYy (heterozygous). • How would the alleles segregate into the F2 • Try the Punnett Square for the cross of F1 hybrids producing and F2 generation.

27. Results • After Mendel counted the offspring of the dihybrid cross, he could make the following conclusion.

28. Mendel Could Now Make A Second Generalization: • Genes for different traits segregate independently during the formation of gametes. • This became known as Mendel’s third principle: Independent Assortment

29. A Summary of Mendel’s Principles • Mendel’s principles form the basis of the modern science of genetics. These principles can be summarized as follows: • The inheritance of biological characteristics is determined by individual units known as genes. Genes are passed from parents to their offspring. • In cases in which two or more forms (alleles) of the gene for a single trait exist, some form of the gene may be dominant and the other recessive (Principles of Dominance). • In most sexually reproductive organisms, each adult has two copies of each gene; one from each parent. These genes are segregated from each other when gametes are formed (Principles of Segregation). • The alleles for different genes usually segregate independently of one another (Principles of independent assortment).

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