1. End Show Slide 1 of 32 Copyright Pearson Prentice Hall Ch. 11: Introduction to Genetics Mendel 11-1 The Work of Gregor Mendel

2. End Show Slide 2 of 32 Traits are characteristics that living things have. They are used to identify living things and to group them. Scientists put living things having the same group traits in the same group. For example, a large mammal that had a trunk would be identified as an elephant. All living things that had these similar traits – large size, being a mammal, having a trunk – would also be identified as elephants. Copyright Pearson Prentice Hall

3. End Show Slide 3 of 32 Organisms within a group share traits but no two are exactly alike. All organisms have individual differences or individual traits. All elephants are large, but some may be larger than others. Copyright Pearson Prentice Hall

4. End Show Slide 4 of 32 Many of an organism’s traits, whether they are group or individual traits, are inherited. That is, they are determined by factors that are passed from parents to offspring. The study of genetics, which is the study of heredity or the delivery of characteristics from parent to offspring, will help us to explain HOW traits are passed from generation to generation. Copyright Pearson Prentice Hall

5. End Show 11-1 The Work of Gregor Mendel Slide 5 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas Gregor Mendel was an Austrian monk whose work was important to the understanding of heredity. Mendel carried out his work with ordinary garden peas. Peas are small and easy to grow. A single pea plant can produce hundreds of offspring. Today we call peas a “model system.” Gregor Mendel’s Peas

6. End Show 11-1 The Work of Gregor Mendel Slide 6 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas Mendel knew that the male part of each flower produces pollen, (containing sperm). the female part of the flower produces egg cells.

7. End Show 11-1 The Work of Gregor Mendel Slide 7 of 32 Copyright Pearson Prentice Hall Male parts: stamen Female parts: pistil

8. End Show 11-1 The Work of Gregor Mendel Slide 8 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas During sexual reproduction, sperm and egg cells join in a process called fertilization. Fertilization produces a new cell. In peas, this new cell develops into a tiny embryo encased within a seed.

9. End Show 11-1 The Work of Gregor Mendel Slide 9 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas Pea flowers are self-pollinating. Sperm cells in pollen fertilize the egg cells in the same flower. The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them. Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.

10. End Show 11-1 The Work of Gregor Mendel Slide 10 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas Mendel wanted to “cross” his stocks of true-breeding plants by joining male and female reproductive cells from two different plants. He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.

11. End Show 11-1 The Work of Gregor Mendel Slide 11 of 32 Copyright Pearson Prentice Hall Gregor Mendel’s Peas This process is called cross-pollination. Mendel was able to produce seeds that had two different parents. The offspring, then, would have traits different from its parents.

12. End Show 11-1 The Work of Gregor Mendel Slide 12 of 32 Copyright Pearson Prentice Hall Genes and Dominance Mendel studied seven pea plant traits, each with two contrasting characters or forms. For example, he studied plant height (the trait), which had 2 different characters, either short or tall. One by one, Mendel crossed plants with contrasting characters for each of the seven traits. He then studied their offspring.

13. End Show 11-1 The Work of Gregor Mendel Slide 13 of 32 Copyright Pearson Prentice Hall Genes and Dominance Each original pair of plants is said to be the P (parental) generation. The offspring are called the F 1, or “first filial,” generation. The offspring of crosses between parents with different traits are called hybrids. Mendel found that the F 1 hybrid plants of his crosses all had the character of only one of the parents.

14. End Show 11-1 The Work of Gregor Mendel Slide 14 of 32 Copyright Pearson Prentice Hall Genes and Dominance Mendel’s F 1 Crosses on Pea Plants From p. 264

15. End Show 11-1 The Work of Gregor Mendel Slide 15 of 32 Copyright Pearson Prentice Hall Genes and Dominance Mendel’s Seven F 1 Crosses on Pea Plants Mendel’s F 1 Crosses on Pea Plants

16. End Show 11-1 The Work of Gregor Mendel Slide 16 of 32 Copyright Pearson Prentice Hall Genes and Dominance Mendel's first conclusion was that inheritance is determined by “factors” that are passed from one generation to the next. Today, scientists call the “factors” that determine traits genes. Mendel’s Conclusions

17. End Show 11-1 The Work of Gregor Mendel Slide 17 of 32 Copyright Pearson Prentice Hall A gene is a portion of a chromosome that determines a certain trait. Remember, chromosomes are found in the cell nucleus and are made up of DNA and proteins. Each chromosome contains many genes.

18. End Show 11-1 The Work of Gregor Mendel Slide 18 of 32 Copyright Pearson Prentice Hall Genes and Dominance Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. Mendel was very lucky in the traits that he chose to study because most traits are determined by several genes working together. In humans, for example, the shape of the eyes or ears is polygenic, controlled by many genes. Also, for each gene, there are usually more than 2 contrasting forms.

19. End Show 11-1 The Work of Gregor Mendel Slide 19 of 32 In any case, the different forms of a gene are called alleles. In Mendel’s pea plants, each true-breeding parent contributed one allele (one form of the gene) at fertilization. The offspring, therefore, received a total of 2 alleles, one of each form (or character). The tall plant, for example, contributed a tall allele and the short plant contributed a short allele. The offspring thus had one tall and one short allele. Copyright Pearson Prentice Hall

20. End Show 11-1 The Work of Gregor Mendel Slide 20 of 32 Copyright Pearson Prentice Hall Mendel’s second conclusion resulted from his observations regarding the offspring of parents with different alleles. It is called the principle of dominance.

21. End Show 11-1 The Work of Gregor Mendel Slide 21 of 32 Copyright Pearson Prentice Hall Genes and Dominance The principle of dominance states that some alleles are dominant and others are recessive.

22. End Show 11-1 The Work of Gregor Mendel Slide 22 of 32 Copyright Pearson Prentice Hall Genes and Dominance Mendel observed that the offspring of his crosses had the character of only one of the parents. In the tall x short cross, for example, the offspring were all tall. He concluded that the tall trait was dominant and the short one was recessive. He concluded that an organism with a dominant allele for a trait will always exhibit that form of the trait and that an organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.

23. End Show 11-1 The Work of Gregor Mendel Slide 23 of 32 All of the offspring in Mendel’s F 1 generation received one dominant allele and one recessive allele. But, because the recessive trait is not exhibited due to the presence of the dominant allele, the plants exhibited only the dominant traits. Copyright Pearson Prentice Hall

24. End Show 11-1 The Work of Gregor Mendel Slide 24 of 32 Copyright Pearson Prentice Hall As you will note in the previous slide, in genetics, dominant alleles are represented by an upper case letter (T) and recessive alleles are represented by a lower case one (t). Since each individual has a total of 2 alleles (one from each parent), their genetic makeup for a trait is represented by two letters. For the Mendel’s F 1 tall plants that would be Tt.

25. End Show 11-1 The Work of Gregor Mendel Slide 25 of 32 Copyright Pearson Prentice Hall Segregation Next Mendel crossed the F 1 generation with itself to produce the F 2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F 2 plants.

26. End Show 11-1 The Work of Gregor Mendel Slide 26 of 32 Copyright Pearson Prentice Hall Mendel's F 2 Generation P Generation F 1 Generation Tall Short F 2 Generation Segregation

27. End Show 11-1 The Work of Gregor Mendel Slide 27 of 32 Copyright Pearson Prentice Hall Segregation Mendel had assumed that a dominant allele had masked the corresponding recessive allele in the F 1 generation. But how could he explain that the trait controlled by the recessive allele showed up in some of the F 2 plants?

28. End Show 11-1 The Work of Gregor Mendel Slide 28 of 32 Copyright Pearson Prentice Hall Segregation The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness. That is, when the F 1 tall plants were crossed, Tt x Tt, the short alleles (t) had somehow segregated from the tall alleles (T) and found each other (tt) to produce a short plant.

29. End Show 11-1 The Work of Gregor Mendel Slide 29 of 32 Copyright Pearson Prentice Hall Segregation Mendel suggested that the alleles for tallness and shortness in the F 1 plants segregated from each other during the formation of the sex cells, or gametes. This is called the principle of segregation.

30. End Show 11-1 The Work of Gregor Mendel Slide 30 of 32 Copyright Pearson Prentice Hall Segregation The alleles separated during gamete formation.

31. End Show 11-1 The Work of Gregor Mendel Slide 31 of 32 Copyright Pearson Prentice Hall Segregation When each F 1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F 1 plant produces two types of gametes—those with the allele for tallness, and those with the allele for shortness. The F 2 generation had new combinations of the alleles.