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Analyzing Inheritance

Section 11-1. Analyzing Inheritance. Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals.

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Analyzing Inheritance

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  1. Section 11-1 Analyzing Inheritance • Offspring resemble their parents. Offspring inherit genes for characteristics from their parents. To learn about inheritance, scientists have experimented with breeding various plants and animals. • In each experiment shown in the table on the next slide, two pea plants with different characteristics were bred. Then, the offspring produced were bred to produce a second generation of offspring. Consider the data and answer the questions that follow.

  2. Parents Long stems  short stems Red flowers  white flowers Green pods  yellow pods Round seeds  wrinkled seeds Yellow seeds  green seeds First Generation All long All red All green All round All yellow Second Generation 787 long: 277 short 705 red: 224 white 428 green: 152 yellow 5474 round: 1850 wrinkled 6022 yellow: 2001 green Section 11-1 • 1. In the first generation of each experiment, how do the characteristics of the offspring compare to the parents’ characteristics? • 2. How do the characteristics of the second generation compare to the characteristics of the first generation?

  3. Section Outline Section 11-1 • 11–1 The Work of Mendel A. Gregor Mendel’s Peas B. Genes and Dominance C. Segregation 1. The F1 Cross 2. Explaining the F1 Cross

  4. Section 11-1 • 11–1 The Work of Mendel • Gregor Mendel (1822-1884) was a monk at a monastery in Brunn, Austria • He taught school and did other chores, but in his spare time he set out to study plant breeding • He started breeding pea plants about 1855 • He wanted to learn the rules of heredity so that better varieties could be produced

  5. Section 11-1 • 11–1 cont. • Mendel’s original pea plants were true-breeding. • When allowed to self-pollinate they produced offspring identical to themselves • Mendel wanted to cross-pollinate the plants and study the results • Mendel studied seven different pea plant traits (specific characteristic that varies from individual to individual) with 2 contrasting characteristics

  6. Figure 11-3 Mendel’s Seven F1 Crosses on Pea Plants Section 11-1 Seed Shape Seed Color Seed Coat Color Pod Shape Pod Color Flower Position Plant Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall

  7. Section 11-1 • 11–1 cont. • Mendel called each original pair of plants the • P Generation (parental) and the offspring of these the F1 generation (first filial) • Mendel produced hybrids when he crossed parents with different traits; and to his surprise the offspring only resembled one of the parents

  8. Principles of Dominance Section 11-1 P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

  9. Principles of Dominance Section 11-1 P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

  10. Section 11-1 • 11–1 cont. • Mendel drew 2 conclusions from this experiment: • Biological inheritance is determined by factors that are passed from one generation to the next; today known as genes • Principle of Dominance which states that some alleles (different forms of a gene) are dominant and others are recessive

  11. Section 11-1 • 11–1 cont. • Dominant alleles will always exhibit that form of the trait and is represented with a capital letter ex. (T) - Tall • Recessive alleles will only exhibit that form of the trait if the dominant allele is not present and is represented with a lower case letter ex. (t) - short

  12. Section 11-1 • 11–1 cont. • Mendel then allowed his F1 hybrid plants to self-pollinate to produce an F2 generation (second filial) • Traits controlled by the recessive alleles reappeared in ¼ of the F2 generation • Mendel explained that the dominant allele masked the recessive allele in the F1 generation and due to the segregation of alleles during gamete formation reappeared in the F2 generation

  13. Principles of Dominance Section 11-1 P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short

  14. F1 Generation T t T t Gametes T t T t F2 Generation T T T t T t t t Section 11-1

  15. Section 11-2 Tossing Coins • If you toss a coin, what is the probability of getting heads? Tails? If you toss a coin 10 times, how many heads and how many tails would you expect to get? Working with a partner, have one person toss a coin ten times while the other person tallies the results on a sheet of paper. Then, switch tasks to produce a separate tally of the second set of 10 tosses.

  16. Section 11-2 • 1. Assuming that you expect 5 heads and 5 tails in 10 tosses, how do the results of your tosses compare? How about the results of your partner’s tosses? How close was each set of results to what was expected? • 2. Add your results to those of your partner to produce a total of 20 tosses. Assuming that you expect 10 heads and 10 tails in 20 tosses, how close are these results to what was expected? • 3. If you compiled the results for the whole class, what results would you expect? • 4. How do the expected results differ from the observed results?

  17. Section Outline Section 11-2 • 11–2 Probability and Punnett Squares A. Genetics and Probability B. Punnett Squares C. Probability and Segregation D. Probabilities Predict Averages

  18. Section 11-2 • 11–2 Probability and Punnett Squares • Probability is the likelihood that a particular event will occur and Mendel realized that the principles of probability could be used in predicting the outcomes of genetic crosses

  19. Section 11-2 • 11–2 cont. • Punnett Squares can be used to predict and compare the genetic variations that will result from a cross • The gametes are shown along the top and left side • Possible gene combinations of the offsping appear in the boxes

  20. Tt X Tt Cross Section 11-2

  21. Tt X Tt Cross Section 11-2

  22. Section 11-2 • 11–2 cont. • Homozygous organisms are true-breeding which means they have two identical alleles for a particular trait Ex. TT • Heterozygous organisms are hybrids that have two different alleles for the same trait Ex. Tt

  23. Section 11-2 • 11–2 cont. • Genotype vs. Phenotype • The genotype is the actual genetic makeup of the traits, while the phenotype is the actual physical characteristics of an organism

  24. Section Outline Section 11-3 • 11–3 Exploring Mendelian Genetics A. Independent Assortment 1. The Two-Factor Cross: F1 2. The Two-Factor Cross: F2 B. A Summary of Mendel’s Principles C. Beyond Dominant and Recessive Alleles 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Polygenic Traits D. Applying Mendel’s Principles E. Genetics and the Environment

  25. 11-3 Exploring Mendelian Genetics • After proving that alleles segregate in the formation of gametes, Mendel wanted to know if they did so independently – did the segregation of one pair of alleles affect the segregation of other pairs? • Two-Factor Cross – F1 • - Mendel crossed homozygous round yellow peas (RRYY) with wrinkled green peas (rryy). • - F1 were all round yellow (RrYy)

  26. Two-Factor – F2 • - Mendel knew F1’s were heterozygous for both traits • - when allowed to self-fertilize the F1 gave rise to F2’s w/ a • definite ratio • - this led Mendel to his Principle of Independent Assortment • - alleles for different characteristics segregate independently of • each other – so as long as genes for two traits are on • different chromosomes every combination of alleles is possible.

  27. Figure 11-10 Independent Assortment in Peas Section 11-3

  28. Alleles are separated during gamete formation “Factors” determine traits Some alleles are dominant, and some alleles are recessive Pea plants Law of Dominance Law of Segregation Concept Map Section 11-3 Gregor Mendel concluded that experimented with which is called the which is called the

  29. Incomplete Dominance • * when neither allele is dominant over the other • - heterozygotes differ phenotypically from either • homozygote • Ex.: Four O’clock plants • - RR = red • - WW = white • - RW = pink • ** this is not blending because red and white • flowers will reappear.

  30. Figure 11-11 Incomplete Dominance in Four O’Clock Flowers Section 11-3

  31. Figure 11-11 Incomplete Dominance in Four O’Clock Flowers Section 11-3

  32. Codominance • * situation in which both alleles contribute to the phenotype • - both are expressed if present • Ex.: chickens – white and black feathers are codominant • - heterozygotes produce both white and black feathers • ** A and B alleles in human blood types

  33. Multiple Alleles • * when a gene has more than two alleles • - each individual still only gets two of the possible alleles that exist in the population • Ex.: Human blood groups • - three alleles = A, B, O • - A and B are codominant • - O is recessive

  34. Polygenic Traits • * traits which are influenced by several genes • - results in a wide range of phenotypes • Ex.: Human height, skin color, color in eyes of fruitflies

  35. Section 11-3 Height in Humans • Height in pea plants is controlled by one of two alleles; the allele for a tall plant is the dominant allele, while the allele for a short plant is the recessive one. What about people? Are the factors that determine height more complicated in humans?

  36. Section 11-3 • 1. Make a list of 10 adults whom you know. Next to the name of each adult, write his or her approximate height in feet and inches. • 2. What can you observe about the heights of the ten people? • 3. Do you think height in humans is controlled by 2 alleles, as it is in pea plants? Explain your answer.

  37. Applying Mendel’s Principles • * Mendel’s principles apply to all organisms • - Thomas Hunt Morgan – began use of fruitflies • - ideal organism • - fast breeder • - lots of young • - many observable traits • - small size • - Environment also affects development of organisms

  38. Sex Linkage • - genes located on autosomes behave as we have studied • - genes located on the sex chromosomes often exhibit a unique • pattern of inheritance. • - in mammals females have two X-chromosomes (XX) • - in mammals males have an X and a Y chromosome (XY) • - the Y chromosome has a gene (SRY) that makes the • embryo male • - but Y has fewer genes than X and so males only get one • copy of some genes on the Y • - genes that males only get one copy of are sex-linked and are • expressed more often in males than females • - color-blindness, muscular dystrophy

  39. The most important fact of mitosis is that each daughter cell has the exact same genetic make-up as the original cell. • Gregor Mendel – The Father of Genetics • - didn’t know where genes were located • - described in detail how genes must move in the • formation of gametes and subsequent fertilization • - each organism must inherit a single copy of every gene from both of its parents • - each offspring therefore has two copies of each gene • - these two copies must be separated to form the gametes of this organism

  40. Chromosome Number • Normal body cells contain two copies of each chromosome • one received from each of the two parents • Homologous chromosomes • Same shape, size, and contain the same genes in same order • Diploid – term used to describe a cell with homologous chromosomes • Symbol – 2N • Found in all normal body (somatic) cells • Haploid – term describing a cell with a single copy of each chromosome • Symbol – N • Found in gametes (sex cells)

  41. Phases of Meiosis • Meiosis is a process of reduction division in which the chromosome number per cell is cut in half through the separation of homologous chromosomes in a diploid cell. (Diploid  Haploid) (2N  N) • - requires two distinct divisions – Meiosis I and Meiosis II • - allows organisms to reproduce sexually and maintain the normal diploid number in the offspring • Meiosis I = Reduction Division (Figure 11.15, pg 276) • - prior to meiosis I the chromosomes replicate

  42. A. Prophase I • - nucleolus, nuclear membrane break down • - centrioles migrate to poles • - homologous chromosomes pair up to form tetrads – 4 • chromatids • - crossing-over occurs • - homologous chromosomes exchange portions of • themselves which results in a mixing of genes between the two chromosomes

  43. Crossing-Over Section 11-4

  44. B. Metaphase I • - tetrads line-up at the equator of the cell • C. Anaphase I • - homologous pairs separate • - sister chromatids stay connected at their centromeres • D. Telophase I • - nuclear membranes reform around the chromosomes and cytyokinesis takes place • - each cell is now haploid

  45. ** No DNA replication takes place before Meiosis II • 2. Meiosis II (Figure 11.15, page 277) • - each cell’s chromosomes consist of two chromatids connected at the centromere • A) Prophase II • - just like prophase in mitosis • B) Metaphase II • - chromosomes align at center of cell • C) Anaphase II • - sister chromatids are separated at the centromere and are pulled to opposite poles • D) Telophase II • - new nuclear envelopes appear • - cytokinesis occurs

  46. Figure 11-17 Meiosis II Section 11-4 Meiosis II Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells.

  47. Results of Meiosis = Four haploid cells which are genetically unique • Gamete formation (Figure 11.17, page 278) • - in male animals and the pollen grains of plants the haploid gametes are called sperm cells • - in female animals generally only one of the cells formed by meiosis develops into an egg • - in female animals uneven cytokinesis at the end of Meiosis I and Meiosis II result in a large egg and 3 polar bodies

  48. Interest Grabber Section 11-4 How Many Chromosomes? • Normal human body cells each contain 46 chromosomes. The cell division process that body cells undergo is called mitosis and produces daughter cells that are virtually identical to the parent cell. Working with a partner, discuss and answer the questions that follow.

  49. Interest Grabber continued Section 11-4 • 1. How many chromosomes would a sperm or an egg contain if either one resulted from the process of mitosis? • 2. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? Do you think this would create any problems in the developing embryo? • 3. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

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