1/9/2014. Introduction to Genetics. The Work of Gregor Mendel THE WORK OF GREGOR MENDEL. Some Definitions:

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1 Introduction to Genetics Chapter 11 Chapter 11 Section 1 THE WORK OF GREGOR MENDEL The Work of Gregor Mendel Some Definitions: Genetics the study of biological inheritance and variation Chromosomes hereditary units of an organism Gene segment of a chromosome that determines traits Alleles different forms of a gene associated with a specific trait Hybrids offspring that result from crosses between 2 parents with different traits Dominant & Recessive some alleles (dominant) will completely prevent the expression of others (recessive) 1

2 The Work of Gregor Mendel Gregor Mendel s Peas Used ordinary garden peas from the monastery garden Each type used was self-pollinating (each flower has both male and female parts) Used several types of pea plants that were each true-breeding for particular characteristics If allowed to self-pollinate, true-breeding plants will produce offspring identical to themselves Tall plants produce only tall plants; short plants produce only short plants Wanted to produce crosses of different types Therefore had to prevent self-pollination The Work of Gregor Mendel Genes and Dominance Mendel studied seven different pea plant traits Each of the seven traits had two contrasting characters Green seed color or yellow seed color Tall plant or short plant Each original plant was called the P generation (parental generation) The offspring were called the F 1 generation (first filial generation) The offspring of parents with different traits are called hybrids (in other words, the F 1 generation) The Work of Gregor Mendel Genes and Dominance (continued) Mendel expected the F 1 offspring to have a blend of characteristics of both parental generation plants But ALL the offspring had the characters of just ONE of the parental types! In each cross, the character of the other parent seemed to have disappeared! 2

3 The Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short The Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short The Work of Gregor Mendel Genes and Dominance (continued) Mendel expected the F 1 offspring to have a blend of characteristics of both parental generation plants But ALL the offspring had the characters of just ONE of the parental types! In each cross, the character of the other parent seemed to have disappeared! Mendel s conclusion was that biological factors are passed from one generation to the next Today those factors are called genes 3

4 The Work of Gregor Mendel Genes and Dominance (continued) Each of the traits Mendel studied was produced by one gene that had two versions Each version of a gene is called an allele Mendel s second conclusion is called the Principle of Dominance Some alleles are dominant and some are recessive An organism with the dominant allele will always exhibit that trait A plant that has an allele for a tall plant will be tall; if does not have the allele for a tall plant, the plant will be short The following table shows the seven traits Mendel studied and the dominant allele for each The Work of Gregor Mendel Mendel s Seven F 1 Crosses on Pea Plants Seed Seed Seed Coat Pod Pod Flower Plant Shape Color Color Shape Color Position Height Round Yellow Gray Smooth Green Axial Tall Wrinkled Green White Constricted Yellow Terminal Short Round Yellow Gray Smooth Green Axial Tall The Work of Gregor Mendel Segregation The F 1 Cross Had the recessive alleles disappeared or were they just masked? Mendel let the F 1 plants produce offspring of their own, an F 2 generation The results were surprising The recessive allele returned! Roughly 25% of the F 2 offspring showed the missing trait from the P generation Explaining the F 1 Cross 4

5 The Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short The Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short The Principles of Dominance P Generation F1 Generation F2 Generation Tall Short Tall Tall Tall Tall Tall Short 5

6 The Work of Gregor Mendel Segregation (continued) Explaining the F 1 Cross The F 1 plants had to have a characteristic from each of the P generation plants The recessive allele in the F 1 plants was masked by the dominant allele for each specific trait During the F 1 plant s own reproduction the alleles for tallness had to be separated from each other, or segregated He concluded that during the formation of the sex cells, or gametes, there had to be segregation of the alleles The Work of Gregor Mendel Each gamete carries only a single copy of each gene Each F 1 plant produces two kinds of gametes Half have the allele for tall plants and have the allele for short plants In the example (left), Red is dominant to White so F 1 is all red and F 2 has both red and white (only 25% white) Real World Example In horses, the black gene uses the designation E and a black coat is dominant to a chestnut coat. A black horse may be EE or Ee A chestnut horse is ee An EE black horse crossed with an ee chestnut horse will always produce an Ee black horse. 6

7 Real World Example Definitions (again) Genotype the genetic makeup of an organism (which alleles it has) Phenotype the physical characteristic of an organism (what it looks like) Examples: Top & Bottom Genotype: EE or Ee Phenotype: black Middle Genotype: ee Phenotype: chestnut Another Real World Example The white color pattern Tobiano in horses (left) is dominant to nonpatterned horses. The Tobiano gene uses the designation T if the gene is present (and n if not). A tobiano horse may be TT or Tn A TT horse (top left) crossed with a solid color horse will always have a tobiano patterned offspring The adult horses pictured are the actual parents of the baby horse (called a foal). Real World Example Many diseases and medical conditions in humans follow the principles of Mendelian genetics They are caused by a single gene and are recessive Sickle-cell anemia Tay-Sachs disease Babies appear normal at birth then nerves begin to be damaged. The baby becomes blind, then deaf, then paralyzed and usually dies before age 4. Cystic fibrosis Albinism 7

8 The Work of Gregor Mendel 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. The Work of Gregor Mendel Parents First Generation Second Generation Long stems short stems Red flowers white flowers Green pods yellow pods All long All red All green 787 long: 277 short 705 red: 224 white 428 green: 152 yellow Round seeds wrinkled seeds All round 5474 round: 1850 wrinkled Yellow seeds green seeds All yellow 6022 yellow: 2001 green 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? Chapter 11 Section 2 PROBABILITY AND PUNNETT SQUARES 8

9 Probability and Punnett Squares Genetics and Probability Since Mendel s results were repeated regardless of the trait he was looking at (and he grew some 29,000 pea plants over the course of his studies), he concluded that probability could be used to explain the results The likelihood that event will occur is called probability 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 Probability and Punnett Squares Genetics and Probability Activity 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. Now answer the questions on the following slide. Probability and Punnett Squares Genetics and Probability Activity 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? 9

10 Probability and Punnett Squares Genetics and Probability Flip a coin, you have a 1 out of 2 chance of it coming up heads Each flip is an independent event so the chance of flipping heads 3 times in a row is: ½ x ½ x ½ = 1/8 How does this relate to genetics? The way in which alleles segregate is completely random so probability can be used to predict the outcome of genetic crosses Probability and Punnett Squares Punnett Squares A diagram that shows gene combinations that might result from a particular cross Quick Lab - Making Connections Find a partner. Each pair of students will receive a paper bag with 4 paper clips inside. They are identical except for color. Three are blue and one is red. What is the probability of picking a red item? Of picking a red item two times in a row? Without looking, pick an item from the bag (replacing it each time) 20 times, then 60 times. Keep track of your results. How many red did you expect for 20? For 60? Did your results equal your calculated probabilities for 20? For 60? 10

11 Probability and Punnett Squares Punnett Squares A diagram that shows gene combinations that might result from a particular cross Organisms that have two identical alleles for a trait are said to be homozygous for that trait Homozygous organisms are true-breeding BB flowers always are purple bb flowers are always white Each combination of genes is the organisms genotype This square shows 3 genotypes: BB, Bb, and bb Probability and Punnett Squares Punnett Squares Organisms that have two different alleles for a trait are heterozygous For a trait that exhibits complete dominance, a homozygous dominant and a heterozygous organism will physically appear the same they will have the same phenotype You can tell which is which only by studying their offspring Probability and Punnett Squares Probability and Segregation Since there is an allele for tall in 3 of the combinations you expect a 3:1 ratio of tall to short plants But 1/3 of the tall plants are homozygous dominant and 2/3 are heterozygous Their phenotype ratio is 3:1 Their genotype ratio is 1:2:1 Is this what we ve found? 11

12 Probability and Punnett Squares Probabilities Predict Averages Probability predicts average outcome of a large number of events It cannot predict the outcome of a specific event Not all families with 4 children have exactly 2 boys and 2 girls Not all families with 2 children have 1 girl and 1 boy The samples are two small! Choose 1000 families with 4 children each and you d find pretty close to 2000 boys and 2000 girls among all of the children The larger the sample, the closer to the expected outcome Quick Lab How are dimples inherited? I. Title: How are Dimples Inherited (page 268) II. Purpose: By using random number to assign a genotype, students will be able to conclude how dimples are inherited III. Safety: none IV. Procedure: 1. Write the last four digits of any phone number. Odd digits represent the dominant allele; even digits represent the recessive allele 2. Use the first two digits to represent the father s genotype. Write his genotype. V. Data/Observations: Quick Lab How are dimples inherited? IV. Procedure: V. Data/Observations: 3. Use the last two digits to represent the mother s genotype. Write her genotype Construct a Punnett square for 4. the cross of these parents. Then, using that Punnett square, determine the probability that their child will have dimples. (remember the allele for dimples is dominant) 5. Determine the class average of the percent of children with 5. dimples. 12

13 Quick Lab How are dimples inherited? VI. Calculations/Questions: 1. How does the class average compare with your results? 2. How does the class average compare with the result of a cross of two heterozygous parents? 3. What percentage of the children will be expected to have dimples if one parent is homozygous for dimples (DD) and the other is heterzygous (Dd)? VII.Conclusion: Summarize your findings. Do you see how a dominant trait, such as dimples, is inherited? Chapter 11 Section 3 EXPLORING MENDELIAN GENETICS Independent Assortment Mendel knew from his studies of pea plants in the F 2 generation that alleles segregated during reproduction but do they segregate independently? Does a round seed always have to be yellow? Mendel designed an experiment to follow two genes as they passed from one generation to another This is called a two factor or dihybrid cross 13

14 Independent Assortment The Two-factor Cross Each allele segregates independently half of all gametes will have each allele For Smooth Yellow seeds, half will be smooth, half not smooth Then half will be yellow, half will be not yellow The F1 plants are heterozygous for both characteristics Proving that genes independently assort can be seen in the results shown using a 4x4 Punnett square Four different phenotypes observed Nine genotypes observed Mendel s F2 plants produced 556 seeds as he studied round vs. wrinkled AND yellow vs. green 315 plants were round and yellow 32 were wrinkled and green 209 were mixed 101 yellow and wrinkled 108 green and round Therefore, the genes had to sort independently of one another! 14

15 Conclusions led to the Principle of Independent Assortment When two traits are considered in the same cross, the segregation of one pair of alleles is independent of the segregation of the other pair of alleles. There could be four possible gametes The Punnett square works on this assumption that each gamete occurs about 1/4 of the time This is why the predicted ratio is 9:3:3:1. NOTE: Independent assortment is not always true but since many do, this accounts for the tremendous variation in living things! A Summary of Mendel s Principles The inheritance of biological characteristics is determined by genes Genes are passed from parents to their offspring during reproduction When two or more alleles for a gene exist, some forms may be dominant and others may be recessive Generally, each organism has two copies of every gene one from each parent The alleles for different genes usually segregate independently of one another 15

16 Beyond Dominant and Recessive Alleles There are exceptions to Mendel s principles! Most genes have more than one allele Many important traits are controlled by more than one gene Just because an allele is dominant doesn t mean it is common Some alleles are neither dominant nor recessive! Incomplete Dominance One allele is not completely dominant over another The heterozygous phenotype is somewhere in between the two homozygous phenotypes Example Four O clock flowers; snapdragons, coat colors in many animals, etc. The F1 generation shows a phenotype that is different than either P generation parents Red and white snapdragons produce all pink flower in F1 F2 generation shows the F1 generation phenotype as well as both P generation phenotypes A C B Example A: C CR C CR double dose of cream gene Example B: C CR C single dose of cream gene Example C: CC fully pigmented; no cream gene 16

17 Co-dominance Both alleles will contribute to the phenotype Example in some chickens there is an allele for black feathers and an allele for white feathers. Chickens with both alleles will NOT have gray feathers but will have both black and white feathers (called erminette) Example blood type genes from both parents combine to establish your blood type A type A father and a type B mother can have offspring with type AB blood Multiple Alleles Most genes have more than two alleles An individual can still have only two alleles though! Some of those alleles can be dominant to others, codominant, incomplete dominant or recessive! Example Blood type there are 3 alleles I A, I B, and i I A and I B are dominant to i but are co-dominant to each other Example (page 273 in text) rabbit coat colors 4 alleles c has no color, producing an albino, recessive to all others; c h restricts color to certain areas of the body (making Himalayan), dominant to c and recessive to all others; c ch shows a partial color change (called chinchilla), partially dominant to c and c h, recessive to C; C is full color and dominant to all others Polygenic Traits Traits controlled by two or more genes Most traits fall under this category! Top right horse is a homozygous black horse (like top left) with TWO modifying genes Bottom right horse is a bay color (normally like lower left color) with 3 alleles of two color modifying genes! 17

18 Applying Mendel s Principles Law of Segregation Law of Independent Assortment Both apply to more than just plants! Humans, plants, insects, animals, etc. Genetics and the Environment Characteristics determined by genes inherited from parents Those same characteristics may be affected by the environment as well Genes provide a plan but how the plan develops is often determined by environment You may have genes for being tall but poor nutrition doesn t let you grow as tall as your genes would allow Chapter 11 Section 4 MEIOSIS General Information about Mendel did not know exactly where genes were located but it was fairly quickly determined to be located on the chromosomes in the nucleus of a cell. Mendel s principles of genetics requires Each organism must inherit a single copy of every gene from both its parents When an organism produces its own gametes, those two sets of genes must separate from each other so that each gamete contains only one set of genes 18

19 Chromosome Number The two sets of chromosomes that every organism has are called homologous chromosomes Chromosomes appear in pairs; one of each pair from each parent Gametes will have half the number of chromosomes The number that represents the number in the gametes is N and is called haploid. All other cells have both sets of chromosomes are so are called 2N or diploid. 2N=46 in humans; fruit flies 2N=8 Phases of During the haploid gamete cells are produced from diploid cells It involves two distinct divisions called meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells I Each chromosome is replicated and the cells begin to divide almost like they do in mitosis Instead of lining up individually in prophase Overview of 19

20 I As in mitosis, cells grow and develop during Interphase I, then duplicate their DNA! I Unlike mitosis, in Prophase I of meiosis, each chromosome pairs with its analogous chromosome to form a tetrad Crossing over occurs here! Crossing over Occurs during Prophase I of I (1) Chromosomes line up in analogous pairs (2) They cross over one another (3) The crossed sections are exchanged This creates recombinant DNA It is different from either parent s chromosomes 20

21 Overview of the process of Crossing over I In Metaphase I, spindle fibers attach to the chromosomes at the centromere They are still lined up in pairs, unlike in mitosis. I In Anaphase I, the spindle fibers pull the homologous chromosomes to opposite ends of cell. Still diploid but starting to become haploid! 21

22 I In Telophase I and Cytokinesis, the nuclear envelope reforms and the cell divides in two. But neither daughter cell has two of each chromosome! They have become haploid! Figure II I Took a diploid cell and created two haploid daughter cells Two haploid daughter cells now enter a second phase of meiotic division called II II Takes each of the I daughter cells and produces two more haploid daughter cells Entire process of I and II takes one diploid cell and makes four haploid daughter cells II Prophase II Metaphase II Anaphase II Telophase II I results in two The chromosomes line up in a The sister chromatids II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. In Prophase II, there is no chromosome replication this time but nuclear envelope disappears and spindles form. 22

23 II Prophase II Metaphase II Anaphase II Telophase II I results in two The chromosomes line up in a The sister chromatids II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. In Metaphase II, chromosomes line up individually like in mitosis II Prophase II Metaphase II Anaphase II Telophase II I results in two The chromosomes line up in a The sister chromatids II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. In Anaphase II, sister chromatids are pulled apart and move to opposite ends of the cell II Prophase II Metaphase II Anaphase II Telophase II I results in two The chromosomes line up in a The sister chromatids II results in four haploid (N) daughter cells, similar way to the metaphase separate and move toward haploid (N) daughter cells. each with half the number of stage of mitosis. opposite ends of the cell. chromosomes as the original. In Telophase II and Cyntokinesis, four total daughter cells are formed; nuclear envelope reappears. 23

24 Gamete Formation In males The four haploid daughter cells at the end of meiosis II are four equally sized sperm In females Generally only one of the four haploid daughter cells at the end of meiosis II forms an egg The egg takes nearly all of the cytoplasm The remaining 3 form polar bodies which are not used in reproduction» The polar bodies pick up the extra sets of chromosomes so that too many are not contained in the egg Comparing Mitosis and Mitosis A diploid cell produces two A diploid cell produces four diploid daughter cells haploid daughter cells Asexual reproduction Sexual reproduction Allows an organism s body Produces gametes only to grow and replace cells Does not occur in organisms that do not reproduce through sexual reproduction! Chapter 11 Section 5 LINKAGE AND GENE MAPS 24

25 Linkage and Gene Maps Gene Linkage When genes for two different traits are always (or nearly always) inherited together Linked genes are close together on the same chromosome Rarely assort independently so they are inherited together Still can be separated but not often Genes that are close together tend to stay together, genes that are far apart tend to separate The chromosomes sort independently but not individual genes! Linkage and Gene Maps Gene Maps Crossing over separates genes on the same chromosome Can sometimes separate linked genes How far apart genes were could be seen in how frequently they were linked The farther apart, the less often they d be linked The rate that linked genes were separated could be used to produce a map showing where on a chromosome a gene is located Linkage and Gene Maps Gene map of chromosome 2 of the fruit fly (as estimated in 1911) Genes are named after the abnormal problem they cause 25

26 Challenge Question The allele that causes galactosemia (g) is recessive to the allele for normal lactose metabolism (G). A normal woman whose father had galactosemia marries a man with galactosemia who had normal parents. They have three children, two normal and one with galactosemia. What are the genotypes of each: The woman, her father, her husband, the husbands parents, and their 3 children? 26