Ch. 14 Mendel & The Gene Idea

1. Ch. 14 Mendel & The Gene Idea Ms. Whipple – Brethren Christian High School

2. 1. What was the “blending” theory of heredity that was popular during the 1800s? • One possible explanation for heredity was a “blending” hypothesis. • This hypothesis proposes that genetic material contributed by each parent mixes in a manner analogous to the way blue and yellow paints blend to make green. • With blending inheritance, a freely mating population would eventually give rise to a uniform population of individuals. • Everyday observations and the results of breeding experiments tell us that heritable traits do not blend to become uniform. Section 14.1

3. 2. Who first pioneered our modern understanding of inheritance? Give me a brief description of his life before discovering the mechanism of inheritance? What contributed to him becoming a scientist? • Modern genetics began in an abbey garden, where a monk named Gregor Mendel documented a particulate mechanism of inheritance. • Mendel grew up on a small farm in what is today the Czech Republic. • In 1843, Mendel entered an Augustinian monastery. • Mendel studied at the University of Vienna from 1851 to 1853, where he was influenced by a physicist who encouraged experimentation and the application of mathematics to science and by a botanist who stimulated Mendel’s interest in the causes of variation in plants. • These influences came together in Mendel’s experiments. • After university, Mendel taught school and lived in the local monastery, where the monks had a long tradition of interest in the breeding of plants, including peas. • Around 1857, Mendel began breeding garden peas to study inheritance. Section 14.1

4. 3. What is a Character? How is this different than a Trait? • Organisms within a species come in many varieties that have distinct heritable features, or characters, with different variant traits. Section 14.1

5. 4. What were the advantages of choosing garden peas for Mendel’s research? How did he control their reproduction? • There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits • Mating can be controlled • Each flower has sperm-producing organs (stamens) and an egg-producing organ (carpel) • Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another Section 14.1

6. EXPERIMENT Figure 14.3-3 P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants Section 14.1

7. 5.What is meant by the term True-Breeding? How do you bring about a true breeding pea plant? • Mendel started his experiments with varieties that were true-breeding. • When true-breeding plants self-pollinate, all their offspring have the same traits as their parents. Section 14.1

8. 6. What was Mendel’s typical breeding experiment? In your explanation, define the P Generation, F1 Generation, & F2 Generation. • In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced Section 14.1

9. EXPERIMENT Figure 14.3-3 P Generation (true-breedingparents) Purpleflowers Whiteflowers F1 Generation(hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple-floweredplants 224 whitefloweredplants Section 14.1

10. 7. How did Mendel’s experiment bring him to the discovery of Dominant & Recessive traits? Give me a specific example in your explanation. • Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids • However, the factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait. Section 14.1

11. 8. Alternate versions of Genes are called: • Alleles!!! • Alternative versions of genes account for variations in inherited characters • For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers • Each gene resides at a specific locus on a specific chromosome. Section 14.1

12. Figure 14.4 Allele for purple flowers Pair ofhomologouschromosomes Locus for flower-color gene Allele for white flowers Section 14.1

13. 9. What is the Law of Segregation? Mendel’s law of segregation states that the two alleles for a heritable character segregate (separate) during gamete production and end up in different gametes. • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis. • If an organism has two identical alleles for a particular character, then that allele is present as a single copy in all gametes. • If different alleles are present, then 50% of the gametes will receive one allele and 50% will receive the other. Section 14.1

14. 10. Review: During the process of Meiosis, how do the homologous chromosomes (one from mom and one from pop) reshuffle their alleles? When do they do this? What is the end result of this while making gametes? • Homologous Chromosomes reshuffle their alleles during prophase I of Meiosis. This reshuffling of alleles called CROSSING OVER and then independent assortment of chromosomes results in non-identical gametes by the end of meiosis. Section 14.1

15. 11. What is a Punnett Square? What is its purpose? Give me an example. • A Punnett square may be used to predict the results of a genetic cross between individuals of known genotype by showing all possible combinations of parental alleles. Section 14.1

16. 12. Define the following terms: • Homozygous: An organism with two identical alleles for a character • Heterozygous: An organism that has two different alleles for a gene • Phenotype: Physical Expression of a Trait • Genotype: Alleles present in an individual for a specific character (i.e. WW; Bb) Section 14.1

17. 13. How can two organisms have the same Phenotype but not the same Genotype? • A Homozygous Dominant Individual for Brown eyes (BB) and Heterozygous Individual (Bb) will both have the same Phenotype – Brown Eyes. Section 14.1

18. 14. What is the purpose of a test cross? • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding themystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous Section 14.1

19. 15. What is a Monohybrid Cross? A Dihybrid Cross? • Monohybrid Crosses look at a single trait and what possible offspring can be produced from two parental genotypes. • Dihybrid crosses involve two traits. Dihybrid crosses are predictions of how two traits will show up in the offspring produced by a mating between two parent organisms. Section 14.1

20. 16. From Figure 14.8, how would the results differ in the F2 generation if alleles assorted dependently or independently? What was the result? • If the alleles assort dependently you would predict the same outcome as a monohybrid cross with a phenotypic ratio of 3:1. • If the alleles assort independently you would predict a phenotypic ratio of 9:3:3:1 • Mendel’s experiments show that alleles assort independently with a ratio of 9:3:3:1. Section 14.1

21. 17. What is the Law of Independent Assortment? Does this law pertain to genes found on separate chromosomes or the same chromosomes (homologous chromosomes)? • Using a dihybrid cross, Mendel developed the law of independent assortment • The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation • Strictly speaking, this law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome • Genes located near each other on the same chromosome tend to be inherited together Section 14.1cc

22. 1. What is the Multiplication Rule? How does it apply to Mendelian inheritance? • We can use the multiplication rule to determine the probability that two or more independent events will occur together in some specific combination. • Compute the probability of each independent event and then multiply the individual probabilities to obtain the overall probability of these events occurring together. • The probability that two coins tossed at the same time will both land heads up is 1/2 × 1/2 = 1/4. • Similarly, the probability that a heterozygous pea plant (Pp) will self-fertilize to produce a white-flowered offspring (pp) is the probability that a sperm with a white allele will fertilize an ovum with a white allele. This probability is 1/2 × 1/2 = 1/4. Section 14.2

23. 2. What is the Addition Rule? How does it apply to Mendelian inheritance? • We can use the addition ruleto determine the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous. • The probability of an event that can occur in two or more mutually exclusive ways is the sum of the individual probabilities of those ways. • The probability of obtaining an F2 heterozygote by combining the dominant allele from the egg and the recessive allele from the sperm is 1⁄4. • The probability of combining the recessive allele from the egg and the dominant allele from the sperm also 1⁄4. • Using the rule of addition, we can calculate the probability of an F2 heterozygote as 1⁄4 + 1⁄4 = 1⁄2. Section 14.2

24. 1. How is Complete Dominance different than Incomplete Dominance? Give me an example of each. • Complete dominanceis exhibited when one allele completely masks the expression of another, this is characteristic of Mendel’s crosses. • However, some alleles show Incomplete dominance, in which heterozygotes show a distinct intermediate phenotype not seen in homozygotes. Section 14.3

25. 2. How is Incomplete Dominance different than Co-Dominance? Give me an example of each. • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties. Example, Flower color (white, red, pink) • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways. Example, blood type (Type A, B, AB, & O) Section 14.3

26. 3. Briefly describe the relationship of Dominance & Phenotype in the expression of Tay-Sach’s Disease. • For any character, dominance/recessiveness relationships depend on the level at which we examine the phenotype. For example, humans with Tay-Sachs disease lack a functioning enzyme to metabolize certain lipids. These lipids accumulate in the brain, harming brain cells and ultimately leading to death. • Children with two Tay-Sachs alleles (homozygotes) have the disease. • Both heterozygotes, with one allele coding for a functional enzyme, and homozygotes, with two such alleles, are healthy and normal. • Thus at the organismal level, the allele for the functional enzyme is dominant to the Tay-Sachs allele. • The activity level of the lipid-metabolizing enzyme is reduced in heterozygotes. • Thus, at the biochemical level, the alleles show incomplete dominance. • Heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules. • At the molecular level, the Tay-Sachs and functional alleles are codominant. Section 14.3

27. 4. What is Multiple Alleles? Give me an example. • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Section 14.3

28. (a) The three alleles for the ABO blood groups and their carbohydrates Figure 14.11 Allele IA IB i none Carbohydrate B A (b) Blood group genotypes and phenotypes Genotype ii IAIA or IAi IBIB or IBi IAIB Red blood cellappearance Phenotype(blood group) A AB O B

29. 5. What is Pleiotropy? Give me an example. • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Section 14.3

30. 6. What is Epistasis? Give me an example. • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in Labrador retrievers and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Section 14.3

31. Figure 14.12 BbEe BbEe Sperm 1/4 1/4 1/4 1/4 Be BE be bE Eggs 1/4 BE BbEE BBEe BbEe BBEE 1/4 bE BbEE bbEe bbEE BbEe 1/4 Be BBEe BBee Bbee BbEe 1/4 be BbEe bbEe bbee Bbee : 3 9 : 4

32. 7. Describe Polygenic Inheritance of Human skin color. Use the term Quantitative Characters in your description. • Quantitative characters are those characteristics that vary in the population along a continuum. • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance Section 14.3

33. 7. Describe Polygenic Inheritance of Human skin color. Use the term Quantitative Characters in your description. • Let’s assume that skin color in humans is controlled by three independent genes. • Imagine that each gene has two alleles, one light and one dark, which demonstrate incomplete dominance. • An AABBCC individual is very dark; an aabbccindividualis very light. • A cross between two AaBbCc individuals (with intermediate skin shade) produces offspring with a wide range of shades. • Individuals with intermediate skin shades are most common, but some very light and very dark individuals may be produced as well. • The range of phenotypes forms a normal distribution if the number of offspring is great enough. • The majority of offspring would have intermediate phenotypes (skin color in the middle range). • Environmental factors, such as sun exposure, also affect skin color and contribute to a smooth normal distribution. Section 14.3

34. Figure 14.13 AaBbCc AaBbCc Sperm 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 Eggs 1/8 1/8 1/8 1/8 Phenotypes: 1/64 6/64 15/64 20/64 6/64 1/64 15/64 Number ofdark-skin alleles: 1 2 3 4 5 0 6

35. 8. What does the phrase “Nature vs. Nurture” mean when talking about genes & their expression? • Phenotype depends on both environment and genes, this is meant by the term “Nature vs. Nurture” • A single tree may have leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. • For humans, nutrition influences height, exercise alters build, sun-tanning darkens skin, and experience improves performance on intelligence tests. • Even identical twins, who are genetically identical, accumulate phenotypic differences as a result of their unique experiences. Section 14.3

36. 9. What is the Norm of Reaction? What does it mean if a character is Multifactorial? Give me examples. • The norm of reaction is the phenotypic range of a genotype influenced by the environment • For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity • Norms of reaction are generally broadest for polygenic characters • Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Section 14.3

37. Figure 14.14

38. 1. What is a Pedigree? How are these useful to society? • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees • This is especially useful for studying human genetics as many other methods for studying genetics are unethical to apply to humans. • A pedigree can help us understand the past and predict the future. • We can use normal Mendelian rules, including the multiplication and addition rules of probability, to predict the probabilities of specific phenotypes. Section 14.4

39. Key Figure 14.15 Mating Affected female Male Affectedmale Female Offspring 1stgeneration Ff ff Ff Ff 1stgeneration Ww ww ww Ww 2ndgeneration 2ndgeneration ff ff ff Ff FF or Ff Ff Ww ww ww Ww Ww ww 3rdgeneration 3rdgeneration ff FForFf WWorWw ww No widow’speak Freeearlobe Widow’speak Attachedearlobe b) Is an attached earlobe a dominantor recessive trait? (a) Is a widow’s peak a dominant orrecessive trait?

40. 2. For recessively inherited disorders, only homozygous recessive individuals exhibit the disorder. Why don’t heterozygotes have the disorder since they have one recessive allele? • Recessively inherited disorders show up only in individuals homozygous for the allele • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents • Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Section 14.4

41. 3. What factors contribute to certain disorders are found in higher prevalence among a certain group of individuals (such as Tay-Sachs disease among Ashkenazi Jews) • Genetic disorders are not evenly distributed among all groups of humans. • This is due to the different genetic histories of the world’s people during times when populations were more geographically and genetically isolated. Section 14.4

42. 4. A taboo on inbreeding (the mating of close relatives) is found in almost every human society on earth. Why do you think this taboo is necessary for a healthy population? What is an example of inbreeding being detrimental to a species? • If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low • Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele • Most societies and cultures have laws or taboos against marriages between close relatives Section 14.4

43. 5. Please describe the evolution of Sickle Cell Anemia and the theorized reason for its prevalence among people of African descent. • Sickle-cell disease affects one out of 400 African-Americans • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • In homozygous individuals, all hemoglobin is abnormal (sickle-cell) • Symptoms include physical weakness, pain, organ damage, and even paralysis Section 14.4

44. 5. Please describe the evolution of Sickle Cell Anemia and the theorized reason for its prevalence among people of African descent. • Heterozygotes (said to have sickle-cell trait) are usually healthy but may suffer some symptoms • About one out of ten African Americans has sickle cell trait, an unusually high frequency of an allele with detrimental effects in homozygotes • Individuals with one sickle-cell allele have increased resistance to the malaria parasite, which spends part of its life cycle in red blood cells. • In tropical Africa, where malaria is common, the sickle-cell allele is both a boon and a bane. • Homozygous normal individuals die of malaria and homozygous recessive individuals die of sickle-cell disease, while carriers are relatively free of both. • The relatively high frequency of sickle-cell trait in African-Americans is a vestige of their African roots. Section 14.4

45. 6. Why do you think a lethal recessive allele would be more reproductively successful than a lethal dominant allele? • A lethal recessive allele will most likely be the most successful because it can “hide” in heterozygote carriers. In this way, it can be passed on without killing the individual with the gene. Section 14.4

46. 7. What characteristic of Huntington ’s disease makes it successful at being passed on even though it’s a dominant trait? What technology is helping to eliminate/lessen the effect of this factor? • The timing of onset of a disease significantly affects its inheritance • Huntington’s disease is a degenerative disease of the nervous system • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age • Once the deterioration of the nervous system begins the condition is irreversible and fatal Section 14.4

47. 8. What is a Multifactorial disease? Why is it so difficult to study the genetic basis of these diseases? • Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases Section 14.4

48. 9. What is the Genetic Information Nondiscrimination Act of 2008? Why was this Act necessary? • Recently developed tests for many disorders can distinguish normal phenotypes in heterozygotes from homozygous dominants. • These results allow individuals with a family history of a genetic disorder to make informed decisions about having children. • Issues of confidentiality, discrimination, and counseling may arise. • The Genetic Information Nondiscrimination Act, signed into US law in 2008, prohibits discrimination in employment or insurance coverage based on genetic test results. Section 14.4

49. 10. Describe the following types of fetal testing: • Amniocentesis • can be used from the 14th to 16th week of pregnancy to assess whether the fetus has a specific disease. • Fetal cells extracted from amniotic fluid are cultured and karyotyped to identify some disorders. • Other disorders can be identified from chemicals in the amniotic fluids. • Chorionic Villus Sampling • allows faster karyotyping and can be performed as early as the 8th to 10th week of pregnancy. • A sample of fetal tissue is extracted from the chorionic villi of the placenta. Section 14.4

50. 10. Describe the following types of fetal testing: • Ultrasound • uses sound waves to produce an image of the fetus • Fetoscopy • a needle-thin tube containing fiber optics and a viewing scope is inserted into the uterus. Section 14.4