southwesternexposure

1. Patterns of InheritanceLecture 10 http://www.southwesternexposure.com

2. Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B. Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

3. Outline • Pollinators and sexual reproduction • In an abbey garden • Pea plant experiments • Mendel’s hypotheses • Principles of inheritance • Probability and its applications • Inheritance of human traits • Family pedigrees • Single-gene recessive and dominant disorders • Words and terms to know • Possible test items

4. Sexual Reproduction in Flowering Plants • Pollinators are known as biotic agents that move pollen from the male to the female parts of flowers to enable sexual reproduction. • The gametes produced in meiosis are contained in the pollen grains and plant ovaries. • The most prolific pollinators are bees, especially domestic honey bees. • Bees have fuzzy bodies and an electrostatic charge that attracts and carries pollen as they search for nectar in travelling from flower-to-flower.

5. Honeybee and Pollen http://earthobservatory.nasa.gov Tiny hitchhikers on a domestic honeybee.

6. Wide Range of Pollinators • While many species of wild bees exist, domestic honeybees perform much of the pollination for agriculture. • Other pollinators include wasps, bats, hummingbirds, some reptiles, mammals, and some species of flies especially at higher elevations. Bats in flight at nightfall http://www.hillcountryadventaure.com

7. Waggle Dance http://www.jesuiscultive.com ‘Language of the bees’—how honeybees communicate the location of a nectar source. http://www.zbp.univie.ac.at

8. Yesterday, Today, and Tomorrow A more pastoral time http://www.newmediaexplorer.org Beekeeping today What does the future hold with colony collapse disorder? http://www.architecturalantiques.com

9. “The Buzz on Honeybees” Domestic honeybees: Bred from European stock introduced in the 1600s Inhabitants per hive: Up to 60,000 bees Agricultural crops tended: About 100 Effort to produce one pound of clover honey: Over 7,000 bee-hours U.S. honey yield (2006): 155 million pounds U.S. diet tied to honeybee services: 33 percent Annual value of pollination: $14.6 billion U.S. beekeepers reporting colony collapse disorder: At least 25 percent Possible causes: Insecticides, parasites, diseases, or a mix of stressors Native bees: They do tend a few crops including apple trees and alfalfa National Geographic Magazine, October 2007

10. We now turn to patterns of inheritance. http://www.retinaaustraliansw.com.au

11. Parakeets http://www.upload.wikimedia.org Parakeets are also known as budgies.

12. Parakeets • Parakeets are small, long-tailed parrots native to Australia, and kept as pets in many countries. • Wild parakeets typically have green underparts and yellow upperparts mixed with black stripes. • Geneticists call the physical traits most commonly found in the natural world, ‘wild-types.’ • Parakeets of other colors are raised in captivity—breeders can predict which colors will result because feather color is an inherited character-istic.

13. Breeding Parents: Wild-type x Wild-type Offspring: Wild-type Parents: Wild-type x Sky-blue http://whatbird.wildbird.com First-generation offspring: Wild-Type Second-generation offspring: 3/4 Wild-Type and 1/4 Sky-blue

14. In an Abbey Garden Gregor Mendel http://kentsimmons.uwinnipeg.ca

15. Gregor Mendel • Gregor Mendel was the first person to analyze patterns of inheritance in a systematic, scientific manner. • During the 1860s, he deduced the fundamental principles of genetics by breeding garden peas in an abbey garden in Brunn, Austria (now part of the Czech Republic). • He was strongly influenced by physics, mathematics, and chemistry in applying experimental techniques and mathematics to the study of pea plants. • Mendel’s studies, like the work of his contemporary Charles Darwin, are a classic in science.

16. Inherited Characteristics • Mendel postulated in a paper published in 1866 that parents pass on factors to their offspring that are responsible for inherited their charac-teristics. • He thought that these heritable factors retain their uniqueness from generation to generation—these factors are now known as genes. • Mendel’s work on inherited characteristics is a foundation of modern biology and genetics. Postulate = to make a claim; to assume or assert the truth.

17. Pea Plants Pea pods http://upload.wikimedia.org Pea flowers http://upload.wikimedia.org

18. Why Study Pea Plants? • Mendel chose garden peas for his experiments because they are easy to grown and have readily distinguishable varieties. • Just as important—he could exercise strict control over the transfer of pollen.

19. Pea Flower Stigma Anther Petals Stamen Filament Pistil Ovary The stamens produce pollen, which contain sperm cells. The stigma, pistil, and ovary make-up the carpel. http://www.mendelweb.org

20. Fertilization • Pea plants usually self-fertilize since the pollen from the stamens settle on the stigma of the same flower. • Mendel assured self-fertilization by covering the flower with a small bag so that no pollen grains from other pea plants could reach the stigma. • He also cross-fertilized by pollinating other pea plants using a small brush. • The precise parentage of the offspring could be controlled through the fertilization process.

21. True-Breeding Varieties • Mendel worked with his pea plants until he was sure he had true breeding varieties. • These are varieties in which many generations of offspring are all identical to their parents. • Mendel chose seven characteristics of pea plants to study, including flower color. • Each of the characteristics occurs in only two forms—for example, purple or white flowers.

22. Seven Characteristics Seed shape Flower position Seed color Flower color Pod shape Stem heighth http://mac122.cu.ac.jp Pod color Dominant to the left and recessive to the right for each characteristic.

23. Hybrids • The offspring of two different true-breeding varieties are called hybrids. • This cross-fertilization is referred to as hybridization, or simply a cross. • The parent plants are the P generation, and their offspring are the F1 generation. • When F1 plants self-fertilize or cross-fertilize, their offspring are the F2 generation.

24. Monohybrid Cross • A monohybrid cross occurs between parent plants that differ in only one characteristic. • The outcome between a pea plant with purple flowers and one with white flowers is a monohybrid cross. • In crossing pea plants with purple and white flowers, Mendel found that the F1 generation had purple flowers. • This is exactly what we saw for a wild-type parakeet bred with a parakeet of another color.

25. Disappearance and Reappearance • Was the inheritance factor for white flowers lost in the F1 generation of purple flowers? • By mating F1 plants, the answer was ‘no,’ one-fourth of the F2 plants had white flowers. • Mendel concluded that the inheritance factor for white flowers did not disappear in the F1 plants, although only the factor for purple flowers was expressed. • He deduced that the F1 plants must be carrying two inheritance factors for flower color (purple and white).

26. Observed Cross P generation: purple flowers x white flowers P x P fertilization F1 generation:purple flowers F1 x F1 fertilization F2 generation:3/4 purple flowersand 1/4white flowers A diagram of Mendel’s most basic experiment with the color of pea flowers.

27. Genotype and Phenotype • Mendel’s experiment led to a conclusion that have been confirmed many times by other biologists and geneticists. • The physical traits of an organism are its phenotype, and its genetic makeup is its genotype. An organism’s appearance does not always reveal its inherited traits, or genetic composition.

28. Mendel’s Hypothesis Alternative forms of genes, known as alleles, determine an organism’s inherited characteristics. An organism has two genes—one from each parent—for each inheritable characteristic. An egg or sperm carries only one allele for each inherited characteristic, which are then paired during fertilization. In each pair, the gene that is fully expressed is the dominant allele and the gene that has no noticeable effect is the recessive allele. Dominant alleles are represented by uppercase letters and recessive alleles by lowercase letters.

29. Punnett Square Parent 2 The Punnett square is a visual tool for showing all combinations of alleles of an inherited characteristic. P p P Pp PP Parent 1 pp p Pp Clockwise rotation by 45o P P Parent 1 Parent 2 PP p p Pp Pp PP—purple flowers Pp—purple flowers pp—white flowers pp Reginald C. Punnett http://www.epidemiology.ch

30. Crossing True Breeding Varieties P p Parent 1 Parent 2 Pp P p Pp Pp Pp All offspring will have a genotype of Pp and a phenotype of purple flowers.

31. Color Cross Revisited P generation: purple flowers x white flowers P x P fertilization F1 generation:purple flowers F1 x F1 fertilization F2 generation:3/4 purple flowersand 1/4white flowers

32. Three Generations of Pea Flowers pp PP Allelles of parents P plants (true-breeding varieties) p P Gametes produced by meiosis X Pp, Pp… Pp Alleles of F1 offspring F1 plants (hybrids) Gametes produced by meiosis p p P X P or or P P Phenotypic ratio 3 purple : 1 white F2 plants PP p p Pp Pp Genotypic ratio 1 PP : 2 Pp : 1 pp pp

33. Principle of Segregation • Mendel found the same type of inheritance pattern occurred for all seven characteristics of peas that he studied. • For true-breeding varieties, one parental trait disappears in the F1 genera-tion and then reappears in one-fourth of the F2 generation. • The underlying mechanism is known as Mendel’s principle of segregation.

34. Principle of Segregation • The principle states that pairs of alleles separate—or segregate—during meiosis, and that the fusion of gametes at fertilization creates allele pairs once again. • Research over the past 140+ years has shown the principle of segregation applies to all sexually-reproducing organisms.

35. Homologous Chromosomes • A pair of chromosomes, as we discussed last week, are said to be homo-logous. • In each pair, one homologous chromosome is from the female parent and the other is from the male parent. • Each homologous pair contains the same genes (such as for flower color) at the same locations, or loci.

36. Homologous Chromosome Fragments Dominant allele (C) P s C Recessive allele (c) P s c Genotypes: ss Cc PP Heterozygous Homozygous for the recessive allele Homozygous for the dominant allele Homo- = same Hetero- = different Genes are shown as three banded colors on the chromosome fragments. The letters for the three gene loci are arbitrary and are only used to convey the concept.

37. Other Pea Plant Characteristics • Mendel also studied seed shape and seed color, among other character-istics. • He found the allele for round seed shape (S) is dominant to the allele for wrinkled shape (s). • And he found the allele for yellow seed color (C) is dominant to the allele for green color (c).

38. Principle of Independent Assortment • What would result from a dihybrid cross, the mating of parents differing in seed shape and seed color? • Mendel found the yield ratios in the F2 generation were the same as the they would have been if seed shape and color were studied as separate monohybrid crosses. • Mendel’s principle of independent assortment states that each pair of alleles segregates independently of other pairs of alleles during gamete formation.

39. Testcross • A testcross is a mating between an individual of an unknown genotype and a homozygous recessive individual (Pp). • The purpose is to determine the unknown genotype by observing the F2 yields. • Mendel used testcrosses to determine if he had true-breeding varieties of pea plants. • Testcrosses are still used by geneticists in determining unknown genotypes P ? Parent 1 Parent 2 p ? Observe Observe Observe Observe

40. Probability • The segregation of allele pairs during gamete formation (meiosis) and the re-forming of allele pairs at sexual fertilization follow the rules of probability. • The same rules apply to tossing a coin, rolling a die, and drawing playing cards. • Just as in probability experiments, Mendel found he needed to obtain large sample sizes of F1 and F2 offspring to determine inheritance patterns since variation exists. Morgan Silver Dollar, 1895 (tails) http://z.about.com

41. Rule of Multiplication • In coin tossing, the probability of heads is 1/2 and the probability of tails is 1/2. • The two events sum to a probability of 1.0 since only two outcomes exist. • Each toss is an independent event since its outcome is unaffected by what happened during previous events. • When two coins are tossed simultaneously, the outcome for each coin is an independent event unaffected by the other coin. • The probability of such a simultaneous event (two heads with two coins) is 1/2 times 1/2, or 1/4. • The rule of multiplication also applies to independent events that occur in genetics.

42. Probability Visualized Pp female Pp male Meiosis F1 genotypes Egg (P or p) Sperm (P or p) (random chance in fertilization) http://www.rpi.edu 1/2 P 1/2 P PP 1/4 1/2 p p 1/2 Pp 1/4 Pp 1/4 F2 genotypes Yields: PP = 1/4 Pp = 1/4 + 1/4 = 1/2 pp = 1/4 pp 1/4

43. Complex Genetic Problems • The results of the rule of multiplication are the same as for a Punnett square. • Complex genetic problems can be solved by applying rules of probability to segregation and independent assortment. • The outcomes of trihybrid crosses involving three different characteristics can be readily determined using probability. • To analyze such a problem with the Punnett tool, would require 64 cells in an 3 x 3 x 3 cube.

44. Inheritance of Human Traits • Mendel’s principles apply to the inheritance of some human traits including those we will discuss next. • Each of these traits is the result of simple dominant-recessive inheritance at one gene locus. • The genetic basis of many human characteristics, such as eye color and hair color, are not as well-understood.

45. Dominant and Recessive Alleles • For the sake of this discussion, the dominant allele is indicated by A and the recessive allele by a. • Recall that phenotype represents the expressed or physical appearance. • The dominant phenotype results from the homozygous genotype, AA, or the heterozygous genotype, Aa. • The recessive phenotype results only from the homozygous genotype, aa.

46. Dominant Phenotype • ‘Dominant’ does not imply that a phenotype is necessarily more common in a population than a recessive phenotype. • Freckles, for example, are the result of a dominant allele, but they are not very common in the general population.

47. Earlobes http://www.jbhs.k12.nf.ca Which is dominant and which is recessive? http://nps.k12.nj.us

48. Hairline http://www.moe.gov.sg Which is dominant and which is recessive?

49. Tongue Roll http://www.moe.gov.sg Which is dominant and which is recessive? What about right- and left-handedness?

50. Handedness and Cerebral Lateralization Right-handed—the left hemisphere contains the processing areas for verbal and math abilities. Left-handed—the right hemisphere often contains the areas for verbal and math abilities. Handedness is not the result of a single gene, and is not fully-understood. From the medical journal, The Lancet http://www.answers.com