Observing Patterns in Inherited Traits

1. Observing Patterns in Inherited Traits Chapter 13

2. Terms and Concepts • Gene • Heritable unit of information about traits • One gene generally codes for one protein • In a diploid cell there are pairs of genes • One of the pair on each of the homologous chromosomes • Locus • Location of a gene on the chromosome

3. Terms and Concepts • Allele • Different molecular forms or traits of the same gene • Arise by mutation • A permanent change in a gene and in the information it carries • Diploid cells have two alleles for each gene

4. Terms and Concepts • Types of alleles • Dominant • Its trait is always expressed • Masks the effect of a recessive allele • Represented with a capital letter in inheritance problems (A) • Recessive • Expressed only when paired with another identical recessive allele • Its trait is masked by dominant alleles • Represented with a lower case letter in inheritance problems (a)

5. Terms and Concepts • Combinations of alleles • Homozygous condition • Homologs carry the same allele • A pair of identical alleles belong to a true-breeding lineage • Individuals can be either homozygous recessive (aa) or homozygous dominant (AA) • Heterozygous condition • Homologs carry different alleles • Individuals are referred to as heterozygous (Aa) or hybrids (result of a cross between two different true-breeding individuals)

6. Terms and Concepts • Gene expression • Process by which a gene’s information is converted to a structural or functional part of a cell • Transcription of DNA to mRNA • Translation of RNA to protein • Determines traits

7. Terms and Concepts • Genotype • Particular alleles that an individual carries • Examples: AA, Aa, aa • Phenotype • Refers to an individual’s traits • Examples: color, shape, size, texture, etc.

8. Terms and Concepts • Genetic crosses • Two individuals are crossed and the resulting offspring are examined to determine inheritance patterns • P stands for the parents • F1 stands for the first-generation offspring of crossed P individuals • F2 stands for the second-generation offspring of intercrossed F1 individuals

9. Questions • Gene An individual’s traits • Locus Heterozygous • Dominant allele Second generation offspring • True-breeding Homozygous • Hybrids An individual’s genes • Genotype A trait that is always expressed • Phenotype Heritable unit of information • F2 generation Location of a gene

10. Genetic Crosses • The following slides will present a genetic cross demonstrating • the use of the above terminology • and the use of punnett squares

11. Genetic Crosses • Whether a person has attached or detached earlobes depends on a single gene with two alleles (We can name the gene with a letter “e”) • Dominant allele is detached ear lobes • Referred to as E (capital for dominant) • Recessive allele is attached ear lobes • Referred to as e (lower case for recessive)

12. Genetic Crosses • Each individual inherits one allele from each parent • Depending on what combination of alleles are inherited will determine the genotype and phenotype of the individual • Inherit two dominant alleles • Genotype = EE or homozygous dominant • Phenotype = detached earlobes • Inherit two recessive alleles • Genotype = ee or homozygous recessive • Phenotype = attached earlobes • Inherit one dominant allele and one recessive allele • Genotoype = Ee or heterozygous • Phenotype = detached earlobes • The dominant allele will always mask the recessive allele’s trait

13. Genetic Crosses • Punnett squares can be used to determine the probability of the genotypes and phenotypes of offspring of any given cross such as the following • If we crossed a homozygous dominant dad with a homozygous recessive mom, what would the offspring genotype(s) and phenotype(s) be?

14. Genetic Crosses • First you need to determine the genotype of the parents • Dad = homozygous dominant = EE • Mom = homozygous recessive = ee

15. Genetic Crosses • Second, determine what each parent’s gametes will be • Based on what we know about meiosis we can determine what allele the gametes will carry (see figure 10.5) • Remember that during meiosis homologous pairs are separated (anaphase I). One of the two alleles is on one of the homologs, the other is on the other homolog. Therefore, during meiosis one “E” will segregate into one gamete, while the other “E” will segregate into the other gamete • Dad’s gametes will be E and E • Mom’s gametes will be e and e

16. Genetic Crosses • Third, place the gametes in a punnett square • Dad’s go vertically in the first column • Mom’s go horizontally across the top e e E E

17. Genetic Crosses • Fourth, determine what the possible outcomes are if either of dad’s gametes fuses with either of mom’s eggs e e E EeEe E EeEe

18. Genetic Crosses • Fifth, determine the probability of the genotypes and phenotypes • Genotype possibilities are • EE, Ee, or ee • Count up how many out of four of each combination are in the punnett square EE : Ee : ee 0 : 4 : 0 Answer

19. Genetic Crosses • Fifth, determine the probability of the genotypes and phenotypes • Phenotype possibilities are • Detached or Attached • Count up how many out of the four of each trait are in the punnett square Detached : Attached 4 : 0 Answer

20. Genetic Crosses • Punnett Square Practice • Cross a heterozygous dad with a homozygous dominant mom • Ee X EE • Cross a heterozygous dad with a heterozygous mom • Ee X Ee • Cross a homozygous recessive dad with a heterozygous mom • ee x Ee

21. Gregor Mendel • Using pea plants Gregor Mendel determined inheritance patterns • Pea plants are self-fertilizing and so develop “true-breeding” varieties (homozygous) • Mendel could open a floral bud of a true-breeding plant and snip out its anthers (contains pollen grains). The buds can then be brushed with pollen from a different true-breeding plant. • Following observable differences between plants Mendel predicted that he would be able to follow certain traits and see if there were patterns in its inheritance.

22. Fig. 10-3, p.154

23. Gregor Mendel • Theory of Segregation • Diploid cells have pairs of genes, on pairs of homologous chromosomes • The two genes of each pair are separated from each other during meiosis, so they end up in different gametes • Mendel used monohybrid crosses to demonstrate segregation

24. Gregor Mendel: Monohybrid Cross • Pea flower color • Cross 1 • True-breeding purple flowering plants were crossed with true-breeding white flowering plants (these are the parental generation, P) • The offspring or F1 generation were all purple flowering • Cross 2 • The F1 generation were allowed to self-fertilize • The offspring or F2 generation had a ratio of 3 purple flowering plants to 1 white flowering plant

25. Gregor Mendel: Monohybrid Cross • Pea flower color • Mendel was able to infer that • Both parents must have two “units” of information • Each parent transferred one of their “units” of information to the offspring • The purple color dominated the white color • The recessive white color shows up in ¼ of the F2 generation

26. Homozygous dominant parent Homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 10-5, p.156

27. Gregor Mendel: Monohybrid Cross • Pea flower color • Purple is dominant = A • White is recessive = a • Genotypes • True-breeding purple genotype = AA • True-breeding white genotype = aa • Punnett square for cross 1 a a A AaAa A AaAa

28. Fig. 10-7b, p.157

29. Gregor Mendel: Monohybrid Cross • Pea flower color • F1 are allowed to self-fertilize • Punnett square for cross 2 A a A AA Aa a Aaaa

30. Fig. 10-7c, p.157

31. Fig. 10-6, p.156

32. Gregor Mendel • Test cross • A method of determining genotype • To determine the genotype of the F1 purple-flowering plants (could be AA or Aa) Mendel could cross them with true-breeding white-flowered plants (aa) • If the F1 is AA, then all of the flowers would be purple • If the F1 is Aa, then half of the flowers would be purple and half white • Try the crosses on a punnett square

33. Gregor Mendel • Theory of Independent Assortment • As meiosis ends, genes on pairs of homologous chromosomes have been sorted out for distribution into one gamete or another, independently of gene pairs on other chromosomes • This is due to random alignment during meiosis

34. Gregor Mendel • Theory of Independent Assortment • Mendel used dihybrid crosses to explain how two pairs of genes are sorted into gametes independently • The following slides will demonstrate the type of dihybrid crosses used

35. Gregor Mendel: Dihybrid Cross • Pea flower color AND plant height • Cross 1 • True-breeding purple flowering tall plants were crossed with true-breeding white flowering dwarf plants (these are the parental generation, P) • The offspring of F1 generation were all purple flowering tall • Mendel’s question was whether purple flowering would always be linked to tall or whether purple could go with dwarf and white with tall. Looking at the F2 generation from cross 2 answered his question.

36. One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: a a A A a a A b b B B B b B b b The resulting alignments at metaphase II: A A a a a a A A B b b B b b B B a a A A b a a b A A b B B b B B c Possible combinations of alleles in gametes: AB ab Ab aB Fig. 10-8, p.158

37. Gregor Mendel: Dihybrid Cross • Pea flower color AND plant height • Cross 2 • The F1 generation were allowed to self-fertilize • The offspring or F2 generation had a ratio of • 9 purple flowering tall plants • 3 purple flowering dwarf plants • 3 white flowering tall plants • 1 white flowering dwarf plant

38. Gregor Mendel: Dihybrid Cross • Pea flower color AND plant height • Mendel was able to infer that • Purple was not linked to tall and white was not linked to dwarf • The two different genes did in fact sort independently

39. Gregor Mendel: Dihybrid Cross • Pea flower color AND plant height • Purple = A and white = a • Tall = B and dwarf = b • Genotypes • True-breeding purple tall genotype = AABB • True-breeding white dwarf genotype = aabb • Punnett square for cross 1 abab AB AaBbAaBb AB AaBbAaBb

40. Gregor Mendel: Dihybrid Cross • Pea flower color AND height • F1 are allowed to self-fertilize • Possible gametes for AaBb are • AB, Ab, aB, ab

41. Gregor Mendel: Dihybrid Cross • Pea flower color AND plant height • Punnett square for cross 2 AB AbaBab AB AABB AABbAaBBAaBb AbAABbAAbbAaBbAabb aBAaBBAaBbaaBBaaBb abAaBbAabbaaBbaabb

42. Fig. 10-9, p.159

43. Questions • T or F: The Theory of Segregation states that the two genes of each pair stay together during meiosis • What type of cross was used to show Mendel’s Theory of Segregation? • What is a Punnett Square? • What genotype and phenotype ratios are seen in the F2 generation? • T or F: The Theory of Independent Assortment states that gene pairs sort independently • What type of cross was used to show Mendel’s Theory of Independent Assortment? • What genotype and phenotype ratios are seen in the F2 generation?

44. Beyond Simple Dominance • Mendel studied traits that have clear cut dominant and recessive forms • Some genes can have alleles that are codominant or incompletely dominant

45. Beyond Simple Dominance • Codominance • Non-identical alleles are both fully expressed even in heterozygotes • Blood type • IA and IB alleles are both dominant • They are always expressed • i is recessive • Genotype Phenotype IAIA and IAi Type A blood IBIB and IBi Type B blood IAIB Type AB blood (codominant, they are both expressed) ii Type O blood

46. Fig. 10-10, p.160

47. Beyond Simple Dominance • Incomplete dominance • One allele isn’t fully dominant over the other allele, so the heterozygote’s phenotype is somewhere between the two homozygotes • Snapdragon flower color • Red flowers = RR • White flowers = rr • Heterozygotes , Rr are pink

48. Fig. 10-11, p.160

49. Beyond Simple Dominance • Epistasis • Some traits are the results of interactions of two or more gene pairs • Labrador coat color • Gene encoding pigment • black is dominant to brown • Gene encoding deposition of pigment • Dominant allele promotes deposition of pigment • Recessive allele reduces deposition • The two genes work together to determine how much of what color pigment ends up in the coat

50. EB Eb eB eb EEBB black EEBb black EeBB black EeBb black EB EEBb black EEbb chocolate EeBb black Eebb chocolate Eb EeBB black EeBb black eeBB yellow eeBb yellow eB eeBb yellow eebb yellow EeBb black Eebb chocolate eb Fig. 10-13, p.161