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Observing Patterns in Inherited Traits Chapter 10.

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1 Observing Patterns in Inherited Traits Chapter 10

2 10.1 Mendel, Pea Plants, and Inheritance Patterns  By experimenting with pea plants, Mendel was the first to gather evidence of patterns by which parents transmit genes to offspring

3 Mendel’s Experiments

4 a Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (stamens). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels). b Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its stamens snipped off. This is one way to assure cross-fertilization of plants. c Later, seeds develop inside pods of the cross- fertilized plant. An embryo within each seed develops into a mature pea plant. d Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants. Fig. 10.3, p.154 carpelstamen

5 Animation: Crossing garden pea plants CLICK HERE TO PLAY

6 Producing Hybrids  Hybrids Offspring of a cross between two individuals that breed true for different forms of a trait  Each inherits nonidentical alleles for a trait being studied

7 Producing Hybrid Offspring

8 fertilization produces heterozygous offspring meiosis II meiosis I (chromosomes duplicated before meiosis) Homozygous dominant parent Homozygous recessive parent (gametes) Fig. 10.5, p.156

9 Heritable Units of Information  Genes Heritable units of information about traits Each has its own locus on the chromosome  Alleles Different molecular forms of the same gene  Mutation Permanent change in a gene’s information

10 Heritable Units of Information

11 b A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles. Fig. 10.4, p.155 a A pair of homologous chromosomes, both unduplicated. In most species, one is inherited from a female parent and its partner from a male parent. c A pair of alleles may be identical or not. Alleles are represented in the text by letters such as D or d.

12 Modern Genetic Terms  Homozygous dominant Has two dominant alleles for a trait (AA)  Homozygous recessive Has two recessive alleles (aa)  Heterozygote Has two nonidentical alleles (Aa)

13 Modern Genetic Terms  Dominant allele may mask effect of recessive allele on the homologous chromosome  Genotype An individual’s alleles at any or all gene loci  Phenotype An individual’s observable traits

14 Animation: Genetic terms CLICK HERE TO PLAY

15 Key Concepts: MODERN GENETICS  Gregor Mendel gathered the first indirect, experimental evidence of the genetic basis of inheritance  His meticulous work tracking traits in many generations of pea plants gave him clues that heritable traits are specified in units  The units, distributed into gametes in predictable patterns, were later identified as genes

16 10.2 Mendel’s Theory of Segregation  Mendel’s Theory of Segregation: Diploid organisms have pairs of genes, on pairs of homologous chromosomes Based on monohybrid experiments  During meiosis Genes of each pair separate Each gamete gets one or the other gene

17 Producing Hybrid Offspring  Crossing two true-breeding parents of different genotypes yields hybrid offspring  All F 1 offspring are heterozygous for a gene, and can be used in monohybrid experiments All F 1 offspring of parental cross AA x aa are Aa

18 A Monohybrid Cross  Crosses between F 1 monohybrids resulted in these allelic combinations among F 2 offspring Phenotype ratio 3:1 Evidence of dominant and recessive traits

19 F 2 Offspring: Dominant and Recessive Traits

20 Trait Studied Dominant Form Recessive Form F 2 Dominant- to-Recessive Ratio Seed shape Seed color Pod shape Pod color Flower color Flower position Stem length 2.98:1 3.01:1 2.95:1 2.82:1 3.15:1 3.14:1 2.84:1 787 tall 277 dwarf 651 long stem 207 at tip 705 purple 224 white 152 yellow428 green 299 wrinkled882 inflated 6,022 yellow2,001 green 5,474 round 1,850 wrinkled Fig. 10.6, p.156

21 Predicting Probability: Punnett Squares

22 female gametes male gametes Fig. 10.7a, p.157 a From left to right, step-by-step construction of a Punnett square. Circles signify gametes. A stands for a dominant allele and a for a recessive allele at the same gene locus. Offspring genotypes are indicated inside the squares. A AA A A AAA A A Aa aa a a a a a a a a

23 a A Aa A a aA aaAa A a aA aaAa AA A a aA aaa A female gametes male gametes Stepped Art Fig. 10-7a, p.157

24 Predicting F 1 Offspring

25 Fig. 10.7b, p.157 A AA Aa a A a aa Aa True-breeding homozygous recessive parent plant F 1 offspring b Cross between two plants that breed true for different forms of a trait. True-breeding homozygous dominant parent plant

26 Predicting F 2 Offspring

27 Fig. 10.7c, p.157 A Aa A a a AAAa aaAa F 2 offspring Heterozygous F 1 offspring Heterozygous F 1 offspring c Cross between heterozygous F1 offspring. aa AA

28 Key Concepts: MONOHYBRID EXPERIMENTS  Some experiments yielded evidence of gene segregation  When one chromosome separates from its homologous partner during meiosis, the pairs of alleles on those chromosomes also separate and end up in different gametes

29 Animation: Monohybrid cross CLICK HERE TO PLAY

30 Animation: F2 ratios interaction CLICK HERE TO PLAY

31 Animation: Testcross CLICK HERE TO PLAY

32 10.3 Mendel’s Theory of Independent Assortment  Mendel’s Theory of Independent Assortment: Meiosis assorts gene pairs of homologous chromosomes independently of gene pairs on all other chromosomes Based on dihybrid experiments  Pairs of homologous chromosomes align randomly at metaphase I

33 Independent Assortment in Meiosis I

34 Fig. 10.8, p.158 One of two possible alignmentsThe only other possible alignment c Possible combinations of alleles in gametes: b The resulting alignments at metaphase II: a Chromosome alignments at metaphase I: A a AB AbabaB a aa a a bb b b b b AA AA bb AABB BB BB aa aa aa b b b b B BB B B B AA AA

35 Dihybrid Experiments  Start with a cross between true-breeding heterozygous parents that differ for alleles of two genes (AABB x aabb)  All F 1 offspring are heterozygous for both genes (AaBb)

36 Mendel’s Dihybrid Experiments  AaBb x AaBb  Phenotypes of the F 2 offspring of F 1 hybrids were close to a 9:3:3:1 ratio 9 dominant for both traits 3 dominant for A, recessive for b 3 dominant for B, recessive for a 1 recessive for both traits

37 Results of Mendel’s Dihybrid Experiments

38 Fig. 10.9, p.159 parent homozygous recessive for white flowers, short stems Gametes at fertilization parent homozygous dominant for purple flowers, tall stems Meiosis, gamete formation in true-breeding parent plants Possible genotypes resulting from a cross between two F 1 plants: meiosis, gamete formation All F 1 plants are AaBb heterozygotes with purple flowers and tall stems. meiosis, gamete formation

39 Key Concepts: DIHYBRID EXPERIMENTS  Some experiments yielded evidence of independent assortment  During meiosis, the members of a pair of homologous chromosomes are distributed into gametes independently of all other pairs

40 Animation: Dihybrid cross CLICK HERE TO PLAY

41 10.4 Beyond Simple Dominance  Other types of gene expression Codominant alleles Incomplete dominance Epistasis Pleiotropy

42 Codominant Alleles  Both expressed at the same time in heterozygotes Example: Multiple alleles in ABO blood typing

43 Fig , p.160 Phenotypes (Blood type): Genotypes: AABBO or ABAOBOOO BBAA or

44 Animation: Codominance: ABO blood types CLICK HERE TO PLAY

45 Incomplete Dominance  An allele is not fully dominant over its partner on a homologous chromosome Both are expressed Produces a phenotype between the two homozygous conditions

46 Incomplete Dominance

47 Fig , p.160 Cross two of the F1 plants, and the F2 offspring will show three phenotypes in a 1:2:1 ratio: homozygous parent (RR) homozygous parent (rr) heterozygous F1 offspring (Rr) x RRRr rr

48 Animation: Incomplete dominance CLICK HERE TO PLAY

49 Epistasis  Interacting products of one or more genes affect the same trait

50 Fig , p.161 9/16 walnut3/16 rose3/16 pea1/16 single comb F2 offspring: RRpp (rose comb)XrrPP (pea comb) F 1 of spring:RrPp(all walnut comb) RrPp RRPP, RRPp, RrPP, or RrPp RRpp or RrpprrPP or rrPprrpp X

51 Animation: Comb shape in chickens CLICK HERE TO PLAY

52 More Epistasis

53 Fig , p.161 EBEbeBeb EB Eb eB eb EeBb black EeBB black EEBb black EEBB black EEBb black EeBB black EeBb black Eebb chocolate EeBb black EEbb chocolate EeBb black Eebb chocolate eeBB yellow eeBb yellow eebb yellow eeBb yellow

54 Animation: Coat color in Labrador retrievers CLICK HERE TO PLAY

55 Pleiotropy  A single gene may affects two or more traits Example: Marfan syndrome

56 Animation: Pleiotropic effects of Marfan syndrome CLICK HERE TO PLAY

57 10.5 Linkage Groups  All genes on the same chromosome are part of one linkage group  Crossing over between homologous chromosomes disrupts gene linkages

58 Linkage Groups and Meiosis  During meiosis, genes relatively close together on a chromosome tend to stay together Few crossover events occur between them  Genes that are relatively far apart tend to assort independently into gametes Greater frequency of crossing over between them

59 Linkage and Crossing Over

60 Fig , p.162 Parental generation F 1 offspring Gametes Most gametes have parental genotypes A smaller number have recombinant genotypes meiosis, gamete formation All AaCc acAC × A A AAaa a a c c cc C C C C

61 Animation: Crossover review CLICK HERE TO PLAY

62 10.6 Genes and Environment  Environmental factors may affect gene expression in individuals Example: Temperature and fur color

63 Animation: Coat color in the Himalayan rabbit CLICK HERE TO PLAY

64 Elevation and Plant Height

65 Fig , p.163 c Mature cutting at low elevation (30 meters above sea level) b Mature cutting at mid-elevation (1,400 meters above sea level) a Mature cutting at high elevation (3,060 meters above sea level) Height (centimeters)

66 Predation and Body Form

67 10.7 Complex Variations in Traits  Polygenic Inheritance When products of many genes influence a trait, individuals of a population show a range of continuous variation for the trait

68 Continuous Variation

69 Fig a, p.164 Range of values for the trait This red graph line of the range of variation for a trait in a population plots out as a bell-shaped curve. Such curves indicate continuous variation in a population. Number of individuals with a measurable value for the trait

70 Animation: Continuous variation in height CLICK HERE TO PLAY

71 Variations in Gene Expression  Gene interactions and environmental factors affect most phenotypes Gene products control metabolic pathways Mutations may alter or block pathways

72 Key Concepts: VARIATIONS ON MENDEL’S THEME  Not all traits have clearly dominant or recessive forms  One allele of a pair may be fully or partially dominant over its nonidentical partner, or codominant with it

73 VARIATIONS ON MENDEL’S THEME (cont.)  Two or more gene pairs often influence the same trait, and some single genes influence many traits  The environment also influences variation in gene expression

74 Video: One Bad Transporter and Cystic Fibrosis CLICK HERE TO PLAY

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